Precipitation Distributions and Green-Ampt Runoff

One-hundred-year and 10-year storm runoff volumes and peak discharges were computed with a Green-Ampt infiltration model and the Soil Conservation Service's (SCS) TR-20 hydrograph model. The results for six different standard design storm temporal distributions were compared with predictions made with the U.S. Geological Survey's (USGS) regional regression equations relating peak discharge to drainage area. Hypothetical watersheds similar to those in eastern Montana were used for the comparisons, which were made over a wide range of hydraulic-conductivity values for drainage areas of one to 400 sq mi. Runoff volumes computed with the Green-Ampt equation vary significantly with the rainfall distribution. The design storms, which included the independent duration (ID) frequency analysis for all durations within the event, resulted in estimates of both volume and peak discharge that were significantly greater than predicted by the USGS equations. The SCS type-I distribution resulted in the best comparison over the range of frequencies and drainage areas examined.