Probable Effect of Summer Weather Modification on Runoff

A stochastic rainfall-runoff simulation procedure was developed and used to estimate effects of hypothetical increases or decreases in May - September rainfall due to weather modification programs on the runoff regime of western South Dakota rangeland watersheds. The distribution function used to simulate daily precipitation was assumed to be the sum of two exponential distributions. The beta function was used to simulate runoff on days when there was precipitation. The study indicated that, if precipitation increased 1.00 in. (25 mm), runoff would be increased 8%–10%, or 0.04 in. - 0.14 in. (1.0 mm – 3.6 mm), depending on watershed soil characteristics. The study also showed that, if precipitation is decreased 10%, runoff would be decreased about 20%.