Evaluating drainage responses in duplex soils in a Mediterranean environment

Abstract

Deep drainage below 0 ·5 m from a sloping duplex soil under a pasture in south-western Australia was evaluated by water balance determinations over durations of a few days. Soil water dynamics were measured with time domain reflectometry, and evapotranspiration with the Bowen ratio technique. These near-continuous measurements enabled accurate determinations of deep drainage to be made at a time scale that was consistent in duration with deep drainage events. From 15 June 1994 to 16 October 1994 at Narrogin (with below average rainfall of 350 mm), deep drainage accounted for 11-21 mm, and occurred entirely within days of one particular rainfall event. This was associated with the only period of prolonged waterlogging at the site, and its occurrence was supported by a rise in the deeper groundwater. Water balance calculations from experimental data at monthly intervals could not detect drainage accurately, due to the large uncertainty associated with ET determinations. The SWIM model predicted 15-25 mm deep drainage, depending on saturated hydraulic conductivity of the B horizon, for the same rainfall event. When errors in rainfall and evapotranspiration measurement were included in the simulation, the range in predicted drainage was 6-31 mm.