Tillage practices for soil and water conservation in the semi-arid zone III. Runoff modeling as a tool for conservation tillage design

A method of runoff calculation and prediction has been developed, based on known soil properties and a probability analysis of historical rainfall data. The method provides an evaluation of the efficiency of any tillage system for conserving runoff water. It consists of four steps: (1) the infiltration characteristics of a particular soil type under a given management regime are determined with the aid of a portable rainfall simulator; (2) the long-term record of actual rainfall of the region is analyzed for storm intensity distribution; (3) runoff amounts and rates are calculated for all rain storms on record by a method which combines the soil infiltration function and storm intensity pattern; and (4) synthetic long-term runoff values are predicted by calculations based on the rainfall probability distribution. The method was verified by comparing predicted results with available data from field experiments. In addition to predicting the probability of runoff under given soil conditions, the method is useful in providing information on the amount of surface storage required for successful performance of the basin tillage (tied-ridge) system, and aids in the evaluation of the efficacy of measures for improving soil infiltrability, e.g., of amendments such as gypsum.