ESTIMATING HYDRAULIC CONDUCTIVITY FOR MODELS OF SOILS WITH MACROPORES

A Simple and efficient method was developed to determine soil macropor osity and hydraulic conductivity for dual-porosity models from measure ments of unconfined infiltration rates. The utility of this method was demonstrated by analyzing unconfined infiltration tests conducted wit h a tension infiltrometer at ponded conditions and at negative 3, 6, a nd 15 cm of water-supply pressure. The conductivity of soil macropores (pores > 1 mm in diameter) was 3.6 times the conductivity of the soil matrix. This contrast in the magnitudes of the hydraulic conductivity may justify the use of dual-porosity models of water flow and solute transport. Positive but small correlation between soil macroporosity a nd hydraulic conductivity of the soil matrix was identified. Soil macr oporosity remained constant near the surface but it decreased with soi l depth. The narrow range of 0 to -15 cm of water pressure may govern water flow and contaminant transport under field conditions because of the rapid decrease of conductivity with water pressure, which was ref lected by a short macroscopic-length scale. The method described in th is paper relied on measurements of unconfined infiltration rates to mi nimize soil disturbance. This method provided several advantages over measuring the macroporosity with double-ring infiltrometer, which requ ires driving a ring into the soil to establish one-dimensional flow.