In-situ characterization of unsaturated soil hydraulic properties of a laterite soil profile in coastal karnataka

Abstract

Unsaturated laterite soil formations play an important hydrologic role in the West Coast region of India. However, very little information on the hydraulic/hydrologic properties of lateritic soils exists in literature. The present study was taken up to characterize the unsaturated soil water retention curves and hydraulic conductivity function of a sandy lateritic soil profile located in the coastal district of Dakshina Kannada, Karnataka State. This was achieved by conducting a field internal drainage test involving measurements of soil moisture contents and soil water pressures at various depths within a 1.5 m deep soil profile over a period of 39 days. Soil moisture contents were measured using a sophisticated Time Domain Reflectrometry (TDR) probe and soil water suction pressures were measured using tensiometers. The water retention curve for each depth layer was characterized by fitting the popular van Genucthen model to observed moisture contents and suction pressures. Depth-wise differences in the shapes of the water retention curves were evident. Observed temporal variations in moisture content and suction pressures during internal drainage were analyzed using well-established procedures to calculate unsaturated hydraulic conductivity values. Results indicated more or less homogeneous depth-wise drainage behaviour and a single relationship based on van Genucthen's model was sufficient to describe the hydraulic conductivity variations with moisture content for the entire profile. Computed hydraulic conductivities showed extreme sensitivity to soil moisture contents and varied from a low value of 0.47 cm/d at moisture content of 0.208 cm3/cm3 to as high as 134.82 cm/d at 0.261 cm3/cm13. Results of this study can provide useful inputs in applications of mathematical models to describe water flow and solute transport in unsaturated lateritic soil media. This may lead to better understanding of the role of the unsaturated soil zone in the hydrologic cycle in the West Coast region and thereby lead to more efficient designs of water conservation structures. 2006 Taylor & Francis Group, LLC.