|Prasad, S. N. - UNIV. OF MS, CIVIL ENGR|
Submitted to: International Soil Conservation Organization Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: May 31, 2002
Publication Date: May 31, 2002
Citation: Prasad, S., Romkens, M.J. 2002. Infiltration into soil with dynamic surface seals. 12th International Soil Conservation Organization Conference Proceedings. 2:92-98. Interpretive Summary: The hydrologic response of watersheds during a rainstorm is determined by many factors, the most important being infiltration. Infiltration is a complicated process that is controlled by the intrinsic properties of the soil, such as the water content/soil water potential relationship and the water content/hydraulic conductivity relationship, but also by the changing nature of the soil surface matric and other antecedent conditions. On unprotected soil surfaces, raindrop impact destroys the soil matrix structure, which leads to the development of a partially sealed condition which, upon drying, becomes a crust. This seal/crust acts as an impedance to water entry or infiltration. In this article, solutions are derived that predict infiltration into soils with surface seals. Two cases were considered: (1) the case of a fixed crust, and (2) the case of a developing surface seal. The approximate integral solutions obtained are expressed in terms of soil parameters, including the seal impedance which, for the dynamic case, is a function of the rainfall intensity and rainfall energy, the subsoil water content, wetting front depth, and cumulative infiltration. These relationships will be very helpful in runoff and infiltration prediction models for situations similar to those considered in this article.
Technical Abstract: For the purpose of developing an infiltration equation under a dynamically sealing soil surface, Richards' equation of unsaturated flow for a layered system is considered. An analytical approach based on the concept of dynamic wetting front is utilized to form the basis of fundamental solutions of Richards' equation applied to the layered system. Since the interface between the layer and the substrate gives rise to a discontinuity in water content, another dynamic variable representing hydraulic conductance is introduced to facilitate the analysis. Thus, we consider a dynamic seal of thickness h(t) to be on top of the soil column, z > h. Both media are homogeneous and isotropic and the seal is much less conductive than the column and h is of the order of a few millimeters. First, the general solution of unsaturated flow with appropriate boundary conditions are developed, which consists of concentration and flux boundary value cases. The dynamic condition at the interfaces between the seal and the substrate permits utilization of these solutions to yield infiltration rate under natural rainfall condition. The same set of solutions also are used to examine the role played by the seal parameters. In particular, infiltration process by the concept of hydrodynamic conductance is examined through the solutions of Richards' equation. The dynamics of cumulative infiltration R is studied at the end by the solution of a set of differential equations in terms of R, wetting front delta, and the interfacial water content Theta 0. An analytical closed form solution for R is also found for certain cases.