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Analysis of Recharge and Recirculation - Phase 2

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Analysis of Recharge and Recirculation - Phase 2
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The Recharge and Recirculation (RR) Phase 2 Report focuses on Edwards Aquifer responses to recharge as simulated by the Aquifer MODFLOW model. Using the model, aquifer water levels and springflows were evaluated by simulating recharge events at eight different locations across the project area. Numerous combinations of recharge timing, volume, and location were modeled. No specific source of recharge water was considered during Phase 2 analysis. The report contains numerous graphics to demonstrate the relative benefits of the scenarios modeled. -- Authors
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Description
The Recharge and
Recirculation (R&R) Phase 2 Report focuses on Edwards
Aquifer responses to recharge as simulated by the Aquifer
MODFLOW model. Using the model, aquifer water levels and
springflows were evaluated by simulating recharge events at
eight different locations across the project area. Numerous
combinations of recharge timing, volume, and location were
modeled. No specific source of recharge water was considered
during Phase 2 analysis. The report contains numerous graphics
to demonstrate the relative benefits of the scenarios modeled.
--
Authors



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Analysis of Recharge and Recirculation Edwards Aquifer Phase 2 Todd Engineers Emeryville, California May 2005 Edwards Aquifer Authority San Antonio, Texas

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Todd Engineers i Table of Contents Page Executive Summary....................................................................................................ES-1 Introduction................................................................................................................... ....1 Background.................................................................................................................1 Scope of Work............................................................................................................2 Background for Model Applications..........................................................................3 Model Summary and Limitations...............................................................................3 Springflow Responses to Enhanced Recharge...............................................................4 Volumetric Responses to Indi vidual Recharge Sites..................................................4 Volumetric Responses to Multiple Recharge Sites.....................................................6 Interpretation of Recharge Sites..................................................................................7 Recharge Effects on Springflows................................................................................7 Comal Springs.............................................................................................................8 Other Springs..............................................................................................................9 Effects of Variations in Recharge Rate.......................................................................9 Increase in Comal Springs with Type 2 Enhancement Recharge.............................10 Water Levels and Flowpath Analysis............................................................................11 Recharge Effects on Well J-17.................................................................................11 Flow Paths from Recharge Sites...............................................................................11 Management Scenarios for Comal Springs..................................................................12 Drought Effects on Comal Springs...........................................................................12 Management Scenarios with Recharge for Comal Springs......................................13 Fixed Volume Recharge Scenarios...........................................................................13 Type 2 Recharge Enhancement Scenarios................................................................14 Well Recirculation Recharge Scenarios....................................................................15 Critical Period Management Rules...........................................................................16 Conclusions.................................................................................................................... ..19 Summary of Findings................................................................................................19 Alternative Recharge Scenarios................................................................................20 Phase 3 Scope of Work.............................................................................................21 References..................................................................................................................... ...22 Appendix A : Response of Springs and Well J17 to Enhanced Recharge Cover Photo from: Alley, W illiam M., Reilly, Thomas E., and Franke, O. Lehn, Sustainability of Ground-Wa ter Resources, U.S. Geologi cal Survey Circular 1186, Denver, CO, 1999.

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Todd Engineers ii List of Tables Table 1. Impacts on Major Springs from Individual Recharge Sites Table 2. Supplemental Water Volumes to Major Springs from Multiple Recharge Sites Table 3. Maximum Spring Fl ow Increases by Recharge Table 4. Supplemental Water Volumes to Ma jor Springs from Recharge Sites Under Varied Recharge Regimes Table 5. Type 2 Recharge Structures Summary Table 6. Maximum Increases by Rechar ge in Well J-17 Water Levels From Enhanced Recharge Table 7. Comparison of No-Flow Conditi ons at Comal Springs for Recharge Scenarios Table 8. Comparison of Days Comal Springs is Dry in Various Recharge Scenarios on Recharge Model 1 Table 9. Comparison of the Number of Da ys in Each Stage of Critical Period for Various Recharge Scenarios List of Figures Figure 1. Location of Type 2 Recharge Sites Figure 2. Distribution of Recharge to Springs Figure 3. Comal Springs Baseline Figure 4. San Marcos Springs Baseline Figure 5. San Antonio Springs Baseline Figure 6. San Pedro Springs Baseline Figure 7. Leona Springs Baseline Figure 8. Impacts to Comal Springs from Variable Recharge at Lower Sabinal Figure 9. Impacts to San Marcos Springs fr om Variable Recharge at Lower Sabinal Figure 10. Impacts to San Antonio Springs fr om Variable Recharge at Lower Sabinal Figure 11. Impacts to San Pedro Springs fr om Variable Recharge at Lower Sabinal Figure 12. Impacts to Leona Springs from Variable Recharge at Lower Sabinal Figure 13. Comal Springs Enhancement from Combined Recharge Figure 14. Groundwater Elevation Well J-17 Figure 15. Impacts to Well J-17 from Vari able Recharge at Lower Sabinal Figure 16. Groundwater Flow Directions Figure 17. Comal Springs Baseline Flow Figure 18. Springflow in Comal Springs Figure 19. Impacts on Comal Springs from Focused Recharge Figure 20. Comal Springs Flow Recircul ation of Groundwater to San Geronimo Figure 21. Critical Period Management on Comal Springs Figure 22. Critical Period Manageme nt Rules and Focused Recharge

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Todd Engineers ES-1 Executive Summary Background In April 2004, Edwards Aquifer Authority (EAA) contracted with Todd Engineers to conduct a multi-phase study on enhanced rech arge and recirculation (R&R) strategies. Phase 1 of that work was completed in September 2004 (Todd Engineers, 2004). The Phase 1 report included a review of existing studies, an analysis of Edwards Aquifer hydraulics, installation and operation of the 2004 U.S. Geological Survey (USGS) MODFLOW model of the Edwards Aquifer, and a pplication of the model for test runs at two hypothetical recharge sites. Test runs indicated that longterm benefits measured in years can be achieved for Comal Springs flow by increasing aquifer recharge. Further, the magnitude and duration of increased springf low was found to vary significantly from site to site. Phase 2 Scope of Work Phase 2 of the R&R study analyzed th e magnitude and duration of increased springflow from enhanced recharge at eigh t sites considered for development by the South Central Texas Regional Water Pla nning Group (SCTRWPG, 2001). Objectives for Phase 2 included a comparison of impacts on a si te-to-site basis for both a single recharge event and yearly enhanced recharge events over time. To achieve these objectives, the approved scope of work for Phase 2 wa s designed with the following tasks: 1. Conduct individual single-year model recharge applications at Type 1 and Type 2 recharge sites as defined by SCTRWPG ( 2001) as shown in Figure 6 of the Phase 1 report. 2. Conduct model recharge applications as in Task 1 above assuming one or more sites have a constant annual recharge from 1946 to 2000. 3. Conduct model recharge applications as in Task 2 above assuming continuous annual recharge proportional to annual natural recharge from 1946 to 2000. 4. Determine for representative recharge site s how much recharge water emerges at the five major springs. 5. Study flow paths of recharge water in the vicinity of the Balcones Fault Zone and Knippa Gap. 6. Evaluate the effect of rech arge when annual pumpage is reduced to selected upper limits. 7. Analyze results of the above model i nvestigations and determine sites and amounts of recharge that appear to be most promising in sustaining Well J-17 levels and Comal Springs flows. 8. Prepare a report summarizing the above ta sks and serving as a proposal to the Edwards Aquifer Authority for work to be performed in Phase 3.

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Todd Engineers ES-2 Application of the USGS Model These analyses were conducted with the application of the recently-available USGS MODFLOW model of the Edwards A quifer (Lindgren, 2004) Although the model represents a much-improved tool with which to analyze R&R scenarios, numerous uncertainties are associated with the model. The model uncertainties and limitations were summarized in the Phase 1 report and are briefl y re-stated in this report to highlight model application obje ctives in Phase 2. In order to prevent the in troduction of more uncertainty in the Phase 2 analyses, all of the model applications began with a ba seline run of the unmodified USGS model, using USGS-generated inputs and boundary c onditions to define model-generated output of water levels and springflow for various ti me periods. Phase 2 applications of the model were used to generate similar data sets of water levels and springflow with modified inputs of recharge or pumping as described for each analysis. These outputs were then compared back to baseline conditions. Most of the results presented in this study are the difference between baseline and Phase 2 appli cations rather than a prediction of actual flows or levels. Enhanced Recharge Model Runs Given the uncertainties associated with the model and the approved scope of work, a series of model runs were devised to analyze the respons e of the aquifer and springflows to various scenarios of enhanced recharge. Preliminary model runs examined the impacts of adding 25,000 AF/yr of water, individually and cumulatively for each of the eight recharge sites previously anal yzed by SCTRWPG (2001). The amount of 25,000 AF/yr was selected for the initial model runs because it was within the range of average recharge amounts estimated for the eight rechar ge sites and was determined in Phase 1 to have a measurable impact on water levels and springflow. Applyi ng the same amount of recharge to each site allowed a comparison of the benefits at one site to the others. Subsequent model runs used actual estim ates for maximum recharge enhancement water under average and drought conditions for the eight recharge sites as summarized by Turner Collie & Braden and LBG Guyton in their 1999 work for EAA (Table 5). Enhanced recharge volumes we re applied both during cert ain months and continuously throughout the year. Although the recharge asso ciated with Type 2 projects may not be available year round, Type 1 projects coul d hold water for recharge when needed. Therefore, continuous recharge scenarios were a way of analyzing impacts from Type 1 projects in addition to Type 2 projects. The effects of adding recharge enhancement available during average conditions continuously were analyzed as well as varying the amount of enhanced recharge during drought conditions. The relationship of timing of recharge to response of springflow was analyzed by adding recharge at the eight sites for one y ear, five years and 27 years, covering various hydrologic time periods. Since the aquifer respon se to recharge was determined to behave uniformly after a certain period of time, th e entire time period of 1946-2000 did not have

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Todd Engineers ES-3 to be run continuously to analyze the res ponse for a given duration of recharge. As a result, the two halves of the model (1946-1973 and 1974-2000) or portions of these model halves were employed as appropriate for various analyses. Impacts of Enhanced Recharge On Springflow Enhanced recharge at each of the eight sites resulted in some increase in springflow at each of the five major springs, although impacts were related to distance to the spring and impacts on some springs were very small (Table 1 and Figure 2). As anticipated, recharge sites closer to Coma l Springs, including Lower Sabinal, Lower Hondo, Lower Verde, San Geronimo, and Cibol o, produced the grea test increase in springflow there, equivalent to about one-h alf of the total recharged amount. Remaining amounts were either discharged through other sp rings, contributed to aquifer storage, or were discharged out of the a quifer at unmonitored locations in the model. The easternmost site, Lower Blanco, was not effective at contributing to spri ngflow at any location except at nearby San Marcos Springs. Other sp rings were not affected due to the Lower Blancos downgradient location and proximity to San Marcos Springs. The two western sites, Indian Creek and Lower Frio produced the largest benefit to Leona Springs in an amount equivalent to about 45 perc ent of the total volume of enhanced recharge (Table 1). Recharge at th ese two sites was less effective at impacting Comal Springs, but did contribute an averag e of about 25 percent of the equivalent volume recharged under various r echarge scenarios. Volumes equivalent to 70 percent or more of the recharge volumes could be acc ounted for in springflow discharge for all except one (Indian Creek) of the recharge site s. For Indian Creek, an equivalent volume of only about one-half of the recharge, conti nuously applied, had been discharged at any of the springs even after a 27 year m odel run. These remaining volumes may be contributing to aquifer storage or, less likely, have been di scharged at an unmonitored location. Two smaller springs, San Antonio Springs and San Pedro Springs, appear to receive little benefit from the modeled enha nced recharge (Table 1 and Figure 2). San Antonio Springs benefits somewhat from rechar ge at centrally-located sites from Lower Sabinal to San Geronimo, but ot her sites are located either too far west or downgradient to the east. San Pedro Springs, a low yieldi ng and intermittent spring, gains little or no benefit from any of the recharge sites (Table 1 and Figure 2). Increases in springflow occur quickly in re sponse to recharge because of the rapid transmission of the pressure wave in the aqui fer. However, in most cases the actual water molecules that are rech arged remain in the aquifer for a long time. Volumetric increases in springflow were very similar for model runs that added water to only one recharge site at a ti me and model runs that added the same corresponding amounts of water cumulative in one run (Table 2). However, continuous recharge results in more uniform springflo w (Figures 8 through 12). Although variations in natural recharge are reflected somewhat in the variability of respons e to recharge at the

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Todd Engineers ES-4 springs, the effect is attenuate d. Increases in springflow are more closely related to the amount of enhanced recharge than to the variability of natural recharge. Although volumes equivalent to large pe rcentages of recharged water were discharged to springs, the enhanced recharge di d not appear to result in large flow rates. For the one-year, five-year, and 27-year model runs with 25,000 AF/yr enhanced recharge, flow rates at Comal Springs were increased a maximum yearly average of about 14 cfs, 23 cfs, and 27 cfs, respectively (San Geronimo site). On a continuing recharge basis, Comal Springs flow increased by 0.9 to 1.1 cfs for each 1,000 AF/yr of enhanced recharge into each of the Lower Sabinal, Lower Hondo, Lower Verde, San Geronimo, or Cibolo sites (see Table 3). Model runs that applied the previously -determined recharge amounts for average and drought conditions at each T ype 2 site (Table 5) showed similar relationships of recharge to springflow as seen in the r uns with 25,000 AF/yr constant rates. This indicates that the gain in flow is propor tional to the magnitude of the recharge. Water Levels and Flowpaths Enhanced recharge at seven of the eight si tes results in raising the water level in Well J-17. Recharge at Lower Blanco does not ap pear to impact the well due to the sites downgradient location. The well-established corr elation between water levels in Well J17 and Comal Springs flow is also seen in the model runs for enhanced recharge. A onefoot rise in water levels at J-17 is equivalent to an increa se in Comal Springs flow of about 5 cfs. The USGS model was used to map the di rections of water movement on a grid over the entire aquifer. Velocity vectors were created using a square grid of five cells (1.25 miles on a side). Excluding inactive cells this totals 3,395 vectors to analyze model-simulated groundwater flow directions. As there is little change in flow direction with time, any instant in the model after several months of r echarge will yield a representative picture of two-dimensional fl ow. Arrows representativ e of the flow field were placed on a map of the aquifer for review (Figure 16). Flows in the central part of the aquifer change directions sharply in response to flow through and around the Balcones Fault zone as previously shown by Maclay (1995). The convergence of western recharge water toward Leona Springs is clear ly demonstrated. In Bexar County, variable flow directions are influenced by concentrated local pumping. Benefits of Enhanced Recharge on Drought Effects In order to simulate severe drought conditi ons that incorporated decreased natural recharge (as occurred in the 1950s drought of record) and increased (current) pumping totals, the USGS model was modified. Natural recharge conditions from the first half of the model (1947-1973) were superimposed on the pumping and other conditions in the second half of the model (1974-2000). Subse quent runs with this modified model, referred to as Recharge Model 1, allowed the analysis of enhanced recharge during these

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Todd Engineers ES-5 extreme conditions. In the baseline run of Recharge Model 1, the number of days when Comal Springs was not flowing, during the drough t of record recharge totaled more than 1,200 days (more extreme than the actual dr ought of record when no-flow conditions occurred over approximately 185 days). Enhanced recharge at all of the sites reduced the number of simulated days with no flow, but the reduction was relatively small given the extreme condition of the simulation. Enhanced recharge at San Geronimo produced the greatest benefit, reducing the no-flow conditions by 271 days (20 %) under the 25,000 AF/yr./site recharge simulations. Because San Geronimo contains only a small amount of enhancement recharge under the previously-d etermined Type 2 analyses (Table 5), the effects were much smaller when those am ounts were simulated (reduction of 66 days compared to 271 days). Under these recharge conditions, Indian Creek, with one of the largest amount of previously-determined e nhancement recharge, reduced the no-flow days by 113 days, even though the site is far to the west. These runs illustrate that the magnitude of the recharge can overcome the long distances to the springs in terms of benefits. Well Recirculation Recharge To analyze a management strategy that involves the recirculation of groundwater from downgradient in the aqui fer to enhanced recharge s ites, a model simulation was devised involving all model wells within a 50-mile radius of Comal Springs. The total volume pumped from these 87 wells was in creased by 50 percent each month if the aquifer was not in critical period. This si mulation produced an average recharge amount of 976 AF/month. Although benefits in increased flow were observed at Comal Springs, the increase in flow rate amounted to less than 10 cfs mo st of the time. Critical Period Management Rules It was originally envisioned that mode l modules being prepared by Hydrogeologic for EAA would be available to facilitate model simulations that incorporated demand management and critical period management (DMCPM) rules. However, the modules were not in the needed format and were of limited use for analysis. An alternative approach was developed using numerous inte ractive model runs to adjust pumping according to EAA critical period rules. A lthough the DMCPM rules are independent of enhanced recharge, the two strategies combin ed were observed to significantly increase springflow and maintain Comal Springs flow longer than under baseline conditions. Alternative Management Scenarios With the responses of Comal Springs to various locations and magnitudes of enhanced recharge developed in Phase 2, it is possible to formulate a number of scenarios that potentially could provide sufficien t water to meet Comal Springs minimum requirements (depending on how they are defi ned). The determination of the quantity of water to be recharged depends not only on the hydraulic feasibili ty of the aquifer, investigated herein, but al so on the source of water and cost of recharge. These considerations lead to a large number of possi bilities, all of which necessitate evaluation

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Todd Engineers ES-6 of legal, political, and economic realities. The next phase will examine some of these alternatives and economic c onsiderations to gain insi ght into how to provide supplemental water to the Edwards Aquife r for the purposes of maintaining Comal Springs. To meet this objective, a proposed scope of work for Phase 3 is presented. Conclusions A summary of the prelimin ary findings of the Phase 2 analyses based on applications of the USGS MODFLOW model are briefly summarized as follows: 1. On a 5-year recharge basis, Comal Spri ngs flow increases by an amount of water equivalent to 45 to 54 percent of water recharged from the Lower Sabinal, Lower Hondo, Lower Verde, San Geronimo, or Cibolo sites. The remaining sites, Indian Creek, Lower Frio, and Lower Blanco, are le ss effective in terms of benefits to Comal Springs (see Figure 2). 2. A flow increase in Comal Springs resu lting from enhanced recharge at an individual site is independent of water r echarged at any other s ite (see Table 2). 3. Year-round recharge yields more uniform springflow than does seasonal recharge (see Figure 8). 4. On a continuing recharge basis, Comal Sp rings flow increases by 0.9 to 1.1 cfs for each 1,000 AF/yr of enhanced recharge in to each of the Lower Sabinal, Lower Hondo, Lower Verde, San Geronimo, or Cibolo sites (see Table 3). 5. Tributary runoffs reaching Indian Creek a nd Lower Blanco recharge sites are the largest of the eight sites but contribute least to Comal Springs flow (see Table 5). 6. Well J-17 shows an increase in water level of about 0.2 foot for each 1,000 AF/yr of continuous recharge into the Lower Sabinal, Lower Hondo, Lower Verde, or San Geronimo sites. 7. Model results indicate that the observed Comal Springs drought of less than 185 days in 1956 would increase to 1,264 days of no flow under the hypothetical situation of 1950s recharge and 1980s pum page (see Table 8). If all annual average available recharge were applied to a single site (L ower Verde), the no flow period would be reduced to 512 da ys, and if DMCPM rules were also in effect, the period would be fu rther reduced to zero days.

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Todd Engineers 1 Analysis of Recharge and Recirculation Phase 2 Introduction This study focuses on the feasibility of implementing enhanced recharge and recirculation (R&R) for the Edwards Aquifer in order to develop an integrated and coordinated approach to water management that will provide adequate flows in Comal Springs during drought conditions. This report covers Phase 2 of a planned four-part study. Background The Edwards Aquifer Authority (EAA) is evaluating management options including enhanced artif icial recharge and recirculation of groundwater from one part of the aquifer to another as part of an inte grated management approach for the Edwards Aquifer. In April 2004, EAA contracted w ith Todd Engineers to conduct a multi-phase study on artificial recharge a nd recirculation strategies. Phase 1 of that work was completed in September 2004 (Todd Engineers, 2004). The Phase 1 report included a review of exis ting studies, an analysis of Edwards Aquifer hydraulics, installation and operation of the 2004 U.S. Geological Survey (USGS) MODFLOW model of the Edwards Aquifer, and a pplication of the model for test runs at two hypothetical recharge sites. Test runs examined the response of the aquifer with a particular emphasis on springflow response at Comal Springs to enhanced recharge of 25,000 acre-feet (AF) at a Medina River recharge site and an Elm Creek recharge site. Model outputs of Comal Springs discharge wi th and without artifi cial recharge were compared for the analysis. Phase 1 test runs indicated that artifici al recharge at the Medina River site increased discharge at Comal Springs by an equivalent of about two-thirds of the recharge amount over an eight-year model r un. At the Elm Creek site, located fifteen miles west of Medina River, increases in di scharge were attenuated, perhaps by faults and flow through Knippa Gap. At this site, discha rge at Comal Springs increased by slightly more than 50 percent of the total artificial recharge volume during the eight-year model run. These results indicate that long-term bene fits measured in years can be achieved for Comal Springs flow by increasing aquifer rech arge. Further, the magnitude and duration of increased springflow vary significantly from site to site. These analyses were conducted with the 2004 USGS model, but ar e generally consistent with previous analyses of artificial recharge with former modeling tools.

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Todd Engineers 2 Scope of Work Results of Phase 1 indicate the need to quantify the magnitude and duration of response at Comal Springs to enha nced recharge at selected si tes. To address this need, the scope of work for Phase 2 included the following tasks: 1. Conduct individual single-year model recharge applications at Type 1 and Type 2 recharge sites as defined by SCTRWPG ( 2001) as shown in Figure 6 of the Phase 1 report. 2. Conduct model recharge applications as in Task 1, above assuming one or more sites have a constant annual recharge from 1946 to 2000. 3. Conduct model recharge applications as in Task 2 above assuming continuous annual recharge proportional to annual natural recharge from 1946 to 2000. 4. Determine for representative recharge site s how much recharge water emerges at the five major springs. 5. Study flow paths of recharge water in the vicinity of the Balcones Fault Zone and Knippa Gap. 6. Evaluate the effect of rech arge when annual pumpage is reduced to selected upper limits. 7. Analyze results of the above model i nvestigations and determine sites and amounts of recharge that appear to be most promising in sustaining Well J-17 levels and Comal Springs flows. 8. Prepare a report summarizing the above ta sks and serving as a proposal to the Edwards Aquifer Authority for work to be performed in Phase 3. The locations used in this R&R study ha ve been selected for proposed Type 2 retention structures. Type 2 st ructures are defined as struct ures located on the recharge zone, consisting of the unconfined portion of th e aquifer, and are designed to retain storm runoff allowing for direct infiltration. Simila rly, Type 1 structures have been designated upstream of Type 2 structures where they ar e intended to store tributary runoff; however, Type 1 structures are planned for streams outside of the Edwards Aquifer where water cannot be recharged directly into the aquifer. Therefore, th e focus of this study will be limited to Type 2 structures that permit water to be recharged into the aquifer. However, the recharge associated with Type 2 projects may not be av ailable year round while Type 1 projects could hold water for recharge when needed. The continuous recharge scenarios analyzed were a way of analyzing impacts fr om Type 1 projects in addition to Type 2 projects. Type 2 structures provide for di rect recharge and are normally dry, impounding water for only a few days or weeks following st orm events. With large recharge rates of 2 to 3 feet per day, the reservoirs minimize evaporation losses and maximize recharge. The relationship of timing of recharge to response of springflow was analyzed by adding recharge at the eight Type 2 sites for one year, five years and 27 years. Since the aquifer response to recharge wa s determined to behave uniform ly after a certain period of time, the entire time period of 1946-2000 did not have to be run con tinuously to analyze the response for a given durati on of recharge. As a result, the two halves of the model (1946-1973 and 1974-2000) or por tions of these model halves were employed as appropriate for various analyses.

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Todd Engineers 3 Model runs used actual estimates for maximum recharge enhancement water under average and drought conditions as summ arized by Turner Collie & Braden and LBG Guyton in their 1999 work for EAA (Tab le 5). Several runs were made by adding enhanced recharge which was based on the st ate of water level and springflow in the aquifer, indirectly based on natural recharge. In several model runs, recharge was varied based on the critical period thre shold (the criteria defined by EAA as the trigger for Stage I of critical period). During thes e select runs, if the aquifer was in critical period, either Drought Type 2 enhancement recharge or no enhanced recharge was used. Background for Model Applications A series of reports on recharge en hancement of the Edwards Aquifer was prepared by HDR Engineering beginning in 1991 (see references). A recent report (SCTRWPG, 2000) defined recharge sites based on the earlier reports. Most important are sites along the nor thern boundary of the Edwards Aquifer that show promise for augmenting recharge. The map in Figur e 1, based on Figure 2.21 in the SCTRWPG report, shows the locatio n of these particular sites. They include stream channels of Indian Creek, Lower Frio, Lower Sabinal, Lower Hondo, Lower Verde, San Geronimo, Cibolo, and Lower Blanco. For modeling purposes, it was assumed that recharge water percolates downward to the aquifer at each Type 2 site, located at the end of a naturally recharging streambed, over a length of ten cells (equivalent to 2.5 m iles) and a width of one cell (0.25 mile). Initial model runs were made assuming that 25,000 AF/yr of water were recharged at each individual site. The benefits of each recharge site were ev aluated in terms of supplemental water increasing flow at the fi ve major springs (Comal, San Marcos, San Antonio, San Pedro, and Leona Springs). Be nefits are first analyzed in terms of volumetric supplements of water to the springs and subsequently in terms of increased flows at the various springs. Model Summary and Limitations The Edwards Aquifer model, used to r un these R&R scenarios, was created by USGS and is documented in a Scientific Inve stigation Report (Lindgr en et al. 2004). The simulation is a finite difference model created using MODFLOW 2000. The model represents the latest information and con ceptualization of the Edwards Aquifer. The conceptual model was developed by a pa nel of advisors, the Ground-Water Model Advisory Panel (GWMAP). Members of th e GWMAP consisted of staff from various local organizations and recognized Karst expert s. The main purpose of the model was to develop an improved understanding of the aqui fer as well as to ev aluate the hydrologic responses to various al ternative proposals. The model is a regional model designed to evaluate variations in spring flow, regional water level changes, and relative co mparison of water management scenarios. The models calibration and tes ting confirm that it is a reas onable representation of the regional ground-water flow system. The m odel, however, cannot be used for local analysis, for example the drawdown effects of a pumping well. The simulation of the

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Todd Engineers 4 conduits as one cell wide also has considerable local scale effects. Because the locations of the conduits are uncertain, the local eff ects of the conduits in the model may not simulate the actual response in the natural system. The conduits simulated in the model as one cell wide (1,320 ft) are up to 50 times larg er than the estimated width of natural conduits occurring in the aquifer. The Edwards Aquifer model is a porous media model used to simulate a karst system. The model cannot simulate turbulent flow occurring in the conduits and locations of the simulated conduits ha ve a strong impact on the ar eas surrounding the conduits. The model, however, can predict regi onal variations in water leve ls and spring flow, although it probably should not be used to predict the fate and transp ort of particles of water or contaminants. The model provided a better simulation of the confined zone than the recharge zone. The model may be limited in predicti ons of head in the recharge area. The hydrogeology of the recharge zone is not well known and data for model construction were limited. The lack of fit in the rechar ge zone is not unique to this model; other previous models have seen similar problems. Future versions of the model may need to model the recharge zone with dual-porosity or as more than one layer to better simulate the varying storativity values. Given these model limitations, the model is a valuable to ol to look at volumetric flow responses in the confined zone, partic ularly the major springs. Although enhanced recharge is added in the recharge zone, the effects are measured in the confined zone through spring discharge and water levels at selected monitoring wells (J-17 and J-27). The observed spring discharge for Comal Springs was well matched by the simulated discharge in the model. Because the simula tions undertaken are consistent with the regional design of the model, the simulated volumetric flow rates in the aquifer are consistent and reproducible at the scale of the model. Springflow Responses to Enhanced Recharge Volumetric Responses to I ndividual Recharge Sites Model calculations were made for enha nced recharge over one year, over five years, and over 27 years. Results for the to tal volume of recharge at each of the eight recharge sites (Indian Creek, Lower Frio, Lower Sabinal, Lower Hondo, Lower Verde, San Geronimo, Cibolo, and Lower Blanco) are listed in Table 1. In Table 1A a one-year enhanced recharge of 25,000 AF is assumed to occur continuously for five months (March-July) in 1974 at a rate of 5,000 AF pe r month. Tabulated values provide the total volume of increased springflow from the 1974 recharge event over the ensuing 27 years (1974-2000). It should be noted that this does not involve molecules of recharge water flow through the aquifer and appearing as molecu les of spring water. Instead the recharge water increases the pressure in the aquifer; the transmission of this pressure in turn causes more groundwater near the spri ngs to be released.

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Todd Engineers 5 The increase in spring volume is based on calculating in the model the difference in springflow with and wit hout recharge. The second half of the model period (19742000) was studied rather than the entire model for convenience in model operation, for pumping conditions that are more representati ve of the current situation, and because a longer time period would have litt le effect on volumetric increases to springs. It is further assumed that actual natural recharge and pum ping occur throughout th e aquifer each year without modification. For illustration, note in Table 1A that recharge of 25,000 AF into Lower Sabinal increased discharge to Coma l Springs by 11,225 AF and a total increase of 24,250 AF at the five major springs during the 27 years. The remaining volume of 750 AF most likely remains in aquifer storage as the model provides no ot her major outlets or changes in pumping. In Table 1A, the Remaining Recharge columns show the volume of water in acre-feet and the percenta ge of 25,000 AF that has not increased flows to the five major springs during the 27-year. Note that generally an equivale nt volume of recharge is discharged at the five springs over the 27 years. At only tw o sites, Indian Creek (19.6% and Lower Frio (12.1%), does less than 90% of the volume recharged contribute to spring discharge. Because of their distance from major springs, a smaller and more delayed impact is anticipated. Almost all of the e quivalent volume of recharge water from San Geronimo influenced the five springs. The ne gative difference for Cibolo shown in Table 1A, suggesting that increased springflow ex ceeded the recharge volume, may be an anomaly of the model. San Geronimo a nd Cibolo, sites closest to Comal Springs, resulted in the largest impact to the majo r springs over the simulated 27-year period. A primary focus of this investigation is to determine enhanced recharge impacts to Comal Springs. The Table 1A data in th e Comal column indicate that the maximum benefit accrues from the San Geronimo s ite, 13,212 AF or 52.8% of the equivalent recharge volume. However, contributions from Lower Sabinal to Cibolo fall within a similar narrow range (44.9% to 52.8%). As c ould be anticipated, the western sites and Lower Blanco result in relatively small im pacts to Comal Springs discharge (24.6% of recharge volume for Indian Creek, 29.5% for Lower Frio, and 3.3% for Lower Blanco). Enhanced recharge in the western sites resu lted in more springflow for Leona springs, while enhanced recharge at Lower Blanco cont ributed to the flow at San Marcos Springs. It is apparent that di stance from recharge site to sp ring and the direction of groundwater flow govern the benefits of enha nced recharge on springflow. Table 1B summarizes results from the s econd set of model runs, where enhanced recharge of 125,000 AF was added to each si te over a five-year period, March 1974 to July 1978 (25,000 AF per year). Data are form atted similar to Table 1A. The difference between the increased discharge at the major springs and the total volume of enhanced recharge is summarized in the Remaining Recharge columns. As shown in Table 1B, the equivalent volume of recharge not discha rged from the aquifer is relatively small. Enhanced recharge at Cibolo results in the greatest equivalent recharge volume discharged in the springs, with only 0.2% of the recharge volume remaining in the aquifer. The most western site, Indian Creek shows the largest volume (21.9%) of enhanced recharge not flowing for the ma jor springs in the 27-year period.

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Todd Engineers 6 Increases in springflow at Comal Springs from the five-year recharge period given in Table 1B are proportional to those f ound in the one-year recharge data of Table 1A. In both cases, the maximum increase in Comal discharge, occurs from enhanced recharge at San Geronimo (67,864 AF or 54.3%). Centrally located recharge sites increase Comal Springs flow from 46.0% up to the 54.3% of the total recharge. Indian Creek and Lower Frio are less efficient (24.6% and 32.4%, respectively) while Lower Blanco has the least impact on Comal, contri buting only 3.3% of the equivalent recharge volume. Again, Leona Springs takes a large frac tion of western enhanced recharge water. The percentage of equivalent volume each s ite contributes to each spring is shown in Figure 2. Results for the third set of model runs are shown in Table 1C. Here recharge was assumed to be 25,000 AF/yr over a five-month period each year for 27 years extending from 1974 to 2000 for each individual recharge site. Thus 675,000 AF were recharged into each site. Similar to other model run results, springflow discharge is increased by a volume equivalent to more than one-half of total recharge. The Remaining Recharge columns in Table 1C shows the equivalent volume of recharge not accounted for by spring discharge. Values range from a low of 9.3% at Lower Blanco to 46.0% at Indian Creek. These results show much larger percen tages of water not reac hing the springs as compared to those previously described for s horter recharge durations. This may be due to recharge and discharge peri ods (27 years) being identical. Water is continuing to enter the aquifer up to the end of the 27-year peri od so that the attenuated response to later enhanced recharge has not fully de veloped throughout the aquifer. Volumetric Responses to Multiple Recharge Sites All of the model results presented in Table 1 assume recharge takes place at a single site for a specified period. Clearl y the maximum benefit to springs can be achieved with recharge originating simultaneously from multiple recharge sites. To evaluate the impacts from operation of multip le recharge sites, recharge was added simultaneously into each of the eight recharge sites at a rate of 25,000 AF per year per site for five years, totaling one million acr e-feet (8 sites x 25,000 AF/yr x 5 years = 1,000,000 AF). The comparison of recharge sites opera ting individually and simultaneously is tabulated in Table 2. Individual Sites, in Table 2, assumes that each recharge site operates independently and sums the volume of increased springflow over the 27-year period (summed from Table 1B). Multiple S ites, in Table 2, assumes that the eight recharge sites are operating together for the same five-year period (a new model run). The small percentage differences listed on th e bottom line of Table 2 indicate that the amount of increased springflow is nearly iden tical for either recharge scenario. This finding is significant because it suggests th at supplemental springf lows are additive and independent of the operation of recharge sites.

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Todd Engineers 7 Interpretation of Recharge Sites Analysis of the model runs documented in Table 1 leads to the conclusion that from a volumetric standpoint, water recharge d into the Lower Sabinal, Lower Verde, Lower Hondo, San Geronimo, and Cibolo will pr ove to be most efficient in increasing flow at Comal Springs. Fi gure 2 summarizes the impacts on Comal Springs from each recharge site as well as cont ributions to other springs. Th e percentage of equivalent recharge volume (from five years of rechar ge 1974-1979) that is discharged in Comal Springs during the 27-year period is shown on Figure 2A. Ranking them in relation to Comal Springs, San Geronimo is first, fo llowed by Lower Verde, Lower Hondo, Cibolo, and Lower Sabinal; however, the differences among these are small so that each should be considered as an important recharge site. Figure 2B shows the percentage of equivalent recharge volume (f rom five years of recharge 1974-1979) that is discharged in each of the major springs during the 27-year period. The western sites, Indian Creek and Lower Frio, have less of an impact on Comal b ecause they are subject to large losses to Leona Springs, while the eastern site, Lower Blanco, is not effective at Comal Springs because of its downgradient location from Comal Springs and its proximity to San Marcos Springs. Recharge Effects on Springflows The above analyses demonstrated that e nhanced recharge in the Edwards Aquifer can increase the volume of water discharged by Comal Springs and other springs. But equally important are the amount of time it takes each spring to respond and the duration of the response. This question of timing can be expressed as the increase in springs flows as a function of time. The MODFLOW mode l was used to calculate daily springflow both with and without enhan ced recharge. Taking the difference between these two allows the increase in flow to be shown as a time function. For baseline comparisons, model simulate d historic flows of the five springs without enhanced recharge are shown in Figur es 3 to 7. The differences between these base flows and calculated springflow from the enhanced model runs provide estimates for the benefits that can be achiev ed by recharge. It is important to note that differences are those between two sets of m odel output; therefore, any disc repancies between actual and modeled springflows are eliminated. Graphs of model outputs for various co mbinations of recharge sites and springs are presented in the appendix and provide the basis for the results pr esented in Table 3. In Appendix A graphs are ordered by each spring as follows: Comal (pages A1-A24), San Marcos (pages A25-A48), San Antonio (pages A49-A72), San Pedro (pages A73A96), and Leona Springs (pages A97-A120). For each spring the recharge sites appear in following the sequence: Indi an Creek, Lower Frio, Lower Sabinal, Lower Hondo, Lower Verde, San Geronimo, Cibolo, and Lower Blanc o. Differences in springflows are plotted from 1974 to 2000 for the three recharge scenarios: 25,000 AFY in 1974, 25,000 AFY for 1974-1978, and 25,000 AFY for 1974-2000. Water is r echarged uniformly from March to June in each year of enhanced recharge. All flow increases are plotted in cubic feet/second (cfs).

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Todd Engineers 8 The flow differences shown graphically in Appendix A, represent increases in flow caused by enhanced recharge. Individual daily dots generated by the model merge to form a solid line. Where a series of dots a ppears at a given time, these represent rapid changes in springflow that may be anomalie s in the model that may have no factual bases. Note that the verti cal scale varies from graph to graph in order to facilitate review. The maximum yearly average (highe st average of a 365-day period) flow increase for each spring, recharge site, and recharge scenario are shown in Table 3. Comal Springs Because of the importance of Comal Sp rings, both in terms of ecosystem support and as an indicator of aquifer response to recharge, model results for this spring are further analyzed. Springflow at Comal Springs as modeled without enhanced recharge is shown in Figure 3 for 1974-2000. Data indicate an average conti nuous flow over the 27year period of about 260 cfs with a range fr om 40 cfs to 500 cfs. An overall downward trend in flow occurs during this time interval. Enhanced recharge at Indian Creek does not increase these flows signi ficantly, adding 0.6, 3.1 cfs, a nd 9.7 cfs for the 1, 5, and 27year recharge periods respectively (Table 3) Recharge at Lower Frio increases Comal Springs flows somewhat w ith respective increases of 1.4, 6.1, and 13.5 cfs. Moving eastward, recharge at Lower Sabinal continue s the trend with resp ective increases of 8.4, 16.7, and 23.2 cfs. The well-defined saw-tooth fl ow pattern indicates a rapid response to the seasonal recharge (see graphs in Appendix A, A-7 th rough A-9). At Lower Hondo flows show gains of 5.7, 15.9, and 23.3 cfs, resp ectively. Similar results are seen at Lower Verde, with increases of 4.1, 14.1, and 22.9 cfs, respectively. Continuing eastward, San Geronimo recharge increases flow at Comal up to 12.9, 22.6, and 29.6 cfs, respectively. At this site, the maximum bene fit to springflow is reached within ten years and is stabilized thereafter (A-18). Comal Spring response to Cibolo is seen immediately, due to the short distance from r echarge site to spring (A-19). Springflow is increased up to 12.1, 19.6, 24.5 cfs. Recharge from Lower Blanco does not result in significant flow increases at Comal Springs, less than 2 cfs for all scenarios. This small contribution occurs because the recharge si te is downgradient of Comal Springs. San Marcos Springs, being closer, receives the most benefit from recharge at Lower Blanco. In summary, enhanced recharge at Lower Sabinal, Lower Hondo, Lower Verde, San Geronimo, and Cibolo are most beneficial with respect to increasing springflow at Comal Springs. Indian Creek and Lower Fr io sites are relatively inefficient at maintaining Comal Springs flow, while Lowe r Blanco recharge does not significantly impact Comal Springs. However, if a large vol ume of enhanced recharge were available at the two western sites, these could be as beneficial as those mu ch closer to Comal Springs. The combined 27-year scenario at al l eight sites results in a maximum increase in Comal Springs of 148 cfs. This findi ng does suggest that su bstantial springflow augmentation at Comal Springs can be ge nerated with Type 2 structures.

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Todd Engineers 9 Other Springs Enhanced recharge effects on springflow in the major springs other than Comal are summarized in Table 3. San Marcos Spring, historically a perennial spring (see Figure 4), receives little or no be nefit from enhanced recharge at any site except Lower Blanco. Because San Marcos Spring is loca ted east and downgradient of Comal Springs, Comal relieves most of the aquifer pressure created from enhanced recharge. Enhanced recharge at nearby sites is th e source of most of the incr eased San Marcos discharge from enhanced recharge. San Antonio Springs, an intermittent sp ring (see Figure 5), has been dry for a most of the time since 1984. It can benefit from centrally located recharge sites from Lower Sabinal to San Geronimo; other site s are located either too far west or downgradient to the east. San Pedro Springs, a low yielding and fr equently intermittent spring (see Figure 6), gains little or no benefit from any of the recharge sites. Mu ch of this can be attributed to the nearby and lower San Antonio Springs, wh ich yields several times as much water. Leona Springs with an average flow of about 60 cfs and almost always perennial (see Figure 7), is unique because of its location in southern Uvalde County and upgradient of Knippa Gap. Geologic evidence suggests that faults as well as igneous intrusions in northern Uval de and Medina Counties rest rict typical west to east groundwater flow, thereby di verting water southward through Knippa Gap. As a consequence substantial quantities of water are discharged from Leona Springs and are lost in terms of usefulness el sewhere in the aquifer. As indicated in Table 3, enhanced recharge at Indian Creek and Lower Frio have more of an impact on Leona Springs than Comal Springs. Enhanced recharge from Sa n Geronimo eastward does not significantly impact Leona Springs. Effects of Variations in Recharge Rate To improve understanding of how recharge rates affect spring flow, two additional model runs were conducted. The first of th ese changed the 25,000 AF/yr recharge from a 5-month intermittent application to a uniform 12-month interval for the Lower Sabinal site. The lines in gray on Figures 8 through 12 show increases in flow for the five major springs with 5-month recharge while the red lines indicate those with uniform recharge and show less annual fluctuation (typically about one-third as much). The overall contribution to each spring remains essentiall y the same; volumetric differences tabulated in Table 4A amount to 3 percent or less. The second model run again assumed a uniform annual recharge but reduced the rate to one-half or 12,500 AF/yr at the Lower Sabinal site. Lines in blue on Figures 8 through 12 illustrate the smalle r increases in springflow. The reduction in recharge reduces the enhanced springflow by 50 percent, as is numerica lly verified in Table 4B. Together these two runs indicate that the increase in springflow is proportional to the recharge quantity and to th e uniformity of recharge.

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Todd Engineers 10 The multi-year fluctuations shown on Figures 8 through 12 imply that even with uniform supplemental recharge, there is te mporal variability in the amount of water reaching the springs. The variable response is due to variable precipitation and natural recharge. Increases in natura l recharge from wet years raise groundwater levels, which affects the discharges at all the springs. Th e effect of natural recharge variation on the benefit of enhanced recharge at the springs is relatively minor. For Comal Springs (see Figure 8), oscillations are 1.5 to 2.5 cfs a bout a mean line, amounting to a 10 to 15 percent variation in flow increases. To illu strate the variability in natural recharge, consider conditions for two time periods 1990-1992 and 1993-1995. For these two time periods basin-wide natural recharge, from U. S. Geol ogical Survey data, was 1,706,000 AF and 506,000 AF, respectively. Although this va riation in natural recharge was more than a factor of three, the response of c ontinued enhanced recharge on Comal Springs flows caused differences of only 5 cfs. Increase in Comal Springs with Type 2 Enhancement Recharge Estimates of the long-term average water volumes that may be available for enhanced recharge at the eight recharge sites were summarized by Turner Collie & Braden with LBG Guyton (1999) and are pres ented in Table 5. This summary was based on HDRs previous work for the Trans-Texa s program (HDR 1998). Recognizing that the gain in flow at Comal Springs is proportional to the magnit ude of recharge, it is possible to make an approximate estimate of how much the sites collectively can enhance the flow of Comal Springs. Adjusting the flow increas es in Table 4 in rela tion to the recharge enhancements of Table 5 provides potentia l increases ranging from 3.5 cfs from San Geronimo recharge to 17 cfs from Lower Sa binal. Accumulating th ese contributions on the basis of contribution to springflow yields the enhancement graph of Figure 13. This shows that the largest contribution of 17 cf s would come from Lower Sabinal alone, while adding Cibolo would increase the maximum flow by 30 cfs. Adding each succeeding smaller benefit leads to the fact th at if all eight recharge sites were in operation, an average ongoing enhancement to Comal Springs would amount to about 71.5 cfs. This analysis is not suggesting a construction sequence of Type 2 projects. However, it illustrates that San Geronimo and Lower Blanco sites have marginal benefit, to Comal Springs, based on the volume of wate r the Type 2 structure would yield. The economics of each project must be considered as well as the relative benefits of where to add other source waters, such as recirculated spring water and imported water. These possibilities will be investigat ed in Phase 3 of this study. Table 5 also lists water volumes that ma y be available for recharge in drought conditions. These amounts ar e about one-half of the l ong-time average enhanced recharge, which would suggest a smaller bene fit to Comal Springs. But as indicated above temporal variations in annual rainfall and natural recharge have only a minor influence on continuing recharge from Type 2 si tes. If additional water were available to recharge, the enhancement to Comal Springs would be proportional to normal conditions. Thus, the drought only affects the quantity of available for re charge, not the hydraulics of the recharged volume in the system.

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Todd Engineers 11 Water Levels and Flowpath Analysis Recharge Effects on Well J-17 Well J-17, a monitoring well in the San An tonio metropolitan area, has served as a useful indicator not only for local water levels but also as a measure of pending drought. There is a good co rrelation between Well J-17 le vels and flows in Comal Springs so that the well is used to pr edict springflow. The MODFLOW model can generate groundwater levels at any location and therefore can be used to define impacts to Well J-17 under various scenarios of enhanced recharge. Model generated water levels at the well with no added recharge are show n on Figure 14. Water levels average about 670 ft above mean sea level (msl) and range from 620 ft to 700 ft. Using the same recharge scenarios described in previous sections, water levels at Well J-17 were determined for the 27-year period 1974-2000. Di fferences in water levels, expressed in feet of increased water levels, are pres ented on graphs in Appendix A (pages A-121 through A-144). Maximum rises of water levels in the well are summarized in Table 6. As shown on Table 6, enhanced recharge at seven of the eight sites results in higher water levels in Well J-17. Recharge at Lower Blanco does not appear to impact Well J-17 water levels, likely due to its downgradient location. Th e most significant impact on water levels (5.6 ft) occurs in respon se to recharge at the San Geronimo site. These results correspond to in creases in springflow shown in Table 3, where the greatest flow increase at Comal Springs (29.6 cfs) also originated from San Geronimo. The maximum response of water levels and spring fl ow to recharge in Tables 3 and 6, show that a one-foot rise in the Well J-17 is e quivalent to about a 5 cfs increase in Comal Springs flow. However, this correlation does not apply to recharge at the Cibolo and Lower Blanco sites, located down gradie nt of Well J-17. Effects on Well J-17 are minimal for recharge at the distant western si tes (Indian Creek and Lower Frio) and at the downstream eastern sites (Cibolo and Lower Blanco). Figure 15 shows the effects of continuous recharge on Well J-17 water levels. Note that the amplitude of annual fluctuati ons for continuous recharge over 12 months (the red line) is approximately one-third of that for the same amount of recharge applied over 5-months (the gray line), indicating that the more uniform the recharge, the more uniform the response. Also in Figure 15 the black line shows th at cutting the annual recharge in half (to 12,500 AF) reduces the we ll response also by one-half. Thus, as was the case with springflow, water levels at Well J-17 correlate with the magnitude and uniformity of recharge. Flow Paths from Recharge Sites Using the MODFLOW model for the Edwa rds Aquifer, general groundwater flow paths were evaluated. These flow lines ar e approximate because the model assumes a porous media aquifer with simulated high permeability conduits. To determine flow paths rather than flow times, velocity vector s were used to indicat e only the direction of flow. Actual flow velocities are highly variab le so that travel times for a given molecule of water from recharge to discharge in the aquifer are the order of days to hundreds of

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Todd Engineers 12 years. To define a flow line from a single point could involve an extended time series of model runs and the result is heavily dependent on locations of conduits in the model. A more practical approach to accomplish the sa me purpose is to map directions of water movement on a grid over the entire aquifer. Velocity vectors were created using a square grid of five cells (1.25 mi) on a side, am ounting to one in every 25 cells. Excluding inactive cells, this tota ls 3,395 vectors. As there is littl e change in flow direction with time, any instant in the model after several mo nths of recharge will yield a representative picture of two-dimensional flows. Based on model velocity vectors, Fi gure 16 illustrates groundwater flow directions for various locations over the confined portion of the Edwards Aquifer. Recharge sites and springs are also shown so as to indicate the general pattern of water migration relating to these inflow and outflow locations. Flows in the central part of the aquifer change directions sharply in re sponse to flow through and around the Balcones Fault Zone as previously shown by Maclay (Maclay, 1995). The c onvergence of western recharge water toward Leona Springs is clear ly demonstrated. In Bexar County variable flow directions are also influen ced by concentrated local pumping. Management Scenarios for Comal Springs Drought Effects on Comal Springs The USGS model was employed to dete rmine what effect current pumping together with a drought comparable to that of 1956 would have on Comal Springs. For this purpose, advantage was taken of the fact that the model was divided for convenience into two sections by the USGS to allow it to be run using the pr e-processor Groundwater Vistas. The first section contains the si mulation of the aquifer from 1947 through 1973 and the second section contains the simulation from 1974 through 2000. In order to simulate an intense drought, similar to the drought of record (1950s), actual natural recharge of the first half of the model (1947-1973) was used to replace the actual natural recharge of the second half of the m odel (1974-2000). Aside from this substitution of recharge, no ot her modifications were made for this hybrid model; initial head, boundary conditions, aquifer characteristic s, and pumping all remained the same in the second half of the model (1974-2000). By changing the recharge amounts in the second half of the model, pumping, boundary conditions and recharge are treated as independent values. In reality the quantit y, distribution, and timi ng of pumping and the boundary conditions are dependent on the fluctua tions of recharge. Also, as the resulting model no longer simulates a real period of time, the in itial head is an arbitrary starting point for these hypothetical model runs. To overc ome these issues in th e analysis, all runs are compared with a baseline model run or to each other. Subsequent model runs are used to assess the relative effects of recharge scenar ios. This allows an estimate of the relative aquifer response to a drought that caused a more serve impact than the impacts that occurred in the 1950s, due to the increase in pumping in the hypot hetical model than originally occurred.

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Todd Engineers 13 The hybrid model, referred to as Rechar ge Model 1 in the remainder of this report, indicates Comal Springs would not flow for 1,940 days (during the period reflected by 1947-1973 recharge and 1974-2000 pu mping) and with no flow occurring intermittently for 1,264 days in the 1952-1959 period as shown by the green line in Figure 17 and also in Table 7. This compares with a computed model no-flow period of 113 days in 1952-1959 period assuming actual rech arge and pumping (actual no-flow in 1956 was observed to around 185 days). It foll ows that repetition of the earlier drought with recent pumping would ha ve made the springflow more vulnerable in terms of the duration and frequency of no-fl ow periods. It should be remembered that this model result is based on hypothetical conditions; ne vertheless it suggests that the historic increase in pumping (from 320,000 AF/yr in 1956 to 500,000 AF/yr in 1996), and particularly the concentrati on of urban pumping upstream of Comal Springs, magnifies the potential for springflow depletion. The blue line in Figure 17 shows the modeled Comal Springs flows for actual recharge and pumping conditi ons in the 1974-2000 period, refe rred to as the Unmodified Model for the rest of the report. Management Scenarios with Recharge for Comal Springs Various management scenarios were tested to determine the effect of enhanced recharge on Comal Springs dur ing the drought of record us ing Recharge Model 1. In addition, similar analyses were conducte d for the drought of 1996 using unmodified second half of the original USGS model. The scenarios include: Adding a fixed volume of water to each recharge site Adding amounts of Type 2 enhancement developed for the Trans Texas program Recirculation of water pumped from wells in the aquifer Fixed Volume Recharge Scenarios As discussed in previous sections, each of the eight main r echarge sites affect Comal Springs in different ways. Because Lo wer Blanco has limited impact to the flow of Comal Springs, it was excl uded from further recharge scenarios. To examine the impact of the recharge sites during drought conditions, 25,000 acre-feet per year of enhanced recharge was added to one site for an individual mode l run. Scenarios using Recharge Model 1, responses can be gauged by the number of days Comal Springs has no flow. The effects of adding recharge are tabul ated in the second line of Table 7 and can be compared with the baseline model run results in the first line. It can be seen that all seven of the recharge sites reduce the no-flow days to be low the no-recharge base of 1,264 days (days of no flow during recharge simulation of 1952-1959); however, benefits overall are not significant. The San Geroni mo site produced the greatest benefit, lowering the no-flow days by 271 days to 993 da ys. Other sites that showed favorable impacts on Comal Springs include Lower Sabi nal, Lower Hondo, and Lower Verde.

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Todd Engineers 14 Additional model runs were conducted to analyze the effect of recharging 25,000 acre-feet per year (equivalent to 2,033 acre-feet per mont h) only during non-critical periods. Here a critical period is defined as when well J-17 level is below 650 ft or Comal Springs is flowing less that 220 cfs. The resu lts in terms of days of no flow in Comal Springs are shown on the third line of Table 7. These model runs were designed to look at the residual impact indivi dual sites may have after enha nced recharge has ended. The seven sites showed similar impacts to Comal ranging from 1,235 (Cibolo) to 1,183 (Lower Verde). The delayed response from the western sites offset the initial high volume response from the intermediate sites. Type 2 Recharge Enhancement Scenarios In the Trans Texas study, estimates we re developed for maximum recharge enhancement from Type 2 structures at each recharge site for both average and drought conditions (HDR, 1998). These quantities (lat er updated and summarized by Turner, Collie and Brandon and LBG Guyton), shown in Table 5, were used in a series of Recharge Model 1 simulations to determ ine the effects on Comal Springs both for average and drought conditions. A set of model runs using the aver age Type 2 enhancement volumes was performed, one on each Type 2 site during all tim e (both critical and non-critical periods) using Recharge Model 1. The resulting nu mber of days Comal Springs had no flow during the drought period (1952-195 9) appears in line four of Table 7. Those sites with larger enhancement recharge, by volume, had greater impacts on Comal Springs. Thus Indian Creek, with over 34,000 acr e-feet of enhancement rech arge per year, reduced no flow at Comal to 1,151 days, 113 days be low the base of 1,264 days, whereas San Geronimo with 3,000 acre-feet of recharge reduc ed it only 66 days. Recharge applied at Lower Sabinal resulted in the fewest numb er of no-flow days in Comal Spring, 1,108 days. Further scenarios were run using Type 2 recharge enhancement, but the enhanced recharge was varied based on the aquifer response. One set of scenarios evaluated applying average Type 2 recharge enhancemen t during months when the aquifer was not in critical period and no enhanced recharge du ring critical period. Applying no enhanced recharge during critical period yi elds no-flow days listed in li ne five of Table 7. Indian Creek proved to decrease the number of no fl ows days in Comal Springs by the most days (63 days). The pressure response from Indian Creek, the most western site, has a delayed response compared to the other sites. This delay combined with the large volume of enhancement recharge available results in the largest benefit to Comal Springs in this scenario. Another set of scenarios was si mulated using average Type 2 enhancement conditions when the aquifer was not in cr itical period and drought Type 2 conditions when the aquifer was in critical period. The resu lts of these scenarios ar e listed in line six of Table 7. Applying the drought Type 2 e nhanced recharge dur ing critical period decreased the number of no-flow days at Co mal Springs by 41 days (San Geronimo) to 95 days (Indian Creek). As expected, a pplying drought Type 2 enhanced recharge showed a greater benefit at Comal Springs th an adding no recharge du ring critical period. The greatest difference is seen in Lower Sabi nal, the number of no-flow days at Comal

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Todd Engineers 15 Springs decreased by an additional 64 days (from the no recharge in critical period scenario) due to the enhanced r echarge during critical period. Another set of model scenarios was perfor med with Type 2 enhancement recharge applied to each of the seven sites simu ltaneously (excluding Lower Blanco). Adding average Type 2 recharge enhancement volumes at all times resulted in only 591 days of no-flow at Comal Springs (Table 8, line 2). Figure 18 shows the baseline run of Recharge Model 1 compared with both continuous recharge of 25,000 AF/yr and Average Type 2 enhancement recharge added at each site simultaneously. A simulation was performed where average Type 2 enhancement recharge was applied only when the aquifer was not in critical period. In this s cenario no water was added duri ng critical period. The result reduced Comal Springs to 1,152 days without fl ow (Table 8, line 3). A third scenario (using all seven times simultaneously) was run where average Type 2 enhancement recharge was applied when the aquifer was not in critical period and drought Type 2 enhancement recharge was applied to each si te during critical period. The results are summarized in Table 8 (line 4). By adding the relatively small amount of enhanced recharge available during drought conditi ons, the benefit to Comal (measured by reduction of no-flow days from baseline simulation) was doubled. Comal Springs had 239 less no-flow days than the baseline simulation when drought Type 2 enhancement volumes were applied and only 112 less no-flow days when no enhanced recharge was applied during critical period. Additional model runs evalua ted the benefits to Comal Sp rings if Type 2 recharge enhancement volumes were transported to one site. Type 2 enhancement recharge for all eight structures (Table 5) was summed and th e total was applied to each of the selected seven sites using Recharge Model 1. In the model runs, average condition amounts were added during months when the aquifer was not in critical period and drought condition amounts were added when the aquifer was in cr itical period. These results appear in the seventh line of Table 7. Note in the table th at the minimum no-flow days decrease from the baseline by 752 days to 512 days, during the drought period (1952-1959) with all enhancement water applied to the Lower Verde site. Well Recirculation Recharge Scenarios Another management scenario that has b een suggested to maintain springflow at Comal is well recirculation. The concept invo lves pumping wells near the springs during times with ample springflow, transporting th at water to a nearby recharge site, and recharging it to provide supplemental water to Comal Springs. A simulation was run using all wells in the model within a 50-mile radius of Comal Springs. The total volume pumped from these 87 wells was increased by 50% each month when not in critical period and the additional volume of water a dded to the recharge site San Geronimo during the same month. San Geronimo was select ed as the recharge site because it is one of the closest sites to Comal Springs and was one of the best sites for flow enhancement at the springs. The recirculation scenar io was run in both Recharge Model 1 and Unmodified Model assumptions.

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Todd Engineers 16 In Recharge Model 1 the increased pump ing yielded an average of 537 AF per month when not in critical period. Recircula tion showed little impact on Comal springs, decreasing the number of no-flow days by four days (1,264 days in baseline runs to 1,260 days in recirculation run). A similar result was shown in Unmodified Model. The same 87 wells were affected and 976 AF per month, wh en the aquifer was not in critical period, was available for recharge. A larger volume of water was available to recharge as the pumping was greater during the 1996 drought be cause the pumping that in the second half of the model that overlaps with th e 1950Â’s drought (1979-1986) was lower. Benefits of the well pumping were negligible as shown in Figure 20 with changes in flow amounting to less than 10 cfs for both model conditions. Critical Period Management Rules Efforts to conserve Comal Springs and to avoid cessation of its flows as happened in 1956 can be accomplished by enhanced recharge as has been described above and also by reducing pumping. In pursuit of the latter approach the EAA has adopted a comprehensive plan, the Demand Management and Critical Period Management Rules (DMCPM rules), for reductions in well pum page throughout the aquifer when water levels fall to certain levels. Rules lim iting the amount of pumping are intended to increase aquifer levels and thereby maintain minimal flows in Comal Springs. Details of the plan are complex and quite specific, leading to a stepwise series of pumping limitations based primarily on water levels in Well J-17. In brief, the plan establ ishes rules for the four stage reductions in pumping as follows: San Antonio Pool Stage I occurs in the San Antonio pool when: o J-17 is less than 650 ft msl o San Marcos 5-day average di scharge is below 110 cfs o Comal 5-day average discharge is below 220 cfs Stage II occurs in the San Antonio pool when: o J-17 is less than 640 ft msl o San Marcos 5-day average discharge is below 96 cfs o Comal 5-day average discharge is below 154 cfs Stage III occurs in the San Antonio pool when: o J-17 is less than 630 ft msl o San Marcos 5-day average discharge is below 80 cfs o Comal 5-day average discharge is below 86 cfs Stage IV occurs in the San Antonio pool when: o J-17 is less than 630 ft msl for more than 30 days o J-17 is less than 627 ft msl

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Todd Engineers 17 Uvalde Pool Stage III occurs in the Uvalde pool when: o J-27 is less than 845 ft msl Stage IV occurs in the Uvalde pool when: o J-27 is less than 845 ft msl for more than 30 days o J-27 is less than 842 ft msl Management modules were developed by H ydrogeologic, a consultant to EAA, to simulate critical period management. Todd Engineers had originally planned to incorporate these modules in the analysis for th is report. However, due to the inability of the format to work with preprocessors, th eir use for these simulated scenarios was limited. Accordingly, an alternative approach was developed to simulate critical period management (DMCPM) rules. This appro ach was iterative and involved running the model multiple times, determining from mode l output when a stage of critical period is entered, and then adjusting pumping for the next stage. This iterative approach to simulate DMCPM rules began with identifying the wells that the rules would a ffect. Information about the wells such as location, county, and model cell were provided by EAA. Wells were divided into two pools, the San Antonio pool (Bexar, Comal, and Medina Counties) and the Uvalde pool (Uvalde County) and two use categories, irrigati on and non-irrigation. Because, pumping is simulated in monthly stress periods in the model modifications were made to pumping on a monthly basis. Pumping was changed on the first day of the month based on the highest DMCPM stage that occurred in the last 15 da ys of the previous month and the first 15 days of the current month. The affected pumping remained the same for the entire month. The first run of the iterative approach was a baseline scenario to determine when each critical period stage woul d be reached if the DMCPM ru les were not in effect. The baseline run identified when Stage I is triggered thereby causing the pumping to be decreased in the model based on the DMCPM rules from that point forward until the stage has ended. Additional runs were made with similar methodology by adjusting pumping based on the DMCPM stage until th e DMCPM rules were simulated over the course of the drought. Wate r rights were assumed equal to the amount pumped; consequently, the volume of water pumped in a stress period was decreased based on the particular DMCPM stage. The amount of pumping decrease for each DMCPM stage is detailed on the following page:

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Todd Engineers 18 Stage I o San Antonio wells decrea se non-irrigation 5% Stage II o San Antonio wells decrea se non-irrigation 10% Stage III o San Antonio wells decrease nonirrigation and irrigation 15% o Uvalde wells decrease nonirrigation and irrigation 15% Stage IV o San Antonio wells decrease nonirrigation and irrigation 23% o Uvalde wells decrease nonirrigation and irrigation 23% The DMCPM rules were applied during th e recharge of the drought of record using the eight-year period 1952-1959 in Rech arge Model 1 superimposed on current pumpage. As indicated in Figure 21, the rules made a substantial improvement in flow at Comal Springs: the spring remained dry fo r only 292 days during the drought compared to 1,264 days without DMCPM rules. It should be noted that there were other days when the spring went dry outside of the eight-year period. Tabulation of Recharge Model 1 run results for the four DMCPM stages appear in Table 9A. The DMCPM rules are independent of e nhanced recharge but when combined they can make a significant difference in flow at Comal Springs. It is noteworthy that, in the hypothetical Recharge Model 1, the DMCPM rules, together with recharge of all Type 2 projects at the Lower Verde site (see Figure 22), resulted in zero no-flow days at Comal Springs compared with decreased condi tions at Comal Springs from 292 days of no-flow with DMCPM rules with no enhan ced recharge (Table 8, lines 6 and 7). The DMCPM rules were also applied, in the Unmodified Model, to the drought that occurred in 1996. Results are summar ized in Table 9B. During the 1996 drought, the baseline run of Unmodified Model resulted in 20 days in Stage III; however, with application of the DMCPM rules, the aquife r did not enter Stage III (Table 9B). However, the implementation of the rules did not show significant improvement over other management scenarios such as using av erage enhancement from Type 2 structures when not in critical period. In summary, the implementation of the DMCPM rules had a large impact on the severe drought simulate d in Recharge Model 1 but had a much smaller impact on the shorte r less severe drought of 1996.

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Todd Engineers 19 Conclusions Summary of Findings The numerous model runs conducted in this Phase 2 of the study provide useful information on how the Edwards Aquifer responds to supplemental recharge and modifications in pumpage with particular emphasis on how these inputs and outputs of water impact flow in Comal Springs. Fo r convenience the chief findings based on the USGS MODFLOW model are brie fly summarized as follows: 1. On a 5-year recharge basis 45 to 54 per cent of water recharged from the Lower Sabinal, Lower Hondo, Lower Verde, San Ge ronimo, or Cibolo sites reaches Comal Springs. The remaining sites Indian Cree k, Lower Frio, and Lower Blanco are less effective (see Figure 2). 2. A flow increase in Comal Springs resulting fr om enhanced recharge at an individual site is independent of water recharged at any ot her site (see Table 2). 3. Year-round recharge yields more uniform spring flow than does seasonal recharge (see Figure 8). 4. On a continuing recharge basis Comal Spri ngs flow increases by 0.9 to 1.1 cfs for a recharge of each 1000 AF/yr into each of the Lower Sabinal, Lower Hondo, Lower Verde, San Geronimo, or Cibolo sites (see Table 3). 5. Tributary runoffs reaching Indian Creek a nd Lower Blanco recharge sites are the largest volumes of the eight sites (see Table 5) but contribute least to Comal Springs flow. 6. Well J-17 shows an increase in water leve l of about 0.2 foot for each 1000 AF/yr of continuous recharge into the Lower Sabina l, Lower Hondo, Lower Verde, or San Geronimo sites. 7. Model results indicate that the observed Comal Springs drought of less than 185 days in 1956 would increase to 1,264 days of no flow under the hypothetical situation of 1950s recharge and 1980s pumpage (see Table 8). If all annual average available recharge were app lied to a single site (Lower Verde), the no flow period would be reduced to 512 days, and if DMCP M rules were also in effect, the period would be further reduced to zero days.

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Todd Engineers 20 Alternative Recharge Scenarios With the responses of Comal Springs to various locations and magnitudes of recharge and pumpage presented in this repor t, it is possible to formulate a number of scenarios that could provide sufficient wate r to meet minimum flow requirements, as determined by EAA, for Comal Springs. The de termination of the quantity of water to be recharged depends not only on the hydraulic feas ibility of the aquifer, investigated herein, but also on the source and cost of recharge. Water from the drainage basins north of the Edwards Aquifer involves costs for Type 2 st ructures and perhaps in some cases also Type 1 structures. Recircul ation of Comal Springs flow and/or imported water from external sources generates costs for diversi ons, pumps, and pipeli nes. Excluding legal and political ramifications, which are beyond the scope of this study, a large number of alternative possibilities exist in cluding consideration of econom ic factors. Phase 3 of this study will consider these in an effort to identify realistic o pportunities for protecting Comal Springs.

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Todd Engineers 21 Phase 3 Scope of Work To gain an insight as how to provide supplemental water to the Edwards Aquifer for the purpose of maintaining Comal Springs flow under drought conditions, a systematic approach can be taken. Variable s include recharge sites, available water, minimum springflow, and costs. A proposed sc ope of work to provide such an evaluation in Phase 3 includes the following tasks: Task 1: For the Type 2 recharge sites of I ndian Creek, Lower Frio, Lower Sabinal, Lower Hondo, Lower Verde, San Geronim o, Cibolo and Lower Blanco, apply known average recharge rates plus impor ted or recirculated water that could be made available to determine alternative combinations that could supply Comal Springs on an ongoing basis with minimum flows (to be specifi ed by EAA for this analysis). Task 2: Apply the findings of Task 1 to external sources of water, either recirculated spring flows or imported from sources identi fied by SCTRWPG, to estimate costs based on known SCTRWPG costs prepared by HDR E ngineering for delivery of water to alternative recharge sites. Task 3 : Estimate the physical and economic fe asibility of guaran teeing Comal Springs flow by well injection of imported water direc tly into the aquifer rather than by use of Type 2 recharge sites. Task 4 : Prepare a report summarizing results of the above tasks and including a proposal to Edwards Aquifer Authority for Phase 4 of this study.

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Todd Engineers 22 References HDR Engineering, Inc. and Ge raghty and Miller, Inc., Nueces River basin regional water supply planning study, Phase 1, Vols. 1, 2, a nd 3, Nueces River Authority, et al., May 1991. HDR, Nueces River Basin Regional Water Supply Planning Study, Phase IIIRecharge enhancement, Nueces River Authority, November 1991. HDR, Guadalupe-San Antonio River basin rech arge enhancement study, Vols. I, II, and III, Edwards Underground Water District, September 1993. HDR, Nueces River Basin, Edwards Aquifer r echarge enhancement project, Phase IVA, Edwards Underground Water District, June 1994. HDR, Nueces River Basin, Edwards Aquifer r echarge enhancement project, Phase IVB, Technical memorandum, Combined impacts of Rio, Sabinal, Hondo, and Verde recharge enhancement projects on downstream water rights, December 1995. HDR, Guadalupe-San Antonio River Basin recharge enhancement study feasibility assessment, Trans-Texas Water Program, We st Central Study Area, Phase II, Edwards Aquifer recharge analyses, San Antonio River Authority et al., March 1998. Maclay, R. W., Geology and hydrology of the Edwards Aquifer in the San Antonio area, Texas, U. S. Geological Survey WRI Rept. 95-4186, 1995. Lindgren, R.J., Dutton, A.R., Hovorka, S.D., Wo rthington, S.R.H., and Painter, Scott, Conceptualization and Simulation of the Ed wards Aquifer, San Antonio Region, U.S. Geological Survey, Scientific In vestigations Report 2004-5277, 2004. South Central Texas Regional Water Planni ng Group, Regional water plan, Vol. III, Technical evaluations of water supply options, 2001. Todd Engineers, Analysis of recharge an d recirculation, Edward s Aquifer, Phase 1, Edwards Aquifer Authority, September 2004. Turner Collie & Braden with LBG Guyton, Technical evaluations of Edwards Aquifer region water supply options, Edward s Aquifer Authority, December 1999.

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Table 1A. One Year of Recharge (1974)Recharge SiteBeginsEndsTotal (AF)ComalLeona San Marcos San Antonio San Pedro Total AF% Indian CreekMar-74Jul-7425,0006,14411,6733781,56034520,100 490019.6 % Lower FrioMar-74Jul-7425,0007,37911,2544612,42646321,983 301712.1 % Lower SabinalMar-74Jul-7425,00011,2256,1157185,38880324,250 7503.0 % Lower HondoMar-74Jul-7425,00011,8094,9707535,61584723,994 10064.0 % Lower VerdeMar-74Jul-7425,00012,4554,6777935,23288124,037 9633.9 % San GeronimoMar-74Jul-7425,00013,2123,0988706,80096324,943 570.2 % CiboloMar-74Jul-7425,00012,3506919,4262,61632125,405 -405-1.6 % Lower Blanc o Mar-74Jul-7425,0008311122,28532523,164 18367.3 % Table 1B. Five Years of Recharge (1974-1978)Recharge SiteBeginsEndsTotal (AF)ComalLeona San Marcos San Antoni o San Pedro Total AF% Indian CreekMar-74Jul-78125,00030,72757,0581,8756,3961,61997,675 27,32521.9 % Lower FrioMar-74Jul-78125,00040,54259,8912,51411,2952,400116,643 8,3576.7 % Lower SabinalMar-74Jul-78125,00057,51930,4313,64422,3493,989117,932 7,0685.7 % Lower HondoMar-74Jul-78125,00061,91125,2403,92123,4934,260118,825 6,1754.9 % Lower VerdeMar-74Jul-78125,00064,66523,4304,08422,4624,314118,955 6,0454.8 % San GeronimoMar-74Jul-78125,00067,86415,5684,43628,9304,836121,635 3,3652.7 % CiboloMar-74Jul-78125,00062,3103,49447,22710,1431,592124,766 2340.2 % Lower Blanc o Mar-74Jul-78125,0004,06354108,43814825112,727 12,2739.8 % Table 1C. Twenty Seven Years of Recharge (1974-2000)Recharge SiteBeginsEndsTotal (AF)ComalLeona San Marcos San Antoni o San Pedro Total AF% Indian CreekMar-74Jul-00675,000113,464218,0226,63220,6025,725364,445 310,55546.0 % Lower FrioMar-74Jul-00675,000179,787252,84910,60435,5589,306488,104 186,89627.7 % Lower SabinalMar-74Jul-00675,000313,832135,93618,69065,03516,630550,123 124,87718.5 % Lower HondoMar-74Jul-00675,000330,952111,20619,71970,79317,862550,531 124,46918.4 % Lower VerdeMar-74Jul-00675,000327,25998,43019,48270,06617,772533,010 141,99021.0 % San GeronimoMar-74Jul-00675,000381,37370,94623,18484,15720,293579,953 95,04714.1 % CiboloMar-74Jul-00675,000327,75216,397232,06427,7756,534610,522 64,4789.6 % Lower Blanc o Mar-74Jul-00675,00021,956257589,225409101611,948 63,0529.3 % Enhanced RechargeIncreased Springflow (AF), 1974-2000Recharge Remaining Enhanced RechargeIncreased Springflow (AF), 1974-2000Recharge RemainingTable 1. Impacts on Major Springs from Individual Recharge Sites, 1/1/1974 12/31/2000 Enhanced Recharge Recharge Remaining Increased Springflow (AF), 1974-2000 Table 1

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Recharge SiteBeginsEndsTotal (AF)ComalLeona San Marcos San Antonio San Pedro Total AF% Individual Sites TotalMar-74Jul-781,000,000389,601215,165176,141125,21623,035929,15870,8427.6% Multiple Sites TotalMar-74Jul-781,000,000378,881207,155178,540126,88422,563914,02385,9778.6% Difference from Individual10,7208,010-2,399-1,66847215,135 Difference %2.8%3.7%-1.4%-1.3%2.1%1.6%Table 2. Supplemental Water Volumes to Major Springs from Multiple Recharge Sites, 1/1/1974 12/31/2000 Enhanced RechargeIncreased Springflow (AF), 1974-2000Recharge Remaining Table 2

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1 yr5 yr27 yr1 yr5 yr27 yr1 yr5 yr27 yr1 yr5 yr27 yr1 yr5 yr27 yr Indian Creek0.63.19.70.00.20.60.61.65.90.00.20.71.46.416.9 Lower Frio1.46.113.50.10.30.80.74.67.50.10.51.02.68.919.4 Lower Sabinal8.416.723.20.30.81.36.67.810.30.71.31.72.15.410.3 Lower Hondo5.715.923.30.20.81.45.28.011.00.41.21.71.24.18.6 Lower Verde4.114.122.90.20.81.42.07.911.40.31.11.60.93.57.8 San Geronimo12.922.629.60.51.21.69.210.313.31.01.72.20.92.75.4 Cibolo12.119.624.57.813.816.64.34.95.10.30.50.70.10.51.3 Lower Blanc o 1.51.82.048.453.156.50.00.10.20.00.00.00.00.00.0 Maximum calculated as highest 365-day average Recharge Site Comal Springs and Duration of RechargeTable 3. Maximum Spring Flow Increases by Recharge (Values in cubic feet/second) Recharge 25,000 AF/yr at each siteSan MarcosSan AntonioSan PedroLeona Table 3

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Table 4A. Comparison of months recharge is applied*Recharge applied to the Lower Sabinal recharge site only Months of Applied Recharge Total Amount (AF) ComalLeona San Marcos San Antonio San Pedro TotalDifference Difference % Mar-Jul (5 months)675,000313,832135,93618,69065,03516,630550,123124,87718.5% Jan Dec (12 months)675,000310,375134,95318,49267,00216,806547,628127,37218.9% Difference 3,457983198-1,967-1762,496-2,496 Difference %1.1%0.7%1.1%-3.0%-1.1%0.5%-2.0% Both runs applied 25,000 acre-feet of recharge per yearTable 4B. Comparison of amount of recharge applied*Recharge applied to the Lower Sabinal recharge site only Amount of Applied Recharge Total Amount (AF) ComalLeona San Marcos San Antonio San Pedro TotalDifference Difference % 25,000 af/yr675,000310,375134,95318,49267,00216,806547,628127,37218.9% 12,500 af/yr337,500156,15367,4379,29833,3558,212274,45563,04518.7% Difference 154,22267,5169,19433,6478,594273,17364,327 Difference %49.7%50.0%49.7%50.2%51.1%49.9%50.5% *Recharge applied 12 months per yearTable 4. Supplemental Water Volumes to Major Springs from Recharge Sites Under Varied Recharge Regimes, 1/1/1974 12/31/2000 (All values in acre-feet) Table 4

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Water Supply Option Number Type 2 Recharge Structure Maximum Pool Capacity (AF) Maximum Recharge Enhancement (AF/yr) Unit Cost ($/AF Recharge) Maximum Recharge Enhancement (AF/yr) Unit Cost ($/AF Recharge) Sustained Yield (AF/yr) Unit Cost ($/AF S. Yield) % Recovery Total Project Cost (Million $) EAA-02 (1)Cibolo Dam #150,00012,8491382,4747191,2691,4015126 EAA-02 (2)Lower Blanco50,00049,76612122,8212642,1652,7861078 EAA-02 (3)San Geronimo14,0003,2313941,4238955522,3083918 EAA-02 (4)Lower Sabinal35,00018,4001554,0127114,22567510540 EAA-02 (5)Lower Hondo28,0009,4202623,25076142,7029158336 EAA-02 (6)Lowr Verde24,0006,2202222,1906311,9866969119 EAA-02 (7)Lower Frio50,00014,4002855,0638105,39076110758 EAA-02 (8)Indian Creek165,00034,50037119,8906437,3611,73737151 416,000148,78622061,12353525,6501,27442427 Reference Turner Collie & Braden/LBG Guyton, 2000Table 5. Type 2 Recharge Structures Summary Drought Conditions 8 Projects Combined Average Conditions Table 5

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One YearFive YearsTwenty Seven Years Indian Creek0.10.61.8 Lower Frio0.31.22.6 Lower Sabinal1.63.24.4 Lower Hondo1.13.04.4 Lower Verde0.82.74.3 San Geronimo2.54.35.6 Cibolo0.81.52.1 Lower Blanc o 0.00.00.0 Maximum calculated as highest 365-day average Water Level Rise for Recharge Scenarios (feet) Recharge SiteTable 6. Maximum Increases by Recharge in Well J-17 Water Levels From Enhanced Recharge Table 6

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Enhanced Recharge Applied in Scenarios Indian CreekLower Frio Lower Sabinal Lower Hondo Lower Verde San GeronimoCibolo No Enhanced Recharge (Baseline)1,2641,2641,2641,2641,2641,2641,264 25,000 Ac-ft per year (all time)1,1771,1461,0431,0351,0419931,064 25,000 Ac-ft per year (non-CP only)1,2211,2151,2231,1951,1831,2131,235 Average Type 2 enhancement recharge (all time)1,1511,1621,1081,1531,1811,1981,150 Average Type 2 enhancement recharge (non-CP only)1,2011,2371,2371,2431,2441,2601,249 Average Type 2 enhancement recharge (non-CP) and Drought Type 2 enhancement recharge (CP) 1,1691,2021,1731,1911,2211,2231,215 All Type 2 enhancement recharge applied to only single site (Focused Recharge) 1,023886675571512532769 Note: Each recharge site evaluated separately with a site-specific model run. Model runs conducted on Recharge Model 1. Number of days Comal is Not Flowing for Each Recharge Scenario *Values should be used to compare between sites and scenarios not as absolute values Table 7. Comparison of No-Flow Conditions at Comal Springs for Recharge Scenarios (all units in days)Drought Period 1952-1959 Pumping Period 1979-1986 Table 7

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Enhanced Recharge Applied in ScenariosDrought (1/52 12/59) Baseline 1,264 Type 2 Enhancement All Sites (all sites, average volumes all time) 591 Type 2 Enhancement All Sites (all sites, average volumes non-CP only) 1,152 Type 2 Enhancement All Sites (Average condition during non-CP and Drought conditions during CP) 1,025All Type 2 Enhancement applied to only Lower Verde (Focused Recharge) 512 DMCPM Rules Only and Lower Verde Focused Recharge0 DMCPM Rules Only292 Recirculation 1,260 CP=Critical Period DMCPM =Demand Management Critical Period ManagementTable 8. Comparison of Days Comal Springs is Dry in Various Recharge Scenarios on Recharge Model 1 (all units in days)*Values should be used to compare between scenarios not as absolute values Table 8

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StageBaseline Average Type 2 Enhancement All Times Average Type 2 Enhancement Non-Critical Period Average and Drought Type 2 Enhancement RecirculationDMCPM Rules All Type 2 Enhancement in Lower Verde DMCPM Rules and Focused Recharge LV Stage 0 113258134190111222306658 Stage I137408135181140339473523 Stage II473767669742471849737772 Stage III246183170141251244270672 Stage VI1924127717851640192012391105269 Stage 0 131573192307139153323499 Stage I Stage II Stage III3043463030308530 Stage VI27322277265525562724271024792364 StageBaseline Type 2 Enhancement All Times Type 2 Enhancement Non-Critical Period RecirculationDMCPM Rules Stage 0 772051765699 Stage I188263161183183 Stage II23353184271236 Stage III2000110 Stage VI00000Table 9. Comparison of the Number of Days in Each Stage of Critical Period for Various Recharge Scenarios (all units in days) Table 9A. Comparison of Various Recharge Scenarios on Recharge Model 1 Table 9B. Comparison of Various Recharge Scenarios on Unmodified Model *Values should be used to compare between scenarios not as absolute values San Antonio Jan 1952Dec 1959 Jan 1996June 1997 San Antonio Uvalde Table 9

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PAGE 63

APPENDIX A Response of Springs and Well J-17 to Enhanced Recharge Todd Engineers Analysis of Recharge and Recirculation Phase 2 5/13/05

PAGE 64

Index of Enhanced Recharge Responses Page Numbers Recharge Scenario Comal Springs San Marcos Springs San Antonio Springs San Pedro Springs Leona Springs Well J17 Indian Creek Recharge 25,000 AFY 1974 A1 A-25 A-49 A-73 A-97 A-121 Recharge 25,000 AFY 1974-1978 A-2 A-26 A-50 A-74 A-98 A-122 Recharge 25,000 AFY 1974-2000 A-3 A-27 A-51 A-75 A-99 A-123 Lower Frio Recharge 25,000 AFY 1974 A4 A-28 A-52 A-76 A-100 A-124 Recharge 25,000 AFY 1974-1978 A-5 A-29 A-53 A-77 A-101 A-125 Recharge 25,000 AFY 1974-2000 A-6 A-30 A-54 A-78 A-102 A-126 Lower Sabinal Recharge 25,000 AFY 1974 A7 A-31 A-55 A-79 A-103 A-127 Recharge 25,000 AFY 1974-1978 A-8 A-32 A-56 A-80 A-104 A-128 Recharge 25,000 AFY 1974-2000 A-9 A-33 A-57 A-81 A-105 A-129 Lower Hondo Recharge 25,000 AFY 1974 A10 A-34 A-58 A-82 A-106 A-130 Recharge 25,000 AFY 1974-1978 A-11 A-35 A-59 A-83 A-107 A-131 Recharge 25,000 AFY 1974-2000 A-12 A-36 A-60 A-84 A-108 A-132 Lower Verde Recharge 25,000 AFY 1974 A13 A-37 A-61 A-85 A-109 A-133 Recharge 25,000 AFY 1974-1978 A-14 A-38 A-62 A-86 A-110 A-134 Recharge 25,000 AFY 1974-2000 A-15 A-39 A-63 A-87 A-111 A-135 San Geronimo Recharge 25,000 AFY 1974 A16 A-40 A-64 A-88 A-112 A-136 Recharge 25,000 AFY 1974-1978 A-17 A-41 A-65 A-89 A-113 A-137 Recharge 25,000 AFY 1974-2000 A-18 A-42 A-66 A-90 A-114 A-138 Cibolo Recharge 25,000 AFY 1974 A19 A-43 A-67 A-91 A-115 A-139 Recharge 25,000 AFY 1974-1978 A-20 A-44 A-68 A-92 A-116 A-140 Recharge 25,000 AFY 1974-2000 A-21 A-45 A-69 A-93 A-117 A-141 Lower Blanco Recharge 25,000 AFY 1974 A22 A-46 A-70 A-94 A-118 A-142 Recharge 25,000 AFY 1974-1978 A-23 A-47 A-71 A-95 A-119 A-143 Recharge 25,000 AFY 1974-2000 A-24 A-48 A-72 A-96 A-120 A-144

PAGE 65

Comal Difference Indian Creek 19740.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 66

Comal Difference Indian Creek 1974-19780.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 67

Comal Difference Indian Creek 1974-20000.00 2.00 4.00 6.00 8.00 10.00 12.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 68

Comal Difference Lower Frio 19740.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 69

Comal Difference Lower Frio 1974-19780.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 70

Comal Difference Lower Frio 1974-20000.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 71

Comal Difference Lower Sabinal 19740.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 72

Comal Difference Lower Sabinal 1974-19780.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 73

Comal Difference Lower Sabinal 1974-20000.00 5.00 10.00 15.00 20.00 25.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 74

Comal Difference Lower Hondo 19740.00 1.00 2.00 3.00 4.00 5.00 6.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 75

Comal Difference Lower Hondo 1974-19780.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 76

Comal Difference Lower Hondo 1974-20000.00 5.00 10.00 15.00 20.00 25.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 77

Comal Difference Lower Verde 19740.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 78

Comal Difference Lower Verde 1974-19780.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 79

Comal Difference Lower Verde 1974-20000.00 5.00 10.00 15.00 20.00 25.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 80

Comal Difference San Geronimo 19740.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 81

Comal Difference San Geronimo 1974-19780.00 5.00 10.00 15.00 20.00 25.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 82

Comal Difference San Geronimo 1974-20000.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 83

Comal Difference Cibolo 19740.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 84

Comal Difference Cibolo 1974-19780.00 5.00 10.00 15.00 20.00 25.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 85

Comal Difference Cibolo 1974-20000.00 5.00 10.00 15.00 20.00 25.00 30.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 86

Comal Difference Lower Blanco 19740.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 87

Comal Difference Lower Blanco 1974-1978-0.50 0.00 0.50 1.00 1.50 2.00 2.50 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 88

Comal Difference Lower Blanco 1974-20000.00 0.50 1.00 1.50 2.00 2.50 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 89

San Marcos Difference Indian Creek 19740.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 90

San Marcos Difference Indian Creek 1974-19780.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140 0.160 0.180 0.200 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 91

San Marcos Difference Indian Creek 1974-20000.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 92

San Marcos Difference Lower Frio 19740.000 0.010 0.020 0.030 0.040 0.050 0.060 0.070 0.080 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 93

San Marcos Difference Lower Frio 1974-19780.000 0.050 0.100 0.150 0.200 0.250 0.300 0.350 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 94

San Marcos Difference Lower Frio 1974-20000.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 95

San Marcos Difference Lower Sabinal 19740.000 0.050 0.100 0.150 0.200 0.250 0.300 0.350 0.400 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 96

San Marcos Difference Lower Sabinal 1974-19780.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 97

San Marcos Difference Lower Sabinal 1974-20000.000 0.200 0.400 0.600 0.800 1.000 1.200 1.400 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 98

San Marcos Difference Lower Hondo 19740.000 0.050 0.100 0.150 0.200 0.250 0.300 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 99

San Marcos Difference Lower Hondo 1974-19780.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 100

San Marcos Difference Lower Hondo 1974-20000.000 0.200 0.400 0.600 0.800 1.000 1.200 1.400 1.600 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 101

San Marcos Difference Lower Verde 19740.000 0.050 0.100 0.150 0.200 0.250 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 102

San Marcos Difference Lower Verde 1974-19780.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 103

San Marcos Difference Lower Verde 1974-20000.000 0.200 0.400 0.600 0.800 1.000 1.200 1.400 1.600 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 104

San Marcos Difference San Geronimo 19740.000 0.100 0.200 0.300 0.400 0.500 0.600 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 105

San Marcos Difference San Geronimo 1974-19780.000 0.200 0.400 0.600 0.800 1.000 1.200 1.400 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 106

San Marcos Difference San Geronimo 1974-20000.000 0.200 0.400 0.600 0.800 1.000 1.200 1.400 1.600 1.800 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 107

San Marcos Difference Cibolo 19740.000 1.000 2.000 3.000 4.000 5.000 6.000 7.000 8.000 9.000 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 108

San Marcos Difference Cibolo 1974-19780.000 2.000 4.000 6.000 8.000 10.000 12.000 14.000 16.000 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 109

San Marcos Difference Cibolo 1974-20000.000 2.000 4.000 6.000 8.000 10.000 12.000 14.000 16.000 18.000 20.000 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 110

San Marcos Difference Lower Blanco 1974-10.000 0.000 10.000 20.000 30.000 40.000 50.000 60.000 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 111

San Marcos Difference Lower Blanco 1974-19780.000 10.000 20.000 30.000 40.000 50.000 60.000 70.000 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 112

San Marcos Difference Lower Blanco 1974-2000-10.000 0.000 10.000 20.000 30.000 40.000 50.000 60.000 70.000 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 113

San Antonio Difference Indian Creek 19740.00 0.50 1.00 1.50 2.00 2.50 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 114

San Antonio Difference Indian Creek 1974-19780.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 115

San Antonio Difference Indian Creek 1974-20000.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 116

San Antonio Difference Lower Frio 19740.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 117

San Antonio Difference Lower Frio1974-19780.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 118

San Antonio Difference Lower Frio 1974-20000.00 5.00 10.00 15.00 20.00 25.00 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 119

San Antonio Difference Lower Sabinal 19740.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 120

San Antonio Difference Lower Sabinal 1974-19780.00 5.00 10.00 15.00 20.00 25.00 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 121

San Antonio Difference Lower Sabinal 1974-20000.00 5.00 10.00 15.00 20.00 25.00 30.00 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 122

San Antonio Difference Lower Hondo 19740.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 123

San Antonio Difference Lower Hondo 1974-19780.00 5.00 10.00 15.00 20.00 25.00 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 124

San Antonio Difference Lower Hondo 1974-20000.00 5.00 10.00 15.00 20.00 25.00 30.00 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 125

San Antonio Difference Lower Verde 19740.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 126

San Antonio Difference Lower Verde 1974-19780.00 5.00 10.00 15.00 20.00 25.00 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 127

San Antonio Difference Lower Verde 1974-20000.00 5.00 10.00 15.00 20.00 25.00 30.00 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 128

San Antonio Difference San Geronimo 19740.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 129

San Antonio Difference San Geronimo 1974-19780.00 5.00 10.00 15.00 20.00 25.00 30.00 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 130

San Antonio Difference San Geronimo 1974-20000.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 131

San Antonio Difference Cibolo 19740.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 132

San Antonio Difference Cibolo 1974-19780.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 133

San Antonio Difference Cibolo 1974-20000.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 134

San Antonio Difference Lower Blanco 1974-0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 135

San Antonio Difference Lower Blanco 1974-19780.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 136

San Antonio Difference Lower Blanco 1974-20000.00 0.20 0.40 0.60 0.80 1.00 1.20 J-74J-79J-84J-89J-94J-99 DateDifference 74

PAGE 137

San Pedro Difference Indian Creek 19740.00 0.01 0.02 0.03 0.04 0.05 0.06 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 138

San Pedro Difference Indian Creek 1974-19780.00 0.05 0.10 0.15 0.20 0.25 0.30 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 139

San Pedro Difference Indian Creek 1974-20000.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 140

San Pedro Difference Lower Frio 19740.00 0.02 0.04 0.06 0.08 0.10 0.12 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 141

San Pedro Difference Lower Frio 1974-19780.00 0.10 0.20 0.30 0.40 0.50 0.60 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 142

San Pedro Difference Lower Frio 1974-20000.00 0.20 0.40 0.60 0.80 1.00 1.20 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 143

San Pedro Difference Lower Sabinal 19740.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 144

San Pedro Difference Lower Sabinal 1974-19780.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 145

San Pedro Difference Lower Sabinal 1974-20000.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 146

San Pedro Difference Lower Hondo 19740.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 147

San Pedro Difference Lower Hondo 1974-19780.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 148

San Pedro Difference Lower Hondo 1974-20000.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 149

San Pedro Difference Lower Verde 19740.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 150

San Pedro Difference Lower Verde 1974-19780.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 151

San Pedro Difference Lower Verde 1974-20000.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 152

San Pedro Difference San Geronimo 19740.00 0.20 0.40 0.60 0.80 1.00 1.20 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 153

San Pedro Difference San Geronimo 1974-19780.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 154

San Pedro Difference San Geronimo 1974-20000.00 0.50 1.00 1.50 2.00 2.50 3.00 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 155

San Pedro Difference Cibolo 19740.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 156

San Pedro Difference Cibolo 1974-19780.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 157

San Pedro Difference Cibolo 1974-20000.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 158

San Pedro Difference Lower Blanco 19740.00 0.00 0.00 0.00 0.00 0.01 0.01 0.01 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 159

San Pedro Difference Lower Blanco 1974-19780.00 0.00 0.00 0.01 0.01 0.01 0.01 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 160

San Pedro Difference Lower Blanco 1974-20000.00 0.00 0.00 0.01 0.01 0.01 0.01 0.01 J-74J-79J-84J-89J-94J-99 DateDifference cfs

PAGE 161

Leona Difference Indian Creek 19740.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 162

Leona Difference Indian Creek 1974-19780.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 163

Leona Difference Indian Creek 1974-20000.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 164

Leona Difference Lower Frio 19740.00 0.50 1.00 1.50 2.00 2.50 3.00 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 165

Leona Difference Lower Frio 1974-19780.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 166

Leona Difference Lower Frio 1974-20000.00 5.00 10.00 15.00 20.00 25.00 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 167

Leona Difference Lower Sabinal 19740.00 0.50 1.00 1.50 2.00 2.50 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 168

Leona Difference Lower Sabinal 1974-19780.00 1.00 2.00 3.00 4.00 5.00 6.00 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 169

Leona Difference Lower Sabinal 1974-20000.00 2.00 4.00 6.00 8.00 10.00 12.00 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 170

Leona Difference Lower Hondo 19740.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 171

Leona Difference Lower Hondo 1974-19780.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 172

Leona Difference Lower Hondo 1974-20000.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 173

Leona Difference Lower Verde 19740.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 174

Leona Difference Lower Verde 1974-19780.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 175

Leona Difference Lower Verde 1974-20000.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 176

Leona Difference San Geronimo 19740.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 177

Leona Difference San Geronimo 1974-19780.00 0.50 1.00 1.50 2.00 2.50 3.00 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 178

Leona Difference San Geronimo 1974-20000.00 1.00 2.00 3.00 4.00 5.00 6.00 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 179

Leona Difference Cibolo 19740.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 180

Leona Difference Cibolo 1974-19780.00 0.10 0.20 0.30 0.40 0.50 0.60 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 181

Leona Difference Cibolo 1974-20000.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 182

Leona Difference Lower Blanco 19740.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 183

Leona Difference Lower Blanco 1974-19780.00 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.01 0.01 0.01 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 184

Leona Difference Lower Blanco 1974-20000.00 0.01 0.01 0.02 0.02 0.03 J-74J-79J-84J-89J-94J-99 DateDiffernce cfs

PAGE 185

J-17 Difference Indian Creek 19740 0.02 0.04 0.06 0.08 0.1 0.12 0.14 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 186

J-17 Difference Indian Creek 1974-19780 0.1 0.2 0.3 0.4 0.5 0.6 0.7 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 187

J-17 Difference Indian Creek 1974-20000 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 188

J-17 Difference Lower Frio 19740 0.05 0.1 0.15 0.2 0.25 0.3 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 189

J-17 Difference Lower Frio 1974-19780 0.2 0.4 0.6 0.8 1 1.2 1.4 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 190

J-17 Difference Lower Frio 1974-20000 0.5 1 1.5 2 2.5 3 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 191

J-17 Difference Lower Sabinal 19740 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 192

J-17 Difference Lower Sabinal 1974-19780 0.5 1 1.5 2 2.5 3 3.5 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 193

J-17 Difference Lower Sabinal 1974-20000 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 194

J-17 Difference Lower Hondo 19740 0.2 0.4 0.6 0.8 1 1.2 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 195

J-17 Difference Lower Hondo 1974-19780 0.5 1 1.5 2 2.5 3 3.5 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 196

J-17 Difference Lower Hondo 1974-20000 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 197

J-17 Difference Lower Verde 19740 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 198

J-17 Difference Lower Verde 1974-19780 0.5 1 1.5 2 2.5 3 3.5 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 199

J-17 Difference Lower Verde 1974-20000 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 200

J-17 Difference San Geronimo 19740 0.5 1 1.5 2 2.5 3 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 201

J-17 Difference San Geronimo 1974-19780 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 202

J-17 Difference San Geronimo 1974-20000 1 2 3 4 5 6 7 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 203

J-17 Difference Cibolo 19740 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 204

J-17 Difference Cibolo 1974-19780 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 205

J-17 Difference Cibolo 1974-20000 0.5 1 1.5 2 2.5 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 206

J-17 Difference Lower Blanco 19740 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 207

J-17 Difference Lower Blanco 1974-19780 0.005 0.01 0.015 0.02 0.025 0.03 J-74J-79J-84J-89J-94J-99 DateDiffernce ft

PAGE 208

J-17 Difference Lower Blanco 1974-20000 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 J-74J-79J-84J-89J-94J-99 DateDiffernce ft