Adaptation of Precipitation Index for the Estimation of Antecedent Moisture Condition in Large Mountainous Basins

The Soil Conservation Service–curve number (SCS-CN) method is widely used in hydrologic practice, but its application often goes beyond the purpose of its original use. Common practice makes the method dependent on the antecedent precipitation index, a simple indicator derived from rainfall depth, which can be used to estimate the antecedent moisture condition (AMC) of soil. By taking data from an unspecified location in the United States, the SCS defined the appropriate AMC level based on the total five-day antecedent rainfall for dormant and growing seasons. These values, originally defined as an example practice at the plot scale, were subsequently adopted for general use without full awareness of scale effects and regional differences. We propose a revision to the amount of antecedent rainfall for the definition of AMC of soil, based on the minimization of the errors between an estimated and an observed runoff volume of 347 rainfall-runoff events in large mountain basins in Italy and Switzerland. A significant correlation between the new thresholds and the topographic index is shown, thus permitting extension of the method to basins not included in this study.