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Water Infiltration and Runoff under Rain Applications¹

ABSTRACT

Experiments to verify the validity and to evaluate the performance of rain infiltration models based on extending the Green and Ampt approach are lacking in the literature. There are severe gaps between theory, validity, and practicality of these models. In this study, a physically-based model for describing infiltration into a homogeneous deep stable soil profile with uniform initial water content distribution is presented. The derivation of the model is based on the concept of mapping the actual wetted soil zone into an equivalent rectangular saturated zone with constant effective hydraulic conductivity and capillary pressure head at the abrupt wetting front. The model takes into account the intial water mobility in the soil profile and predicts the system response under any rainfall pattern. The proposed model is further simplified, for practical purposes, by grouping three basic input parameters into a single parameter, the Surface Saturation Index.

The validity of the model was tested and strongly supported by the results of laboratory experiments conducted on samples of three soils, two silt loams and a silty clay loam, under five different (variable and constant) water application rate patterns. It was found that the amount of runoff from a nearly symmetrical convex variable rainfall pattern was very close to that which resulted from its equivalent average constant application rate pattern, regardless of the significant differences between the times at which runoff begins and the characteristics of infiltration rates thereafter.

¹ Contribution from the Utah Agric. Exp. Stn. (Project 591), Utah State Univ., Logan, UT 84322.

² Postdoctoral Fellow, and Associate Professor, respectively, Dep. Agric. and Irrig. Eng., Utah State Univ., Logan.

Received for publication December 13, 1976. Accepted for publication March 25, 1977.