Flow of water under transient conditions in unsaturated soils

Abstract

An experimental investigation of the behavior of soil water movement under unsaturated transient conditions is reported for the case of vertical infiltration into a sandy loam and silt loam soil material. Water was allowed to enter at a small constant suction into air-dry columns of soil and its subsequent distribution followed with a gamma radiation attenuation device. An analytic expression of water content as a function of depth and time was obtained by multiple regression analysis from which it was possible to determine the instantaneous flux and the water concentration gradient at given water contents. During the early stages of infiltration the relationship between the flux and gradient was linear as prescribed by the Darcy equation. At later times when the gradient became less steep linear proportionality broke down. Non-linearity at low water gradient was evidenced for both soils throughout a wide range of water contents. The magnitude and direction of the departure from linearity was similar for both soils indicating the deviations were possibly not due to specific soil properties, but rather to an inherent characteristic of the flow system itself. An empirical flow equation modeled after the Darcy equation fits the data very well. The behavior of the equation parameters was strongly reminiscent of those of the Darcy equation. Where flux was proportional to the gradient, the equation reduced to the Darcy equation. If flux were not proportional to the water gradient then the term representing the diffusivity of diffusion analysis became a function of both the water gradient and water content.