Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2007
Publication Date: 2/1/2008
Citation: Stone, J.J., Paige, G.B., Hawkins, R.H. 2008. Rainfall intensity-dependent infiltration rates on rangeland rainfall simulator plots. Transactions of the ASABE. 15(1): 45-53. Interpretive Summary: Runoff models which use a soil physics based infiltration approach are widely used to predict runoff from agricultural watersheds. According to the theory, the infiltration rate decreases with time or cumulative depth of infiltration. Data collected using a rainfall simulator that can applied six different rainfall intensities (1 to 7 in/hr) show that the infiltration rate can increase with increasing rainfall rate contrary to conventional infiltration theory. The reason proposed for this is that the infiltration capacity of an area is not uniform such that a low intensities only those portions of the area which have an infiltration capacity less than the rainfall rate contribute to runoff. As the rainfall rate increases those areas have a higher capacity begin to contribute. The effect is that the infiltration rate increases. A model was developed to account for this process and reproduced the runoff process better than with a model which uses the conventional approach.
Technical Abstract: Most implementations of infiltration equations with rainfall-runoff models use a hydraulic conductivity parameter which 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. 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 a variable intensity rainfall simulator. The experiments were conducted on rangeland vegetation-soil associations at the Walnut Gulch Experimental Watershed in southeastern Arizona. The results from the experiments 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 and the absolute difference between the dry and wet moisture condition runoff start times was small, 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 suggest that the infiltration rates become constant quicker than infiltration theory would suggest.