The amounts of nitrogen and phosphorus travelling off land into water depend to a large extent on how the land is used. In heavily forested areas, for instance, there are few animals producing nitrogen-rich urine and phosphorus-rich dung, while the tree roots hold the soil on the land. In areas dominated by pasture, especially dairy pasture, nutrient loss rates are much higher. Consequently, when land uses change, the amount of nitrogen and phosphorus lost from land to water also changes.
Much of the Commissioner’s work in this area has been based on the amalgamation of two models: Land Use in Rural New Zealand (LURNZ) and Catchment Land Use for Environmental Sustainability (CLUES).
The Commissioner revisited this aspect of water quality in a recent update report. That report contains new modelling information showing that land conversion to dairy farms has continued, but a predicted increase of forested land has not – and that is bad news for water quality.