Effects of antecedent soil moisture on runoff modeling in small semiarid watersheds of southeastern Arizona

This study presents unique data on the effects of antecedent soil moisture on runoff generation in a semi-arid environment, with implications for process-based modeling of runoff. The data were collected from four small water- sheds measured continuously from 2002 through 2010 in an environment where evapo-transpiration approaches 100 % of the infiltrated water on the hillslopes. Storm events were gen- erally intense and of short duration, and antecedent volumet- ric moisture conditions were dry, with an average in the up- per 5 cm soil layer over the nine year period of 8 % and a standard deviation of 3 %. Sensitivity analysis of the model showed an average of 0.05 mm change in runoff for each 1 % change in soil moisture, indicating an approximate 0.15 mm average variation in runoff accounted for by the 3 % stan- dard deviation of measured antecedent soil moisture. This compared to a standard deviation of 4.7 mm in the runoff depths for the measured events. Thus the low variability of soil moisture in this environment accounts for a relative lack of importance of storm antecedent soil moisture for mod- eling the runoff. Runoff characteristics simulated with a nine year average of antecedent soil moisture were statisti- cally identical to those simulated with measured antecedent soil moisture, indicating that long term average antecedent soil moisture could be used as a substitute for measured an- tecedent soil moisture for runoff modeling of these water- sheds. We also found no significant correlations between measured runoff ratio and antecedent soil moisture in any of the four watersheds.