Potential deep drainage under wheat crops in a Mediterranean climate. II. Management opportunities to control drainage

High rates of deep drainage in Western Australia are contributing to groundwater recharge and secondary salinity. Strategies are being sought to increase water use in cropping systems and to reduce deep drainage. Quantifying potential drainage through measurements is hampered by the high degree of complexity of these systems as a result of diverse soil types, a range of crops, and in particular the inherent seasonal variability. Simulation models can provide the appropriate means to extrapolate across time and space. The Agricultural Production Systems Simulator (APSIM) was used to explore the effect of alternative agronomic practices on wheat production and deep drainage for representative soils and rainfall regions of the central wheatbelt of Western Australia. Soil water profiles were reset each year to the lower limit of plant-available water, assuming maximum water use in the previous crop. The long-term simulation studies showed that management practices with N fertiliser directed at yield increase were most effective in achieving these aims in the medium to high rainfall regions. The corresponding effect for drainage reduction was marginal. The small effect on drainage control associated with production increase can be traced to the effect of rainfall distribution with major occurrences of both rainfall and drainage during winter (June-August) coinciding with the lowest potential atmospheric demand for evapotranspiration, in combination with low water-holding capacity soils. Nitrogen-induced increases in crop transpiration were in conjunction with reduced soil evaporation, which increased water use efficiency and occurred mostly after the main drainage period, but had little effect on deep drainage within the season. Similar outcomes of enhanced productivity with minor impact on deep drainage were noted with crops sown at different times and with a hypothetical wheat crop having a deeper rooting system. Simulations without resetting soil water each year enabled the quantification of potential carryover effects on long-term average deep drainage. The carry-over of soil water left behind at crop harvest reduced the water storage capacity of the soil in a subsequent year and could increase long-term deep drainage substantially, depending on soil type. Improved management increased late water use in the high rainfall region, in particular on better water-holding soils, and could largely reduce this carry-over effect. The current wheat-based cropping systems, even with alternative management practices, continue to be a major threat to sustainability on the low water-holding soils in the wheatbelt of Western Australia, as a main cause of secondary salinity.