USE OF THE APSIM WHEAT MODEL TO PREDICT YIELD, DRAINAGE, AND NO3(-) LEACHING FOR A DEEP SAND

High rates of drainage and leaching of nitrates in deep sands in Weste rn Australia are contributing to groundwater recharge and soil acidifi cation in this region. Strategies are being sought to increase water a nd nitrogen (N) use in the legume-based cropping systems. Choice of ap propriate management strategies is complicated by the diversity of soi l types, the range of crops, and the inherent season to season variabi lity. Simulation models provide the means to extrapolate beyond the bo unds of experimental data if accurate predictions of key processes can be demonstrated. This paper evaluates the accuracy of predictions of soil water content, evapotranspiration, drainage, inorganic N content in soil, nitrate (NO3-) leaching, wheat growth, N uptake, and grain yi elds obtained from the Agricultural Production Systems Simulator (APSI M) model when this was initialised with appropriate information on soi l properties and wheat varieties commonly grown on deep sands in the 5 00 mm rainfall zone west of Moora in Western Australia. The model was found to give good predictions of soil water content, evapotranspirati on, deep drainage, and overall NO3- leaching. Temporal changes in inor ganic N in soil were simulated, although the small concentrations in s oil inorganic N precluded close matching of paired observed and predic ted values. Crop growth and N uptake were closely predicted up to anth esis, but a poor fit between observed and predicted crop growth and N uptake was noted post anthesis. Reasons for the discrepancies between modelled and observed values are outlined. The model was run with hist orical weather data (81 years) and different initial soil water and in organic soil N profiles to assess the probability of drainage and NO3- leaching, and the grain yield potentials for wheat grown on deep sand s in the region west of Moora. Simulation showed that the soil water a nd the soil inorganic N content at the beginning of each season had no effect on grain yield, implying that pre-seed soil NO3- was largely l ost from the soil by leaching. There was a 50% probability that 141 mm of winter rainfall could drain below 1.5 m and a 50% probability that 53 kg N/ha could be leached under wheat following a lupin crop, where initial soil water contents and soil NO3- contents used in the model were those measured in a deep sand after late March rainfall. Simulate d application of N fertiliser at sowing increased both grain yield and NO3- leaching. Splitting the N application between the time of sowing and 40 days after sowing decreased NO3- leaching, increased N uptake by wheat, and increased grain yield, findings which are consistent wit h agronomic practice. The high drainage and leaching potential of thes e soils were identified as the main reasons why predicted yields did n ot approach the French and Schultz potential yield estimates based on 20 kg grain yield per mm of rainfall. When the available cater was red uced by simulated drainage, simulated grain yields for the fertilised treatments approached the potential yield line.