INFILTRATION FROM SURFACE PONDS INTO SOILS OVERLYING A VERY PERMEABLESUBSTRATUM

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 th e wetting front reaches the capillary fringe above the water table. Th e soil below the pond then rapidly becomes saturated at a positive soi l-water pressure with the water table joining with the free water in t he surface pond, and the infiltration rate is reduced to a new steady- state value. For a circular infiltration pond, a shape factor was defi ned to relate this steady rate to the pond radius, the depth of soil t o the substratum, the hydraulic conductivity of the soil and the preva iling artesian pressure. Shape factors for a range of ratios of soil d epth to pond radius were found from an analytical solution and also fr om numerical solutions of Laplace's equation, using boundary condition s that approximated to those of the flow problem. An empirical express ion for the shape factor is suggested. Infiltration from a circular su rface pond on a silt loam soil overlying a raised beach terrace showed a reduction of the steady infiltration rate when the advancing wettin g front joined with the underlying capillary fringe. Hydraulic conduct ivity values deduced from the two steady rates were in agreement.