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Streamflow report for the Quebrada Cuecha in Monteverde, Costa Rica : June 2004-April 2006

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Title:
Streamflow report for the Quebrada Cuecha in Monteverde, Costa Rica : June 2004-April 2006
Translated Title:
Reporte de corriente para la quebrada Cuecha en Monteverde, Costa Rica: Junio 2004-Abril 2006 ( )
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Book
Language:
English
Creator:
Yeung, June K. .. et al.
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Subjects / Keywords:
Water-supply--Costa Rica--Quebrada Cuecha
Water flow studies
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Books/Reports/Directories
Books/Reports/Directories
letter   ( marcgt )

Notes

Summary:
Study of streamflow and monitoring of it on the Quebrada Cuecha.
Summary:
Estudio de la corriente y monitoreo de la Quebrada Cuecha.
Language:
English

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usfldc doi - M37-00226
usfldc handle - m37.226
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SFS0000926:00001


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Streamflow Report for the Quebra da Cuecha in Monteverde, Costa Rica June 2004 – April 2006 Technical Report submitted to the Monteverde Institute Yeung, June K.1, Andrew J. Guswa2,4*, Amy L. Rhodes3,4 1 September 2006 1 Class of 2007, Picker Engineering Program, Smith College, Northampton, MA, USA 2 Assistant Professor, Picker Engineering Program, Smith College, Northampton, MA, USA 3 Associate Professor, Department of G eology, Smith College, Northampton, MA, USA 4 Research Associate, Monteverde Institute, Monteverde, Costa Rica corresponding author, aguswa@email.smith.edu

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 ii Executive Summary Monitoring streamflow on the Quebrada Cuecha in Monteverde, Costa Rica began in June 2004 and continues to this day. A stream gaging station consisting of a staff gage and pressure transducers was established to obtain a continuous record of discharge. Details of the measurement methodology are provided in Appendix A. Streamflow records from the following periods are included in this study: Period 1. 11 Jun – 1 Sep 2004 Period 2. 25 Jun – 28 Jul 2005 Period 3. 6 Jan – 1 May 2006 The first two periods fall during the rainy season, and investigators from Smith College were in Monteverde making hydrologic measurements for most of this time. During the third period, which straddles the transitional and dry seasons, two pressure transducers were used to record stage (for redundancy), and investigators from Smith College were on-site to make discharge measurements in January and April. While the focus of this report is on these three time periods, Appendix B contains a record of all direct discharge measurements, including those outside of these time periods. The distribution of flow rates for the Quebrada Cuecha is highly skewed due to a few large events of relatively short duration, and the streamflow data reflect the effects of both natural hydrologic processes and water withdrawals for human use and consumption. Average streamflows were 230 liters/second, 280 liters/second, and 100 liters/second for time periods 1, 2, and 3, respectively, and median streamflows were 210 liters/second, 240 liters/second, and 80 liters/second. The 7-day minimum flow is of particular utility in the assessment of water resources because it indicates the lowest flow in a stream or river. To determine the 7-day minimum flow, a 7-day moving average of discharge is calculated for a time period of interest, and the smallest value is selected. From 6-Jan-06 to 1-May-06, the 7-day minimum flow was 40 liters/s, which is less than one-third of what is observed in the rainy season. The data, statistics, and figures presented in this report reflect the conditions for the periods of measurement. Given the seasonal and year-to-year variability in precipitation, however, these data should not be presumed to be representative of streamflow for all time. Continued monitoring and measurement is necessary to fully charac terize streamflow for the Quebrada Cuecha.

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 iii Table of Contents 1 Introduction 1 1.1 Monteverde, Costa Rica 1 1.2 Smith College Involvement 1 1.3 Hydrological Concepts 2 1.4 Streamflow Monitoring of the Quebrada Cuecha 3 2 Methodology 3 3 Results 6 Acknowledgments 13 References 13 Appendix A: Methods for Discharge Determination 14 Appendix B: Discharge Measurements at QC200 17 Appendix C: Rating Curve Determination 18 Appendix D: Error Assessment for Discharge Predictions 20 Appendix E: Daily Average Streamflow Data 21

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 iv List of Figures Fig. 1. Map of Costa Rica. Fig. 2. The Rio Guacimal watershed and the San Luis watershed in Monteverde, Costa Rica are outlined with the dashed line. Quebrada Cuecha and the site of the stream gaging station, QC200, are located above the center of the map. Fig. 3-5. QC200 direct discharge measurements and rating curves for the three periods of interest. Fig. 6-8. Hydrographs and precipitation data. Fig. 9. Frequency distributions for QC200 streamflow. Fig. 10. Flow-duration curves. Fig. A1. Photo of flowmeter wand and digital meter. Fig. A2. Illustration of the numerical method used to calculate discharge through a channel cross section. Fig. A3. Photo of staff gage and of two types of pressure transducers (levelogger and barologger) installed at QC200. List of Tables Table 1. Summary of streamflow statistics. Table B1. Discharge measurements at QC200. Table C1. Summary of the rating-curve selections for QC200. Tables E1-E3. Daily average streamflow for the three periods of record.

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 1 of 23 1 Introduction 1.1 Monteverde Monteverde, Costa Rica (8448’ W Long., 1018’ N Lat.), is home to tropical montane cloud forests in the volcanic mountain range of the Cordillera de Tilarn. The rich biological diversity sustained by the regional climate and habitat has attracted many visitors to the community, and Monteverde has experienced an economic shi ft from agriculture and milk production to ecotourism over the past two decades. While the community has experienced tremendous growth, numerous efforts strive to promote sustainable development and preserve Monteverde’s valuable resources. Figure 1. Monteverde, Costa Rica, is located on the Pacific sl ope of the Cordillera de Tilarn. The Costa Rican climate is governed by the geographic position of the Intertropical Convergence Zone (ITCZ), a low-pressure region near the equator marked by heavy precipitation. The climate is typically divided into three seasons: the rainy season (May through October), when the ITCZ is positioned over Costa Rica, and the transitional (November through January) and dry (February through April) seasons when the ITCZ migrates south. Since water for supply is withdrawn directly from streams and springs, the seasonality of precipitation has major implications for the water resources of Monteverde. Although annual precipitation totaled 2404 mm in 2004 and 3791 mm in 2005, only 9% of the total precipitation in 2004 and 3% in 2005 fell during the dry season (Johnson et al. 2005, Guswa et al. 2006). An improved understanding and quantification of the temporal variability in streamflow can inform resource management and decision-making. 1.2 Smith College Involvement A hydrological and geochemical investigation of the Guacimal Watershed (Figure 2) was initiated in 2001 by Professor Amy Rhodes (Geology) and Professor Andrew Guswa (Engineering) of Smith College in Northampton, Massachusetts, USA. The overarching purpose of this project is to improve understanding of the hydrologic and geochemical processes at work in the region. Activities have included the installation of a meteorological station on the roof of

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 2 of 23 the Monteverde Institute (reports and data are available at www.mvinstitute.org and www.science.smith.edu/~aguswa/research.html ), the quantification of throughfall variability (Guswa and Rhodes, 2004), geochemical characterization of stream water (Rhodes et al., in press a), and determination of the importance of dry-season orographic precipitation to streamflow (Rhodes et al., in press b). Current projects involve mapping the geology of the region to determine the geologic and anthropogenic processes affecting stream chemistry and the investigation of rainfall-runoff relations hips to develop quantitative hydrologic models. Figure 2 The Rio Guacimal watershed and the San Luis watershed in Monteverde, Costa Rica are depicted with the dashed boundaries. Triangles indicate sites of stream sample collection and squares indicate sites of precipitation collection and measurement. The Quebrada Cuecha and the site of the stream gaging station, QC200, are located above the center of the map. 1.3 Hydrological Concepts Water flowing in a stream or river originates as precipitation, and the area of land that drains to a given point on a stream is known as the drainage basin or watershed associated with that point. The different paths taken by water as it moves to a stream channel affect both timing and magnitude of the stream response. During a precipitation event, some water moves quickly into the channel, leading to a rapid response in water level and discharge. Elevated streamflow induced by a precipitation event in this way is known as stormflow. Some of this stormflow is

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 3 of 23 generated by water that quickly runs off over the ground surface. Other water infiltrates into the ground and makes its way to the stream more slow ly. Between precipitation events, streamflow is sustained by the release of this stored water. Streamflow that is not directly associated with a specific precipitation event is known as baseflow. The fate of rain falling on a land surface depends on factors such as land cover, soil hydraulic properties, and the geomorphology of the drainage basin. For example, the stormflow response for a basin with thick, vegetative cover and well-drained soil might be small, as much of the rain is absorbed by the soil. For the same storm, the rapid stormflow response for a steep basin with large areas of saturated or impervious surfaces could be much greater as it is more conducive to runoff. The balance between rain that is stored in a watershed and rain that causes a rapid but shortlived response affects both the vulnerability of a community to flooding and the long-term supply of water to its streams. In Monteverde, flooding is less of an issue as the topography is steep and the stream channels are deeply incised. Due to the seasonality of precipitation, streamflow during the dry season is strongly dependent on the slow drainage of water that arrived during the rainy season. 1.4 Streamflow Monitoring of Quebrada Cuecha The Quebrada Cuecha becomes the Rio Guacim al when it crosses under the road in Monteverde (Figure 2). As part of our scientific investigations, discharge on this stream has been measured since June 2004. A gaging station consisting of a staff gage and pressure transducers for acquiring continuous stage (water level) measurements has been set up and maintained just upstream of the road at a site named QC200. Upstream of this monitoring point, the area of land from which water drains to the Quebrada Cuecha is approximately 1.7 km2. Within this watershed, water is withdrawn for supply, some of which is transferred out of the basin. This report summarizes the streamflow behavior at QC200 from June 2004 through April 2006, and these records reflect both natural hydrologic processes and the effects of the withdrawals. 2 Methodology Stream discharge is the volume of water flowing through a channel cross section per unit of time; it is the product of the channel velocity and the cross-sectional area (Equation 1). Discharge = Average Velocity Area (1) While discharge can be determined by measuring depth and velocity at discrete points across a channel, the method is time consuming and impractical for long-term, continuous monitoring of streamflow. Water level (stage) is a more easily measured parameter. Measuring stage and discharge concurrently enables the development of correlations between these variables. Continuous records of stage can then be converted to discharge using these correlations known as rating curves. Detailed methods of acquiring stage and discharge records and the equipment involved are documented in Appendix A. Changes in the channel profile and morphology in the vicinity of the stream gaging station could alter the stage-discharge relationship developed for a channel. Therefore, rating curves must be reassessed from time to time by making di rect measurements and comparing the data with the existing rating curve. If a shift is apparent, a new rating curve must be developed. From June 2004 to April 2006, manual discharge measurem ents were performed primarily in January,

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 4 of 23 June, and July. A record of all direct measurements of discharge at QC200 is included as Appendix B. Since discharge is inferred from automated stage records for most of the year, changes in channel conditions could render the stage data unreliable. Retaining only the most reliable data, the following time periods were included in this study: Period 1. 11 Jun – 1 Sep 2004 Period 2. 25 Jun – 28 Jul 2005 Period 3. 6 Jan – 1 May 2006 The first two periods fall during the rainy season, and investigators from Smith College were in Monteverde making hydrologic measurements for most of this time. During the third period, which straddles the transitional and dry seasons, two pressure transducers were used to record stage (for redundancy) and investigators from Smith College were on-site to make discharge measurements in January and April. While the focus of this report is on these three time periods, Appendix B contains a record of all direct discharge measurements made at QC200, including those outside of these time periods. Rating curves for the three periods were developed based on both a power-law equation and a second-order polynomial (quadratic) equation: Quadratic Model Discharge = a(stage)2 + b(stage) + c (2) Power Law Discharge = a(stage – c)b = a(fitted stage)b (3) Results from both models were compared to determine the more appropriate fit for the data. The resulting rating curves for each period are presented in Figures 3-5 below. Note that the rating curves were developed for the stage and discharge measurements in English units (ft and cubic feet per second). Details of the rating-curve determination and the associated uncertainty are available in Appendix C.

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 5 of 23 4.6 4.7 4.8 4.9 5 5.1 5.2 5.3 0 5 10 15 20 25 30 Relative Stage (ft)Discharge (cubic feet per second)11-Jun-04 to 1-Sep-04 Discharge = 19.765 Stage2-164.98 Stage+344.93 Figure 3. QC200 direct discharge measurements and rating curve for the period 11-Jun04 to 1-Sep-04. The points indicate the direct discharge measurements performed to develop the rating curve (solid line) for the stated period (see Table B1 in Appendix B). The uncertainty in the rating-curve predicti ons is given by the dashed lines. Stage is measured relative to a consistent point on a staff gage (see Appendix A). 0.6 0.8 1 1.2 1.4 1.6 0 5 10 15 20 25 30 Fitted Stage (ft)Discharge (cubic feet per second)25-Jun-05 to 28-Jul-05 Discharge = 8.936 Fitted Stage2.304 Figure 4. QC200 direct discharge measurements and rating curve for the period 25-Jun05 to 28-Jul-05. The points indicate the direct discharge measurements performed to develop the rating curve (solid line) for the stated period (see Table B1 in Appendix B). The uncertainty in the rating-curve predicti ons is given by the dashed lines. Stage is measured relative to a consistent point on a staff gage (see Appendix A).

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 6 of 23 4.3 4.4 4.5 4.6 4.7 4.8 4.9 0 1 2 3 4 5 6 7 8 9 10 Relative Stage (ft)Discharge (cubic feet per second)6-Jan-06 to 1-May-06 Discharge = 16.643 Stage2-142.68 Stage+307.31 Figure 5. QC200 direct discharge measurements and rating curve for the period 6-Jan06 to 1-May-06. The points indicate the di rect discharge measurements performed to develop the rating curve (solid line) for the stated period (see Table B1 in Appendix B). The uncertainty in the rating-curve predicti ons is given by the dashed lines. Stage is measured relative to a consistent point on a staff gage (see Appendix A). 3 Results Figures 6-8 show the time series of discharge for the three periods of interest. The stage records were derived from pressure-transducer measurements, and the discharge records were generated using the appropriate rating curves. The circles indicate direct discharge measurements and the horizontal lines indicate the maximum and minimum discharge measurements within each period. Discharge estimates beyond the range of measured flows require the extrapolation of the rating curves and, therefore, are associated with larger uncertainties. Note that the vertical scale for 2005 (Figure 7) differs from the other periods because of a large rain event and corresponding high flow in July 2005. Daily average streamflow data are presented in tabular form in Appendix E. Precipitation data are from the meteorological station on the roof of the Monteverde Institute (Johnson et al. 2005, Guswa et. al. 2006).

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 7 of 23 11-Jun-2004 26-Jun-2004 11-Jul-2004 26-Jul-2004 10-Aug-2004 25-Aug-2004 0 20 40 60 80 Daily Precipitation [mm]Date 11-Jun-2004 26-Jun-2004 11-Jul-2004 26-Jul-2004 10-Aug-2004 25-Aug-2004 0 400 800 1200 1600 Discharge [l/s] Figure 6. Time series of discharge at QC200 for the period 11-Jun04 to 1-Sep-04. The circles represent direct discharge measurements; the horizontal lines indicate the maximum (710 l/ s) and minimum (130 l/s) direct discharge measurements for the period. The average streamflow over this period was 230 l/s.

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 8 of 23 25-Jun-2005 09-Jul-2005 23-Jul-2005 0 30 60 90 120 150 Daily Precipitation [mm]Date 25-Jun-2005 09-Jul-2005 23-Jul-2005 0 500 1000 1500 2000 2500 Discharge [l/s] Figure 7. Time series of discharge at QC200 for the period 25Jun-05 to 28-Jul-05. The circles represent direct discharge measurements; the horizontal lines indicate the maximum (560 l/ s) and minimum (130 l/s) direct discharge measurements for the period. The average streamflow over this period was 280 l/s.

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 9 of 23 15-Jan-2006 29-Jan-2006 12-Feb-2006 26-Feb-2006 12-Mar-2006 26-Mar-2006 09-Apr-2006 23-Apr-2006 0 20 40 60 80 Daily Precipitation [mm]Date 15-Jan-2006 29-Jan-2006 12-Feb-2006 26-Feb-2006 12-Mar-2006 26-Mar-2006 09-Apr-2006 23-Apr-2006 0 400 800 1200 1600 Discharge [l/s] Figure 8. Time series of discharge at QC200 for the period 6Jan-06 to 1-May-06. The circle s represent direct discharge measurements; the horizontal lines indicate the maximum (200 l/s) and minimum (40 l/s) direct discharge measurements for the period. The average streamflow over this period was 100 l/s.

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 10 of 23 On all three figures, the streamflow response to precipitation is apparent. Precipitation is typically coupled with a rapid rise in discharge followed by a recession as stormflow subsides and the stream returns to baseflow conditions. Figure 8 shows a gradual decrease of baseflow over several months due to the relatively low precipitation during the transitional and dry seasons. Further work is required to understand the interaction between rainfall, runoff, and the groundwater storage capacity within the region. Table 1 provides summary statistics for streamfl ow and precipitation during the three periods of interest. The representative streamflow for each period can be quantified by the median, which is the value of discharge for which half of the time the flowrate is higher and for half of the time the flowrate is lower. The mean, or average, discharge is useful as a measure of the total amount of water discharged by the stream, but is le ss helpful as a measure of a typical flowrate due to a few large events of relatively short duration that skew the distribution of flows. The 7day minimum flow is of particular utility in the assessment of water resources because it indicates the lowest flows in a stream or river. To determine the 7-day minimum flow, a 7-day moving average of discharge is calculated for a time period of interest, and the smallest value is selected. From 6-Jan-06 to 1-May-06, the 7-day minimum flow was 40 liters/second, which is less than one-third of what is observed in the rainy season. Table 1. Summary statistics for discharge at QC 200 for the three periods of interest. 11 Jun 04 – 1 Sep 04 25 Jun 05 – 28 Jul 05 6 Jan 06 – 1 May 06 Season Rainy Rainy Transitional/Dry Average Precipitation (mm/day) 7.2 13.3 2.1 Mean Streamflow (l/s) 230 280 100 Median Streamflow (l/s) 210 240 80 7-day Minimum Streamflow (l/s) 150 130 40 Max Direct Discharge Measurement (l/s) 710 560 200 Min Direct Discharge Measurement (l/s) 130 130 40 Figure 9 shows the frequency distributions of streamflow on the Quebrada Cuecha for the three time periods. As shown on the figure, the discharge at QC200 is much lower during the dry and transitional season than during the rainy season. The effect of stormflow explains the positive skewness of the data (i.e., the few very large values of discharge) shown in Figure 9. During and immediately after a rain event, streamflow is very high and then quickly subsides. Not surprisingly, the highest flows are observed during the rainy season.

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 11 of 23 0 200 400 600 800 0 0.2 0.4 Discharge (l/s)Frequency11-Jun-04 to 1-Sep-04 Rainy Season 0 200 400 600 800 0 0.2 0.4 Discharge (l/s)Frequency25-Jun-05 to 28-Jul-05 Rainy Season 0 200 400 600 800 0 0.2 0.4 Discharge (l/s)Frequency6-Jan-06 to 1-May-06 Transitional/Dry Seasons Figure 9. Frequency distributions for QC200 streamfl ow for the three periods of interest. The width of each bar represents 50 liters/s. Flow-duration curves (Figure 10) present the same information as the frequency distribution curves (Figure 9), but present it in a different manner. In a flow-duration curve, stream discharge is plotted against the frequency of time for which streamflow is greater than or equal to that value Figure 10 presents flow-duration curves for the Quebrada Cuecha. This figure shows that streamflow in excess of 200 l/s is not uncommon during the rainy season; discharge at QC200 met or exceeded 200 l/s 56% of the time during period 1 and 62% during period 2. During the transitional and dry seasons of 2006, however, the median flow rate was 80 l/s and discharge exceeded 200 l/s only 7% of the time. The different geometries of the flow-duration curves are due to the different distributions of the streamflow data (see Figure 9); the slope for the dry season in 2006 is steeper than those for the other two periods because the range of flows was relatively small, suggesting that streamflow did not deviate significantly from baseflow conditions. Note that the median streamflow is the discharge that corresponds to a frequency of 0.5 on Figure 10. The data, statistics, and figures presented in this report reflect the conditions for the periods of measurement. Given the seasonal and year-to-year variability in precipitation, however, these data should not be presumed to be representative of streamflow at QC200 for all time. Continued monitoring and measurement is necessary to fully characterize streamflow for the Quebrada Cuecha.

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 12 of 23 0 100 200 300 400 500 600 700 800 0 0.2 0.4 0.6 0.8 1 Exceedance FrequencyDischarge (l/s)11-Jun-04 to 1-Sep-04 Rainy Season 0 100 200 300 400 500 600 700 800 0 0.2 0.4 0.6 0.8 1 Exceedance FrequencyDischarge (l/s)25-Jun-05 to 28-Jul-05 Rainy Season 0 100 200 300 400 500 600 700 800 0 0.2 0.4 0.6 0.8 1 Exceedance FrequencyDischarge (l/s)1-Jan-06 to 1-May-06 Transitional/Dry Seasons Figure 10. Flow-duration curves for QC200. T hese plots present di scharge versus the frequency with which that value was exceeded during each of the three periods. The vertical lines represent the maximum and minimum manual discharge measurements; these values correspond to 710 and 130 liters/second for 2004, 560 and 130 liters/second for 2005, and 200 and 40 liters/second for 2006. The ‘staircase’ pattern in the curves is due to the finite re solution of the stage measurements.

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 13 of 23 Acknowledgments The authors would like to thank the many individuals who contributed to the completion of this report. The Rockwell family provides access to site QC200 through their property. The Monteverde Institute provides logistical support during our visits to Monteverde. Smith students Silvia Newell ‘04, Elizabeth Koenig ’05, Ilona Johnson ’06, Mai Kobayashi ’06, and Merilie Reynolds ’08 made manual discharge measurements at QC200 and helped to generate the rating curves. We are grateful for all of these specific contributions and the general support of the Monteverde community. References Cited Guswa, Andrew J. and A. L. Rhodes, 2006. Meteorology of Monteverde, Costa Rica 2005. Technical Report submitted to the Monteverde Institute, 34 pages. Guswa, Andrew J., and A. L. Rhodes, 2004. Wet-season throughfall in primary and secondary tropical montane cloud forests, Monteverde, Costa Rica. Eos Trans. AGU 85(47), Fall Meeting Suppl., Abstract H54C-08. Johnson, Ilona J. '06, A. J. Guswa, A. L. Rhodes, 2005. Meteorology of Monteverde, Costa Rica, November 2003-November 2004. Technical Report submitted to the Monteverde Institute, 23 pages. Rhodes, Amy L., A. J. Guswa, S. Dallas, E. M. Kim ‘02, S. Katchpole ‘02, A. Pufall, in press a. Water quality in a tropical montane cloud forest, Monteverde, Costa Rica. In: Bruijnzeel, L.A., Juvik, J., Scatena, F.N., Hamilton, L.S., and Bubb, P. (eds), Forests in the Mist: Science for Conservation and Management of Tropical Montane Cloud Forests University of Hawaii Press. Rhodes, Amy L., A. J. Guswa, and S. E. Newell ‘04, in press b. Seasonal variation in the stable isotopic composition of precipitation in the trop ical montane forests of Monteverde, Costa Rica, Water Resources Research

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 14 of 23 APPENDIX A. Methods for Discharge Determination Discharge can be determined by measuring the depth and the vertically-averaged flow velocity at discrete points along a channel cross section. In this study, a Swoffer flowmeter (Figure A1) is used to measure velocity and depth. Velocity is typically measured twice at each location and the results are averaged; velocity measurements are accurate to approximately 3%. At each location, velocity is measured at six-tenth of the total depth because the velocity at this depth is theoretically equal to the vertical average. These measurements are typically performed at 1520 locations across a channel cross section. M easurements from these locations are then numerically integrated to determine total discharge (Figure A2). Figure A1. Photo of flowmeter wand and digital meter. The flowmeter consists of a metal rod with marked increments for depth measurement and a propeller for velocity measurement.

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 15 of 23 Figure A2. Illustration of the numerical method used to calculate discharge through a channel cross section. Numbers 2-17 represent locations of depth and velocity measurements; the tagline of numbers 1 and 18 are recorded to indicate the banks. Example of the numerical method: The dept h and velocity measurements at 8 are assumed to be representative of the depth and velocity of the entire shaded rectangle; the product of the shaded area and the repres entative velocity measurement at 8 is the discharge through the rectangle (Equation 1). The width of the rectangle is determined by summing of the distance between 7 and 8 and of the distance between 8 and 9. The sum of all such rectangles across the channel is the total discharge through the entire cross section. Depth and velocity are typically measured at 15-20 locations across a channel. Direct discharge measurements are time consuming and field conditions do not allow for the implementation of this method when flows are extremely high or low. A more practical approach is to establish an empirical relationship relating discharge and a more easily-monitored parameter such as water level (stage). Manual staff gage readings are obtained when direct discharge measurements are performed. Based on direct discharge measurements and corresponding manual staff-gage readings, rating curves are developed to relate stage and discharge. Automated measurements of stage can then be converted to records of discharge using the rating curves. To obtain a continuous record of stage at QC200, a gaging station consisting of a staff gage and pressure transducers has been maintained since June 2004. The staff gage (Figure A3) is a large metal ruler with marked increments of 0.01 ft (3.0 mm); it is installed within the channel and water level above an arbitrary point can be read from the gage. There are two pressure transducers: a levelogger and a barologger (Figure A3). The levelogger is installed near the staff gage in a vertical PVC pipe with slits that allow for water to pass through; this instrument measures both the atmospheric pressure and water pressure above the point of measurement. The barologger is installed out of the water; it measures only atmospheric pressure. The subtraction of the barologger readings from the levelogger readings yields the water pressure; the latter can be converted to equivalent sta ff gage readings by comparing with available manual staff-gage measurements. Both logger s were manufactured by Solinst, Inc.; the accuracies of the levelogger and barologger measurements are 0.1% and 0.3%, respectively.

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 16 of 23 Figure A3. [Left] The water level above an arbitrar y point is measured using a staff gage. [Right] Two pressure transduc ers (levelogger and barologger) installed at QC200. The subtraction of the barologger readings from the levelogger readings yields the water pressure; the latter can be converted to equivalent staff-gage readings by comparing with manual staff-gage measurements.

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 17 of 23 APPENDIX B. Discharge Measurements at QC200 Table B1. Record of all direct measurements of di scharge (to the nearest 10 liters/second) made at QC200 from June 2004 through June 2006. Date Time (CST) Discharge (liters/s) Date Time (CST) Discharge (liters/s) Date Time (CST) Discharge (liters/s) 11-Jun-04 16:45 280 28-Feb-05 14:45 90 23-Jun-06 8:30 180 13-Jun-04 14:00 260 15-Mar-05 13:47 40 23-Jun-06 13:02 160 14-Jun-04 13:50 240 31-Mar-05 10:45 20 28-Jun-06 13:21 500 15-Jun-04 13:30 220 23-Jun-05 8:42 200 28-Jun-06 15:17 450 16-Jun-04 10:30 250 25-Jun-05 10:00 560 29-Jun-06 16:38 310 18-Jun-04 13:38 210 27-Jun-05 13:57 460 30-Jun-06 8:40 700 19-Jun-04 10:30 180 28-Jun-05 9:20 300 30-Jun-06 8:59 790 21-Jun-04 13:35 240 5-Jul-05 14:21 290 30-Jun-06 9:21 810 22-Jun-04 11:45 200 7-Jul-05 11:03 280 30-Jun-06 11:16 840 23-Jun-04 15:40 180 8-Jul-05 9:06 210 30-Jun-06 15:20 870 24-Jun-04 10:40 300 13-Jul-05 15:48 160 25-Jun-04 10:22 200 18-Jul-05 15:19 170 26-Jun-04 12:00 250 22-Jul-05 9:50 170 28-Jun-04 10:45 210 22-Jul-05 12:40 160 29-Jun-04 13:15 210 25-Jul-05 14:55 130 30-Jun-04 13:55 710 27-Jul-05 13:13 160 01-Jul-04 9:30 250 6-Jan-06 8:57 70 02-Jul-04 13:13 220 6-Jan-06 10:01 70 03-Jul-04 11:43 310 8-Jan-06 14:34 200 05-Jul-04 11:20 170 9-Jan-06 14:13 140 06-Jul-04 13:40 170 10-Jan-06 8:46 90 07-Jul-04 14:50 140 11-Jan-06 13:38 90 08-Jul-04 8:50 180 3-Apr-06 13:28 50 10-Jul-04 11:20 130 5-Apr-06 9:55 40 12-Jul-04 13:20 240 9-Jun-06 9:00 140 13-Jul-04 15:13 170 10-Jun-06 13:34 120 14-Jul-04 11:55 220 10-Jun-06 14:30 140 15-Jul-04 14:45 190 11-Jun-06 15:35 210 20-Jul-04 11:45 190 13-Jun-06 10:11 100 22-Jul-04 12:00 270 14-Jun-06 13:36 200 22-Jul-04 12:01 260 15-Jun-06 10:32 90 04-Nov-04 13:45 170 16-Jun-06 9:33 320 10-Jan-05 12:25 720 16-Jun-06 12:39 270 10-Jan-05 13:45 720 16-Jun-06 14:59 240 30-Jan-05 15:10 200 19-Jun-06 8:18 90 16-Feb-05 14:25 90 19-Jun-06 16:22 90 23-Feb-05 14:30 220 21-Jun-06 8:16 70 23-Feb-05 15:25 170 21-Jun-06 14:45 60 24-Feb-05 13:15 160 22-Jun-06 17:20 560

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 18 of 23 APPENDIX C. Rating Curve Determination Two mathematical models were assessed to describe the relationship between water level and stream discharge: Quadratic Model Discharge = a(stage)2 + b(stage) + c (2) Power Law Discharge = a(stage – c)b = a(fitted stage)b (3) The quadratic model (Equation 2) is purely empirical; it is based solely on the correlation of the measured stage and discharge data. This model could be desirable because it is not based on any inherent assumptions about the data. The power law model (Equation 3) is based on a theoretical assumption; the parameter ‘c’ indicates the staff-gage level which corresponds to a discharge of zero. The choice of this parameter is based on qualitative comparisons of the data with the existing rating curves. Different values of the parameter ‘c’ can be used within a period of interest. If any changes in the channel morphology in the vicinity of t he stream gage are known and their impact on the water level is quantifiable, ‘c’ could be altered to reflect this change. This was applicable in period 2 (25-Jun-05 to 28-Jul-05); a flow obstruction was in place downstream of the staff gage from 12-22 Jul 05 and stage was consequently elev ated throughout the period. A new ‘c’ was used for the period when the obstruction was in place and another was used for when it was removed. The initial and the final parameters within the period are not equal (Table C1) because the system did not return to its initial conditions. Model assessment was based on a compar ison between the correlation factors (R2), the root mean square (RMS) error, and a graphical assessment of residuals vs. time and residuals vs. discharge. The results of the model selection are summarized in Table C1 below. Continuous discharge records for each period of interest were generated using the chosen rating curves. Figures 3-5 illustrate the direct discharge measurements and the rating curves selected for each period. Note that the rating curves were developed for the stage and discharge measurements in English Units (ft and cubic feet per second).

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 19 of 23 Table C1. Summary of the rating-curve selections for QC200. Assessment was based on a comparison between the correlation factors (R2), the root mean square (RMS) errors, and a graphical assessment of residuals vs. time and residuals vs. discharge. The quadratic model was selected for the first and third periods, and the power-law model was selected for the second. The parameters, a, b, and c, refer to those in Equation 2 (quadratic) and Equation 3 (power law). More than one parameter ‘c’ was used in the second period because a flow obstruction was in place downstream of the stream gaging station from 1222 Jul 05. 11 Jun 04 – 1 Sep 04 25 Jun 05 – 28 Jul 05 6 Jan 06 – 1 May 06 Power Law Quadratic Power Law Quadratic Power Law Quadratic Parameters A 17.009 19.765 8.936 24.218 7.802 16.643 B 2.149 -164.98 2.304 -207.49 1.77 -142.68 C 3.38 344.93 3.73 for 25-Jun to 12-Jul 447.22 3.48 307.31 3.57 for 12-Jul to 22-Jul 3.66 for 22-Jul to 28-Jul Statistics R2 0.92 0.97 0.99 0.93 0.95 0.97 Root Mean Square (cfs) 0.65 0.65 0.51 1.19 0.46 0.31 Mean of the Residuals (cfs) 0.0 0.0 0.0 0.0 0.0 0.0 Any trends in… Residual vs. Discharge None None None None None None Residual vs. Time Weak, negative correlation (R2 = 0.47) Weak, negative correlation (R2 = 0.47) None None None None Chosen Model Quadratic Power law Quadratic

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 20 of 23 APPENDIX D. Error Assessment for Discharge Predictions There are two major sources of uncertainty associated with discharge values predicted from a rating curve: 1. Variability of the direct di scharge measurements about the rating curve and 2. Uncertainty in discharge predicted by the rating curves due to uncertainty in the continuous record of stage. The former is quantified by the root mean square (RMS) error, and the latter is calculated as follows: S dS S dQ S Qs ) ( ) ( (4) where Q is the discharge predicted from a rating curve, Q is the uncertainty associated with the predicted discharge, S is stage, and S is the uncertainty in the continuous stage record (a conservative estimate of 0.04 ft was used) The larger of the two errors at each value of stage was used to quantify the uncertainty in the rating-curve predictions; the calculated uncertainties are indicated by the dashed lines on Figures 3-5.

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 21 of 23 APPENDIX E. Daily Average Streamflow Data Table E1. Daily average streamflow at QC200 (reported to the nearest 10 liters/second) from 12-Jun-04 through 31-Aug-04 determined from a c ontinuous record of stream stage. Date liters/s Date liters/s Date liters/s 12-Jun-04 450 12-Jul-04 270 11-Aug-04 140 13-Jun-04 270 13-Jul-04 210 12-Aug-04 140 14-Jun-04 240 14-Jul-04 240 13-Aug-04 140 15-Jun-04 260 15-Jul-04 230 14-Aug-04 170 16-Jun-04 250 16-Jul-04 200 15-Aug-04 240 17-Jun-04 220 17-Jul-04 180 16-Aug-04 250 18-Jun-04 200 18-Jul-04 170 17-Aug-04 210 19-Jun-04 210 19-Jul-04 190 18-Aug-04 160 20-Jun-04 260 20-Jul-04 220 19-Aug-04 160 21-Jun-04 240 21-Jul-04 230 20-Aug-04 190 22-Jun-04 200 22-Jul-04 350 21-Aug-04 170 23-Jun-04 180 23-Jul-04 220 22-Aug-04 140 24-Jun-04 250 24-Jul-04 230 23-Aug-04 160 25-Jun-04 240 25-Jul-04 230 24-Aug-04 160 26-Jun-04 260 26-Jul-04 210 25-Aug-04 160 27-Jun-04 230 27-Jul-04 200 26-Aug-04 150 28-Jun-04 190 28-Jul-04 220 27-Aug-04 140 29-Jun-04 200 29-Jul-04 210 28-Aug-04 150 30-Jun-04 400 30-Jul-04 190 29-Aug-04 160 1-Jul-04 280 31-Jul-04 200 30-Aug-04 280 2-Jul-04 210 1-Aug-04 180 31-Aug-04 300 3-Jul-04 240 2-Aug-04 170 4-Jul-04 200 3-Aug-04 180 5-Jul-04 180 4-Aug-04 190 6-Jul-04 200 5-Aug-04 230 7-Jul-04 180 6-Aug-04 200 8-Jul-04 190 7-Aug-04 180 9-Jul-04 170 8-Aug-04 160 10-Jul-04 160 9-Aug-04 160 11-Jul-04 420 10-Aug-04 150

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 22 of 23 Table E2. Daily average streamflow at QC200 (reported to the nearest 10 liters/second) from 26-Jun-05 through 26-Jul-05 determined from a continuous record of stream stage. Date liters/s 26-Jun-05 650 27-Jun-05 500 28-Jun-05 390 29-Jun-05 340 30-Jun-05 270 1-Jul-05 300 2-Jul-05 780 3-Jul-05 630 4-Jul-05 370 5-Jul-05 300 6-Jul-05 260 7-Jul-05 270 8-Jul-05 240 9-Jul-05 240 10-Jul-05 250 11-Jul-05 290 12-Jul-05 250 13-Jul-05 210 14-Jul-05 280 15-Jul-05 260 16-Jul-05 220 17-Jul-05 180 18-Jul-05 160 19-Jul-05 160 20-Jul-05 160 21-Jul-05 160 22-Jul-05 160 23-Jul-05 120 24-Jul-05 90 25-Jul-05 120 26-Jul-05 150

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Streamflow Report: Quebrada Cuecha, 2004-2006 Yeung et al., 2006 page 23 of 23 Table E3. Daily average streamflow at QC200 (reported to the nearest 10 liters/second) from 7-Jan-06 through 30-Apr-06 determined from a co ntinuous record of stream stage. Date liters/s Date liters/s Date liters/s Date liters/s 7-Jan-06 90 6-Feb-06 140 8-Mar-06 60 7-Apr-06 40 8-Jan-06 190 7-Feb-06 140 9-Mar-06 60 8-Apr-06 40 9-Jan-06 170 8-Feb-06 110 10-Mar-06 50 9-Apr-06 40 10-Jan-06 100 9-Feb-06 90 11-Mar-06 50 10-Apr-06 40 11-Jan-06 80 10-Feb-06 80 12-Mar-06 50 11-Apr-06 40 12-Jan-06 80 11-Feb-06 80 13-Mar-06 50 12-Apr-06 40 13-Jan-06 70 12-Feb-06 70 14-Mar-06 50 13-Apr-06 40 14-Jan-06 120 13-Feb-06 100 15-Mar-06 50 14-Apr-06 40 15-Jan-06 940 14-Feb-06 140 16-Mar-06 50 15-Apr-06 40 16-Jan-06 380 15-Feb-06 150 17-Mar-06 50 16-Apr-06 40 17-Jan-06 180 16-Feb-06 110 18-Mar-06 50 17-Apr-06 40 18-Jan-06 160 17-Feb-06 110 19-Mar-06 50 18-Apr-06 40 19-Jan-06 210 18-Feb-06 100 20-Mar-06 50 19-Apr-06 40 20-Jan-06 140 19-Feb-06 100 21-Mar-06 40 20-Apr-06 40 21-Jan-06 110 20-Feb-06 80 22-Mar-06 50 21-Apr-06 40 22-Jan-06 100 21-Feb-06 80 23-Mar-06 50 22-Apr-06 40 23-Jan-06 90 22-Feb-06 70 24-Mar-06 40 23-Apr-06 50 24-Jan-06 90 23-Feb-06 80 25-Mar-06 50 24-Apr-06 40 25-Jan-06 90 24-Feb-06 80 26-Mar-06 50 25-Apr-06 40 26-Jan-06 90 25-Feb-06 100 27-Mar-06 40 26-Apr-06 40 27-Jan-06 190 26-Feb-06 80 28-Mar-06 50 27-Apr-06 40 28-Jan-06 230 27-Feb-06 90 29-Mar-06 90 28-Apr-06 40 29-Jan-06 210 28-Feb-06 80 30-Mar-06 100 29-Apr-06 40 30-Jan-06 130 1-Mar-06 70 31-Mar-06 80 30-Apr-06 40 31-Jan-06 140 2-Mar-06 70 1-Apr-06 60 1-Feb-06 180 3-Mar-06 60 2-Apr-06 50 2-Feb-06 130 4-Mar-06 60 3-Apr-06 50 3-Feb-06 110 5-Mar-06 60 4-Apr-06 50 4-Feb-06 110 6-Mar-06 60 5-Apr-06 50 5-Feb-06 100 7-Mar-06 60 6-Apr-06 40


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