Theory of rainfall uptake by soils initially drier than their field capacity and its applications

The Darcian approach to moisture transfer in unsaturated soils yields a flow equation that was utilized for formulating a theory of rainfall infiltration. Analytical considerations applied to this equation yield a qualitative description of the infiltration process, as well as the necessary and sufficient conditions for the occurrence of ponding and the absence of soil‐parameter hysteresis. Numerical solutions of the flow equation, obtained by means of finite‐difference methods, furnish quantitative information on the moisture profiles and infiltration rates during nonponding, preponding, and ponded‐rainfall infiltrations. The theory is not utilizable when the soil exhibits significant air compression, parameter hysteresis,fabric transformations, or areal heterogeneity. For soils with known moisture parameters and rains of a given intensity‐pattern, the theory, whenever applicable, can furnish information useful in dealing with problems of runoff control or sprinkler irrigation, e.g., estimates of excess rainfall rates and of water uptakes at incipient ponding.