WRRCTR No.143 Water Conduction in Hawaii Oxic Soils

Abstract

Oxic soils on Oahu were studied to develop and test simplified methods of determining the hydraulic conductivity of unsaturated soils, to test some simple infiltration models, and to assess the utility of soil survey mapped units in defining hydrologically similar soils. Field measurements of water infiltration and redistribution were accomplished on 21 sites located on the Lahaina, Molokai, and Wahiawa soil series. Water retention curves measured on undisturbed soil cores from the Ap1, Ap2, and B horizons of each site provided a means of determining the downward flux of water during redistribution from soil water suction measurements over time. These data allowed calculation of hydraulic conductivities (by a detailed Darcy analysis) of soil at various depths in the soil profile and for a range of water contents and suctions. The detailed analysis and field infiltration data provided a means of evaluating two new simplified methods of determining hydraulic conductivity functions of well-drained soils; the new methods are sufficiently accurate and economical to be used in watershed characterization. Also, field measured sorptivity and water redistribution data were used to successfully predict cumulative infiltration with the Talsma-Parlange and Green-Ampt equations, respectively. Statistical analysis of field and laboratory data suggested that soil maps of central Oahu would not be particularly useful in delineating soil areas of relative homogeneity with respect to hydrologic properties. These results further emphasize the need for simple methods to characterize hydrologic properties of importance.