Estimating Hydraulic Conductivity for Models of Soils with Macropores

Abstract

A simple and efficient method was developed to determine soil macroporosity and hydraulic conductivity for dual-porosity models from measurements of unconfined infiltration rates. The utility of this method was demonstrated by analyzing unconfined infiltration tests conducted with a tension infiltrometer at ponded conditions and at negative 3, 6, and 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 and hydraulic conductivity of the soil matrix was identified. Soil macroporosity remained constant near the surface but it decreased with soil 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 reflected by a short macroscopic-length scale. The method described in this paper relied on measurements of unconfined infiltration rates to minimize soil disturbance. This method provided several advantages over measuring the macroporosity with double-ring infiltrometer, which requires driving a ring into the soil to establish one-dimensional flow.