Loss of nitrate from farmland to freshwater can harm the environment and diminish drinking water quality. In response to this, and the National Policy Statement for Freshwater Management (2011), regional authorities across New Zealand are developing plans to manage water quality. Many of these plans include use of OVERSEER®, a computer model originally designed to assist farmers and their advisors with on-farm nutrient use, for estimating nitrate losses from individual pastoral farms. Given this emerging and new regulatory role for OVERSEER®, and the relatively recent addition of crop modelling to the programme, the Foundation for Arable Research (FAR) commissioned a review of the arable cropping model of OVERSEER®. The purpose of the review was to assess the strengths, weaknesses and further developments that would improve the usefulness and usability of the cropping model of OVERSEER®. To do this FAR assembled an expert panel to review the available information regarding development and testing of the OVERSEER® crop model and compare this to other plant/soil interaction models. The review concluded that OVERSEER® is the best tool currently available for estimating nitrate leaching losses from the root zone across the diversity and complexity of farming systems in New Zealand. More specific findings included: OVERSEER‘s® crop model has been developed using simplifications that are consistent with the approach taken in modelling pastoral systems but that contrast with other cropsoil interaction models. Whilst the crop model has been tested to a limited extent, further testing is needed to determine whether these simplifications impair its ability to predict long-term average nitrate leaching losses from arable systems. OVERSEER‘s® user interface for crops is relatively under-developed compared to the pastoral model and in need of further attention before it will be able to deal effectively with complex crop rotations.
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