Effective Rainfall Estimation Methods

Numerous methods for estimating effective rainfall have been proposed in the past, including: direct measurement techniques; empirical methods; and soil water balance methods. The best estimates of effective rainfall can be obtained by conducting soil water balance computations. A soil water balance model (SWBM) for estimating effective rainfall was used to test the accuracy of the United States Department of Agriculture Soil Conservation Service (USDA‐SCS) and the Hershfield effective rainfall estimation methods for a well‐drained soil and for a poorly drained soil. Estimates of mean annual monthly effective rainfall by the USDA‐SCS and estimates of mean annual growing season effective rainfall by the Hershfield method were found to compare closely with estimates from the SWBM for the well‐drained soil but not for the poorly drained soil. Effective rainfall estimates by these two methods for either soil condition did not compare well with the SWBM estimates for annual events with return periods higher than the mean annual event.