Watershed-scale AMC selection for hydrologic modeling

The Natural Resources Conservation Service curve-number (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, this may not adequately represent soil water conditions for watershed modeling purposes. The objectives of this study were to evaluate the use of individual-event watershed-scale AMC values to adjust field-scale CN, and to assess which hydrologic parameters would provide the best estimate of individual-event AMC. Landsat Thematic Mapper images from 1997 and 1998 were used to obtain 10 landcover classes for Red Rock Creek watershed, Kansas. The canopy growth of crops was used to provide temporal adjustment of CNs in the watershed. Stream-flow data for 1997-1999 was collected from a U.S. Geological Survey gaging station near the watershed outlet, and base flow was separated to obtain surface-runoff amounts. Watershed-average AMC factors were estimated from measured rainfall and surface runoff amounts for each of 23 events and used to adjust CNs in the AGNPS watershed model. For individual runoff events, calibration was achieved with AMCs that 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 sub-humid Kansas watershed. Generally, AMC increased with 5-day antecedent rainfall above 5 mm. Soil moisture and 5-day antecedent rain were found to be significantly correlated to AMC.