An improved model for linking phosphorus loads in runoff to climate, soil and agricultural management

Abstract: Soil phosphorus (P) is one of the nutrients that contribute to eutrophication of waterways, algal blooms and damage to coral reefs. Reducing P movement from farm paddock to the broader environment often requires comprehensive, catchment-wide approaches to management. The substantial complexity of the physical and managerial causes of P pollution has led to a role for simulation models in evaluating the efficacy of regulations and management changes. In Queensland, the Reef Plan program (http://www.reefplan.qld.gov.au) addresses issues of sediment, nutrient and pesticide impact on the health and future of the Great Barrier Reef. Some of the associated regulations are concerned with the use of P fertilizer. The HowLeaky? model is one of few biophysical models that represent agricultural management, biophysical conditions and P exports. As reported at MODSIM07, information about P export was added to an existing model of water balance and sediment export (HowLeaky?) to create a useful model of P export from agriculture. However, the predictive power of the model was modest, especially over short periods (e.g. individual days). To improve the predictive power of the model, several changes have been made, including (i) additional empirical functions for estimating the enrichment of total P in sediment, and (ii) additional functions for estimating concentration of soluble P in runoff (mg P/L). Soil adsorption of P (P buffering), which affects the soluble P concentration in runoff, is now estimated from the widely available phosphorus buffering index test (PBI) rather than phosphorus buffering capacity (PBC). Large, high-quality datasets are used to evaluate these relationships. A case study is presented where HowLeaky? has been used to estimate P loads generated at a site in the GBR catchment. The Brigalow Catchment Study is a long-term study of the effects of land clearing and agriculture on the water balance and water quality. It is located in central Queensland, has a subcatchment that is cropped, has representative soils, and is part of the Fitzroy Catchment. HowLeaky? reproduced daily runoff amounts very well, and the revised P model in HowLeaky? was acceptably accurate at predicting daily P loads. Modelling predicted the long-term totals (6 years) of soluble and particulate P export from the field site with less than 50% error. This is a better-than-expected result with an un-calibrated model, as P exports are notoriously difficult to predict