Understanding the distribution and fate of nitrogen and phosphorus in OVERSEER

OVERSEER® Nutrient Budgets (Overseer) is an agricultural management support tool that examines the flow of nutrients in a farming system. There is increasing pressure from a range of users for transparency of the way Overseer functions, particularly the modelling of nitrogen (N) and phosphorus (P) loss to water. The aim of this paper is to provide a conceptual description of the way Overseer models the distribution and fate of N and P in a pastoral system, to support user understanding and correct model use. The core of Overseer is a nutrient budget, which accounts for the flow of nutrient into, around and off the farm. The key strength of Overseer is its ability to model these nutrient transfers around the farm, identifying how much, where and when nutrients move. Other parts of the model then estimate the fate of these nutrients. Nitrogen and P cycle differently around the farm, which is reflected in the way they are modelled. This paper is intended to be a support document for understanding the way Overseer models N and P, and where more detailed information is required, it may be found on the Overseer website (www.overseer.org.nz). Keywords: Nutrient budget, model, leaching, run-off