Rainfall Intensity-Dependent Infiltration Rates on Rangeland Rainfall Simulator Plots

Abstract

Most implementations of infiltration equations with rainfall-runoff models use a hydraulic conductivity parameter that is constant for a given rainfall event. However, plot data from rainfall simulator experiments and natural rainfall events have shown that infiltration rates can increase with increasing rainfall rate instead of decreasing with time or infiltrated depth, as predicted by infiltration models. This has been hypothesized to be a function off the spatial variability of the infiltration capacity across the area. In this article, an exponential model relating steady-state infiltration rate with rainfall intensity and the average areal infiltration rate when the area under consideration is contributing to runoff is evaluated using data from variable-intensity rainfall simulator experiments. The experiments were conducted on five rangeland vegetation-soil associations at the Walnut Gulch Experimental Watershed in southeastern Arizona. The results from 19 rainfall simulation runs show that the increase in infiltration rate with increasing rainfall intensity can be significant and that the exponential model represents the relationship between steady-state infiltration and rainfall intensity. The exponential model coupled with a kinematic wave model also represents the hydrographs better than the Green-Ampt Mein-Larsen infiltration model coupled with the same routing model. The time to the start of runoff is influenced more by rainfall intensity than by initial soil moisture conditions, particularly when the initial rainfall intensity was high. The rapid time to steady-state runoff at the beginning of the simulation run of the observed runoff hydrographs suggests that the infiltration rates become constant more quickly than infiltration theory would suggest.