WATER FLOW IN CLAY SOIL BENEATH A TENSION INFILTROMETER

Abstract

Tension infiltrometers have been used to demonstrate the contribution of macropore flow to total flow by comparing measured infiltration rates at various supply water potentials. Hydraulic conductivity may be extracted from infiltration data with theoretical analyses of uniform water flow beneath the tension infiltrometer. By-pass or macropore flow, though, is nonuniform flow of water. We wished to determine whether a combination of hydraulic conductivities determined from traditional analysis of tension infiltrometer data and laboratory-determined water capacities could be used to simulate the infiltration on the soil from which the hydraulic conductivity data was collected. We observed water flow patterns beneath tension infiltrometers placed at 0.5-m depth in highly structured clay soils after infiltration with water containing a dye. The water was allowed to infiltrate under two series of increasing supply water potentials ψ o ; -0.24, -0.12, -0.06, -0.03 m and -0.24, -0.12, -0.06, -0.03, -0.02, -0.01, 0 m. Infiltration rates from the tension infiltrometers reached steady state within a few minutes after the supply water potential of the infiltrometer was adjusted. Hydraulic conductivities were calculated from traditional theory based on the steady state infiltration rates. Specific water capacities were determined in the laboratory from soil cores. Infiltration was simulated with an axisymetric finite difference model. Dye stain patterns showed that water infiltrating at both -0.24 m ≤ ψ o ≤ -0.03 m and -0.24 m ≤ ψ o ≤ 0 m passed first through fissures between slickenside surfaces and between ped surfaces, and through biopores, and then into the soil matrix. We found dye stain at much greater depths than we simulated with the finite difference model that was based on uniform flow. We conclude that models using hydraulic conductivity-water potential relationships derived from present analysis of tension infiltrometer data may not give desired results when combined with laboratory-determined capacities.