Estimating nitrate leaching under a sugarcane crop using APSIM-SWIM

The Lower Burdekin Delta (LBD) is located on the dry-tropical coastal strip in North Queensland, Australia. The region is Australia’s largest sugar producing area with approximately 38,000 hectares of land under sugarcane. However, the LBD also borders the Great Barrier Reef World Heritage Area (GBRWHA). Thus industry, community, and federal, regulatory and environmental organisations are interested in ascertaining the magnitude of nitrate leaching from the root zone and potential implications for the GBRWHA. Direct measurement of nitrate leaching is problematic, thus it is likely that modelling will play an ever-increasing role in guiding experimental work and decision-making. The difficulty in monitoring nitrate leaching also raises concerns with respect to capturing data for the purpose of calibrating and verifying models. However, other crop/soil parameters that directly affect the amount of nitrate leaching are more readily measured (e.g., soil hydraulic properties, matric potential, crop yield, fertilizer application timing, and initial soil N). Thus, parameterising/calibrating a model to such data raises the expectation that associated model outputs related to nitrate leaching will be useful. We used APSIM (Agricultural Production Systems Simulator), with constituent crop-growth, soil-water, and nitrogen transformation modules (Sugar, APSIMSWIM, Soiln2) to model the 2001/2002 sugarcane ratoon crop at Kalamia, field 48, within the LBD and found the measured and modelled soil water nitrate loads at 1.5 m depth to be of a similar order. This result suggests that the modelled nitrate leaching values are likely to be indicative of the magnitude of nitrate leaching that occurs under field 48. Subsequent sensitivity analyses identified the timing and mode of fertilizer application as key parameters over which management control might be exploited to minimise the flux of nitrate to groundwater.