FIELD-EVALUATION OF DRAINMOD FOR PREDICTING WATERLOGGING INTENSITY AND DRAIN PERFORMANCE IN SOUTH-WESTERN AUSTRALIA

Waterlogging is common on sloping duplex soils in south-western Austra lia and causes damage to non-irrigated cereal crops and pastures. The factors which affect the performance of surface seepage interceptor dr ains installed to reduce this waterlogging are complex because the soi ls are very variable and have preferred pathways for groundwater flow. We compared DRAINMOD's predictions with field measured waterlogging i ntensity and drain flow over 3 years near Mt Barker and Narrogin in We stern Australia. DRAINMOD failed to accurately predict waterlogging in tensities and drain flows because water can move through macropores wh ich bypass the soil matrix. At Mt Barker, DRAINMOD overpredicted water logging intensity by between 120% in a wet year and 650% in a very dry year. Drain flows were underpredicted by 148% in the driest year. At Narrogin, DRAINMOD underpredicted waterlogging intensity each year (ra infall was below average each year) and drain flow in the driest two y ears. However, by increasing K-s of the topsoil and adjusting K-s of t he subsoil clay, DRAINMOD predictions agreed with measured responses. DRAINMOD can be used to predict waterlogging intensities and drain flo ws in duplex soils in the >450 mm annual rainfall areas of south-weste rn Australia provided adjustments are made to the field point-measured K-s. Reliability increases with increasing rainfall during the growin g season.