Modelling phosphorus losses from pastoral farming systems in New Zealand

A model was developed for incorporation into the computer model “Overseer® nutrient budgets 2” to estimate phosphorus loss from pastoral farms to surface waters. The model sums potential risk of phosphorus loss from soil (background) with incidental losses attributed to the application of fertiliser or effluent to fields or waterways. The overall risk is estimated as the amount of P lost annually (kg P ha–1 yr–1). Data for 23 sites of grazed pasture (sheep, beef, and dairying) ranging from small plot trials to catchments were used to validate each component of the model. A good relationship (R2 = 0.97***) was obtained between measured and estimated soil P losses for systems without much incidental P loss. Similarly, a good relationship was observed between measured and estimated overall (soil + incidental) P losses for all systems (R2 = 0.96***). The estimated loss of P is categorised into low, medium, high or extreme risk to the environment (<1, 1–2, 2–4, and >4, respectively) and best management strategies are suggested accordingly. The model shows promise in directing responsible P management for New Zealand pastoral farms. However, inadequacies are likely when scaling up the model for use in large catchments where land use and soils vary greatly.