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.