EVALUATING DRAINAGE RESPONSES IN DUPLEX SOILS IN A MEDITERRANEAN ENVIRONMENT

Deep drainage below 0.5 m from a sloping duplex soil under a pasture i n south-western Australia was evaluated by water balance determination s over durations of a few days. Soil water dynamics were measured with time domain reflectometry, and evapotranspiration with the Bowen rati o technique. These near-continuous measurements enabled accurate deter minations of deep drainage to be made at a time scale that was consist ent in duration with deep drainage Events. From 15 June 1994 to 16 Oct ober 1994 at Narrogin (with below average rainfall of 350 mm), deep dr ainage accounted for 11-21 mm, and occurred entirely within days of on e particular rainfall event. This was associated with the only period of prolonged waterlogging at the site, and its occurrence was supporte d by a rise in the deeper groundwater. Water balance calculations from experimental data at monthly intervals could not detect drainage accu rately, due to the large uncertainty associated with ET determinations . The SWIM model predicted 15-25 mm deep drainage, depending on satura ted hydraulic conductivity of the B horizon, for the same rainfall eve nt. When errors in rainfall and evapotranspiration measurement were in cluded in the simulation, the range in predicted drainage was 6-31 mm.