Systems analysis of wheat production on low water-holding soils in a Mediterranean-type environment: II. Drainage and nitrate leaching

In Mediterranean-type environments, the concentration of rainfall in winter months results in average winter rainfall that is in excess of evaporative demand. Cropping coarse textured soils in such regions results in a risk of drainage below the root zone, and associated with this, nutrient leaching. We used the APSIM-Nwheat simulation model to quantify the magnitude and variability of drainage and nitrate–N leaching under wheat crops for six locations and three soil types in the northern sandplain region of the Western Australian wheat belt and to assess the impact of varying crop management on drainage and leaching. Overall, the combination of a high concentration of rainfall in the winter months and coarse soil types resulted in a significant risk of drainage and leaching events of considerable magnitude even at the driest sites considered: the assumption that leaching and drainage are low in areas of low rainfall is an over-simplification. For some locations, simulated drainage was high, exceeding 100 mm for two locations on two soils; the sand and the acid loamy sand. Across the six locations considered, drainage was linearly related to average growing season rainfall. Leaching varied markedly between the soil types, with loamy sand having only one fifth the leaching that was calculated for the acid loamy sand or the sand. This emphasises the importance of small differences in soil type for the risk of drainage and leaching, and hence the potential for negative off-site effects, when cropping light soils in a Mediterranean-type environment. Although sandy soils are held to present the most scope for reducing drainage through agronomic management, the analysis suggested the potential improvements are likely to be small. Consistent with experimental results from other parts of the Western Australian wheat belt, modification of rooting depth appears to present the best option to reduce drainage beneath annual crops.