Modified Green-Ampt Infiltration Model for Steady Rainfall

The Green-Ampt equation describing the infiltration of water into soil requires that the rainfall intensity is greater than the infiltration capacity of the soil at all times. The well-known Mein-Larson model modifies the Green-Ampt equation and attempts to avoid this restriction for infiltration of water into a homogeneous soil under rainfall of constant intensity. The rainfall intensity is considered to be less than the infiltration capacity of the soil in the initial phases of a storm. The Mein-Larson model assumes that until the time to ponding of water over the soil surface, the infiltration rate is constant and equals the rainfall intensity. This study shows that the Mein-Larson model underestimates the ponding time and that the cumulative infiltration predicted by the model is always less than the cumulative infiltration potential of the soil. To avoid these drawbacks, an alternative modification of the Green-Ampt equation is presented. In the proposed modification, the ponding time is considered to occur when the cumulative infiltration potential of the soil is fully satisfied. Beyond the ponding time, both the infiltration rate and cumulative infiltration follow the classical Green-Ampt equations. The modification is achieved by using asymptotic matching technique to render the model applicable within a wide range of rainfall rates greater or less than the soil infiltration capacities. This modification will help extend the ability to analyze hydrologic problems related to water infiltration into soil in different environments of high or low rainfall intensities. Case studies are provided to demonstrate the behavior of the proposed model and compare it with the Mein-Larson modification. (C) 2014 American Society of Civil Engineers.