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USE of RAINFALL-SIMULATOR DATA in PRECIPITATION-RUNOFF MODELING STUDIES by Gregg C. Lusby and Robert W. Lichty U.S. GEOLOGICAL S

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USE of RAINFALL-SIMULATOR DATA in PRECIPITATION-RUNOFF MODELING STUDIES by Gregg C. Lusby and Robert W. Lichty U.S. GEOLOGICAL S USE OF RAINFALL-SIMULATOR DATA IN PRECIPITATION-RUNOFF MODELING STUDIES By Gregg C. Lusby and Robert W. Lichty U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 83-4159 Prepared in cooperation with the U.S. BUREAU OF LAND MANAGEMENT Denver, Colorado 1983 UNITED STATES DEPARTMENT OF THE INTERIOR JAMES G. WATT, Seer atary GEOLOGICAL SURVEY Dallas L. Peck, Director For additional information Copies of this report can be write to: purchased from: Project Chief Open-File Services Section Water Resources Division. Central Region Western Distribution Branch U.S. Geological Survey U.S. Geological Survey Box 25046, Mail Stop 412 Box 25425 Denver Federal Center Denver Federal Center Denver, Colorado 80225 Denver, Colorado 80225 (Telephone: (303) 234-5888) CONTENTS Page J.TIU L OCIU.C L ion~~~~~~""" lr UL pO S w dllCl S COp w~ rlGt ilOClS Ot S tllQy~~ D J_L w Description of the study watershed 3 Delineation of hydrologic-response units and development of test plots 5 Development of runoff data from rainfall simulator 7 Model application to plot runoff 9 Overland-flow routing 10 Results of model calibration 12 Calibration using rainfall-simulator data 12 Calibration using observed rainfall-runoff data 33 Comparison of results of plot calibrations 40 Watershed modeling North Fork Willow Gulch 40 Partitioning the watershed 40 Comparison of observed and simulated runoff events 44 O O U I. C- WO O i g JJ J^Q Jj ^ HH 0 0 0 0 0 H__ HH » ____ » » » » ____ » » » » » «__ .___ .___ » .___ .___ .___ .___ H__ .___ .___ «__ ^ V^ r*L-wHL-.J-U-0-LL/Ho /^ r^^-»T ^ic?~i ^^T^C? _ _ . _ _ _.« ».__ _.»»._____».___.«_ __. _ __ 4/ Q,7 ILLUSTRATIONS Page Figure 1. Map of Willow Gulch drainage basin and location of instrumentation 4 2-4. Photographs showing: 2. Grass cover on plot 1, Willow Gulch study watershed 6 3. Close-up view of sod cover on plot 3, Willow Gulch study watershed 6 4. Rainfall simulator in operation, plot 2, Willow Gulch study watershed 7 5. Graph showing model calibration of simulator 6. Graph showing model calibration of simulator T-nnoJ- Lll.io j nlni-JJJ.UL £.9 , J-Z/OJ.1QR1. _ _ ____ __ _ __. _ _______ ________. j_t(.1 /i 7-12. Diagrams showing the sensitivity of calculated runoff: 7. To changes in model parameters and precipitation 15 8. To changes in KSAT 17 9 r.LP^-k \-> L-r* T-illdll^wo o t-» rr £i o JLll-i -n "Pi "~ "~ .».._ J_OT Q 10. To changes in WINT 19 U TrtJ. w L-/-»Tno-r»rr^o lid 11^ Co -T-nJ.1I. OCTT"DT? U IA.P _ __ ____ _ _ _ __ _ _ __ _______ __ _ _«_ _ £*\J0 C\ 12. To changes in FRIG 21 iii ILLUSTRATIONS CO itinued Page Figure 13-18, Graphs showing model calibrati on of simulator runs: 13. Plot 3, 1981 24 14. Plot 4, 1981 25 15. Plot 5, 1981 26 16. Plots 1 and 2, summer 1982 28 17. Plots 3, 4, and 5, summer 1982 29 18. Plots 1 and 3, fall 1982 30 19-22, Graphs showing model calibration using observed data: 19. Plot 1 35 20. Plot 2 - 36 21. Plot 3 37 22. Plot 4 38 23. Graph showing precipitation in observed storms at Willow Gulch, 1982 39 24. Map showing hydrologic-respon e units, subbasins, and drainage network in North Fork Willow Gulch 42 25. Diagram showing conceptual view of North Fork Willow Gulch watershed showing flow planes, channel segments, and juncti ons 43 26-34, Graphs showing modeling results of North Fork Willow Gulch: 26. June 25, 1982, using para meters from simulation runs on plot 45 27. June 25, 1982, using parameters from storm data on plots 46 28. July 26, 1982, using para meters from simulation runs on plot s~ 47 29. July 26, 1982, using para meters from storm data on plots 48 30. August 10, 1982, using pa rameters from simulation runs on plot s 50 31. August 10, 1982, using p rameters from storm data on plots 51 32. June 25, 1982. KSAT par meter lowered slightly and precipitation raised 40 percent - 53 33. July 26, 1982. KSAT pare meter obtained from simulation plots and pi ecipitation lowered 10 percent 54 34. August 10, 1982. KSAT p rameter lowered slightly and precipitation raised by 10 percent - 55 35, Photograph showing trees depo ited in channels by tornado 56 36 Photograph showing tree lodgec above supercritical flow flume on North Fork low Gulch - 56 iv TABLES Page Table 1. Statistical summary of bulk density data 8 2. Parameters used in simulation model of plot runoff 11 3. Moisture storage in the 0- to 20-inch soil profile before and after simulator run of August 3, 1981, plot 1 22 4. Summary of fitted values of hydraulic conductivity for rainfall-simulator runs of 1981 23 5. Moisture storage in the 0- to 20-inch soil profile before and after simulator runs 31 6. Range in fitted values of hydraulic conductivity for summer and fall runs 33 7. Summary of available data for three rainfall-runoff events during 1982 34 8. Summary of fitted values of hydraulic conductivity for observed rainfall-runoff events during 1982 40 9. Range in fitted values of hydraulic conductivity for both rainfall-simulator runs and observed rainfall-runoff events 41 10. Summary of predicted and observed runoff and peak flow from rainstorms, North Fork Willow Gulch, 1982 52 CONVERSION FACT RS For use of readers who prefer to use me ric units, conversion factors for terms used in this report are listed below: Multiply by_ To obtain inch (in) 25.40 millimeter (mm) foot (ft) 0.3048 meter (m) mile (mi) 1.609 kilometer (km) acre ,047 square meter (m2 ) acre 0.4047 hectare square inch (in2 ) 6.452 square centimeter (cm2 ) square foot (ft2 ) 929.0 square centimeter (cm2 ) square foot (ft 2 ) 0.09294 square meter (m2 ) square mile (mi2 ) 2.590 square kilometer (km2 ) foot per second (ft/s) 0.3048 meter per second (m/s) inch per hour (in/h) 25.40 millimeter per hour (mm/h) cubic foot per second (ft 3 /s) 0.02832 cubic meter per second (m^/s) (°F-32) degree Fahrenheit (°F) degree Celsius (°C) 1.8 VI DEFINITIONS FRIG Surface roughness. H Pressure head at the entry surface. HRU Hydrologic response unit. KSAT Hydraulic conductivity of the transmission zone. P Effective pressure head at the wetting front. PRMS Precipitation runoff modeling system. SURF Surface retention capacity. WINT Uniform initial moisture content. WWET Uniform moisture content of the transmission zone above the wetting front. vii USE OF RAINFALL-SIMULATOR DATA IN PRECIPITATION-RUNOFF MODELING STUDIES By Gregg C. Lusby and Robert W. Lichty ABSTRACT Results of a study using a rainfall simulator to define infiltration parameters for use in watershed modeling are presented. During 1981-82, a total of 23 rainfall-simulation runs were made on 5 small plots (about 2,500 square feet) located on 4 representative soil-vegetation types of the Willow Gulch watershed, located about 50 miles east of Denver, Colorado. During the summer of 1982, data for 3 observed rainfall-runoff events were recorded by gages on 4 of the plots. Runoff data from both simulator runs and observed rainstorms were used to develop best-fit parameters of the Green-Ampt infiltration equation. In all fitting attempts, the hydraulic conductivity term, KSAT, grossly ontrolled the goodness of fit. High variability in soil-water uptake found from soil samples taken before and after simulator runs confirms the empir­ ical nature of KSAT. Best-fit values are plot-average values that reflect both the limitations of the Green-Ampt equation, and the inherent (natural) variability of soil-water properties of field soils. Results of fitting KSAT to reproduce runoff from rainfall-simulator runs, and results of fitting KSAT to reproduce runoff from observed rainfall-runoff events are inconsistent. Summer runs on plots located in the upland area of ponderosa pine give little indication of runoff potential from observed rainstorms. In contrast, results for plots located in the lowland prairie area are in reasonable agreement with results from observed rainstorms. Fall runs on upland plots indicate that cooler soil temperatures may influence the infiltration process. In contrast, fall runs on lowland plots show no consistent effect of cooler soil temper­ atures; reasons for these anomalous results are unknown. The drainage area of the North Fork Willow Gulch watershed was partitioned into homogeneous hydrologic-response units (HRUs), and a conceptual flow-routing network of plane and channel segments was developed to characterize required input to a precipitation-runoff modeling system, PRMS. The application of PRMS to three storms in 1982, using estimates of KSAT based on simulator runs, produced predicted runoff volumes that were 70 percent less than those observed in the first two cases and 40 percent more than that observed in the third case. Using estimates of KSAT based on observed rainfall events on plots 1-3 improved the prediction for two events, and degraded the result for the third event. Adjustments in KSAT specfications, and adjustments to the storm rainfall confirmed that accuracy of predicting peak flow rates is controlled by the amount of water that falls on the ground, and the amount of water that infil­ trates. Runoff routing is adequately represented by the conceptual network of plane and channel segments. INTRODUCTION Prediction of overland flow generated by precipitation has been the object of intense study by hydrologists for many years.
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