Infiltration during a rainfall event can be divided into two distinct stages: a stage with surface ponding and a stage without surface ponding. Few of the infiltration models in current use are suitable to describe infiltration for both stages. In this paper the Green and Ampt equation was applied to determine the time that separates these two stages so that infiltration for the different stages can be treated separately. To obtain an integrated form of the Green and Ampt equation, it is traditionally assumed that the cumulative infiltration is zero at the time when surface ponding starts. But in a rainfall event the cumulative infiltration equals the water infiltrated into the soil profile prior to the ponding time, which is usually not zero. Therefore a modification in the traditional Green and Ampt equation is needed to describe infiltration during a rainfall event. It is shown in this paper that this modification is equivalent to a shift of the time scale by an amount which is referred to as the pseudotime by the author. The modified version of the Green and Ampt equation was applied to determine rainfall excess and to predict total runoff for three major storms recorded by the Agricultural Research Service from 1957 to 1959 on a watershed near Oxford, Mississippi. A comparison of the prediction and the measured total runoff appeared to be promising.