Potential deep drainage under wheat crops in a Mediterranean climate. I. Temporal and spatial variability

High rates of deep drainage (water loss below the root-zone) in Western Aus tralia are contributing to groundwater recharge and secondary salinity. How ever, quantifying potential drainage through measurements is hampered by th e high degree of complexity of these systems as a result of diverse soil ty pes, a range of crops, different rainfall regions, and in particular the in herent season-to-season variability. Simulation models can provide the appr opriate means to extrapolate across time and space. The Agricultural Produc tion Systems Simulator (APSIM) was used to analyse deep drainage under whea t crops in the Mediterranean climate of the central Western Australian whea tbelt. In addition to rigorous model testing elsewhere, comparisons between simulated and observed soil water loss, evapotranspiration, and deep drain age for different soil types and seasons confirmed the reasonable performan ce of the APSIM model. The APSIM model was run with historical weather records (70-90 years) acros s 2 transects from the coast (high rainfall zone) to the eastern edge of th e wheatbelt (low rainfall zone). Soils were classified as 5 major types: de ep sand, deep loamy sand, acid loamy sand, shallow duplex (waterlogging), a nd clay soil (non-waterlogging). Simulations were carried out on these soil types with historical weather records, assuming current crop management an d cultivars. Soil water profiles were reset each year to the lower limit of plant-available water, assuming maximum water use in the previous crop. Re sults stressed the high degree of seasonal variability of deep drainage ran ging from 0 to 386 mm at Moora in the high rainfall region (461 mm/year ave rage rainfall), from 0 to 296 mm at Wongan Hills in the medium rainfall reg ion (386 mm/year average rainfall), and from 0 to 234 mm at Merredin in the low rainfall region (310 mm/year average rainfall). The largest amounts of drainage occurred in soils with lowest extractable water-holding capacitie s. Estimates of annual drainage varied with soil type and location. For exa mple, average (s.d.) annual drainage at Moora, Wongan Hills, and Merredin w as 134 (73), 90 (61), and 36 (43) mm on a sand, and 57 (64), 26 (43), and 4 (18) mm on a clay soil, respectively. These values are an order of magnitu de higher than drainage reported elsewhere under native vegetation. When no t resetting the soil each year, carry-over of water left behind in the soil reduced the water storage capacity in the subsequent year, increasing long -term average deep drainage, depending on soil type and rainfall region. Th e analyses revealed the extent of the excess water problem that currently t hreatens the sustainability of the wheat-based farming systems in Western A ustralia.