An empirical method for determining average soil infiltration rates and runoff, Powder River structural basin, Wyoming

Abstract

This report describes a method to estimate infiltration rates of soils for use in predicting runoff from small basins containing these soils. Data requirements are: (1) Rainfall frequency, (2) soils maps, and (3) rainfall-runoff data. The emphasis of this report is development of the theory and verification of the method. Three simplifying assumptions are: (1) Initial water uptake includes interception by vegetation, surface storage, and channel storage; (2) some average antecedent moisture condition exists; and (3) rainfall for a storm event is uniform both in space and time. Average rainfall intensity is plotted against storm duration on log-log paper. All rainfall events are designated as runoff or nonrunoff events. A power-decay-type curve is visually fitted to separate the two types of rainfall events. This separation curve is an incipient-ponding curve and its equation describes infiltration parameters for a soil. The parameters are used to define a series of infiltration-rate curves for that soil. For basins with more than one soil complex, only the incipientponding curve for the soil complex with the lowest infiltration rate can be defined using the separation technique. Incipient-ponding curves for soils with infiltration rates greater than the lowest curve are defined by ranking the soils according to their relative permeabilities and optimizing curve position using measured runoff as a control for the fit. A comparison was made between the sums of computed runoff and of measured runoff for all available runoff events in the six basins studied. Summation of the computed runoff events ranged from 16.6 percent less to 2.3 percent more than the summation of the measured runoff events.