Estimating future scenarios for farm-watershed nutrient fluxes using dynamic simulation modelling – Cac on-farm BMPs really do the job at the watershed scale?

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 illustrate watershed P flux and to predict future P loss rates under a range of management scenarios. Input parameters were sourced from 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. P-retention characteristics of a variety of management practices were determined from field trials where available and published values where not. 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 this is projected to rise to 1300 tpa if current land management practices continue. Even if broad-scale BMP implementation occurs, P losses are likely to increase to approximately 200 tpa. This has significant implications for both future land use and subsequent water quality in the watershed