Estimating nutrient loss to waterways—an overview of models of relevance to New Zealand pastoral farms

Abstract

Reliable estimation of nutrient losses from farmland is of increasing interest, driven by both economic and environmental concerns. Routine direct measurement of nutrient losses is currently impractical given the scale and variability of the problem. Simulation models are the best alternative and their use for assessing potential nutrient losses has been increasing worldwide. In New Zealand, there are a considerable number of models in use, or that are being developed, aiming to estimate N and P losses from pastoral fields. This range of alternative models reflects both the different level of detail and scale at which N and P losses can be estimated, and the diverse range of purposes assumed during the model development. Thus, it is important to understand the differences between models in order to select the one that will produce estimates appropriate to the intended use. This work presents an overview of the principal models for estimating nutrient loss being used or developed in New Zealand. It emphasises models that deal with N and P losses from pastoral farming systems, particularly via leaching, and that may allow the handling of different farm management procedures. Most of the models have gone through some testing and are supported by published works, although some are not fully operational yet and others need further evaluation. There is, in general, a lack of organised information about how several of these models work and what their main purposes are. We aim to supply some basic information about the available tools, sorting them into categories to highlight their primary differences and similarities. This is intended to assist discussions about model selection as well to highlight where information gaps about particular models need to be addressed.