WATERSHED–SCALE AMC SELECTION FOR HYDROLOGIC MODELING

Abstract

The Natural Resources Conservation Service curvenumber (CN) method commonly uses three discrete levels(1, 2, and 3) of antecedent moisture condition (AMC) to describe soil moisture at the time of a runoff event. However, thismay not adequately represent soil water conditions for watershed modeling purposes. The objectives of this study were toevaluate the use of individualevent watershedscale AMC values to adjust fieldscale CN, and to assess which hydrologicparameters would provide the best estimate of individualevent AMC. Landsat Thematic Mapper images from 1997 and 1998were used to obtain 10 landcover classes for Red Rock Creek watershed, Kansas. The canopy growth of crops was used toprovide temporal adjustment of CNs in the watershed. Streamflow data for 19971999 was collected from a U.S. GeologicalSurvey gaging station near the watershed outlet, and base flow was separated to obtain surfacerunoff amounts.Watershedaverage AMC factors were estimated from measured rainfall and surface runoff amounts for each of 23 eventsand used to adjust CNs in the AGNPS watershed model. For individual runoff events, calibration was achieved with AMCsthat averaged 1.5 and ranged from 0.9 to 2.4. Therefore, an AMC of 2, as used in many hydrologic models, would overestimate the surface runoff amounts in this subhumid Kansas watershed. Generally, AMC increased with 5-day antecedent rainfallabove 5 mm. Soil moisture and 5day antecedent rain were found to be significantly correlated to AMC.