Estimating future scenarios for farm–watershed nutrient fluxes using dynamic simulation modelling

A dynamic model of phosphorus (P) movement through the Peel-Harvey watershed in South Western Australia was developed using STELLA dynamic modelling software. The model was developed to provide a means to illustrate watershed P flux and of predicting future P loss scenarios. Model input parameters were sourced from extensive surveys of local agricultural practices and regional soil testing data. Model P-routing routines were developed from the known interactions between the various watershed P compartments and fluxes between various P stores. The model simulated a 200 year time-frame to reflect 100 years to the present day since initial land development, and forecast 100 years into the future. Although the watershed has an annual P loss target of 70 tonnes per annum (tpa), the measured present day loss is double this amount (140 tpa) and is projected to rise to 1600 tpa if current land management practices continue. This has significant implications for both future land use and subsequent water quality in the watershed.