Antecedent Retention Indexes Predict Soil Moisture

Two models for calculating antecedent moisture index values are evaluated by comparisons with measured soil moisture. One model, based on the traditional antecedent precipitation index equation, depletes the index values exponentially. The second model depletes the index values by an evapotranspiration-soil moisture relation. Index values computed by the traditional exponential model with recommended K values of 0.88 and 0.92 compare poorly with measured soil moisture. Derived K values show a distinct seasonal trend and range from 0.98 in the spring and fall to 0.92 in the summer when representing the 0- to 12-in. soil depth, and 0.98 to 0.96 when representing the 0- to 36-in. soil depth. Index values by both models compare favorably with available soil moisture when derived K and PET values are used. The K and PET values are related to pan evaporation to facilitate their prediction at other locations.