Infiltration from surface ponds into soils overlying a very permeable substratum

Abstract The effect of an underlying very permeable substratum below a uniform soil on steady infiltration rates from surface ponds is discussed. The steady infiltration rate is the same as that for a deep soil until the wetting front reaches the capillary fringe above the water table. The soil below the pond then rapidly becomes saturated at a positive soil-water pressure with the water table joining with the free water in the surface pond, and the infiltration rate is reduced to a new steady-state value. For a circular infiltration pond, a shape factor was defined to relate this steady rate to the pond radius, the depth of soil to the substratum, the hydraulic conductivity of the soil and the prevailing artesian pressure. Shape factors for a range of ratios of soil depth to pond radius were found from an analytical solution and also from numerical solutions of Laplace's equation, using boundary conditions that approximated to those of the flow problem. An empirical expression for the shape factor is suggested. Infiltration from a circular surface pond on a silt loam soil overlying a raised beach terrace showed a reduction of the steady infiltration rate when the advancing wetting front joined with the underlying capillary fringe. Hydraulic conductivity values deduced from the two steady rates were in agreement.