Nutrient Trading in Lake Rotorua: Determining Net Nutrient Inputs

Lake Rotorua is experiencing increasing nutrient-related water quality problems. This paper is one in a series that explores the idea of creating a nutrient trading system as part of the ongoing policy response to this problem.1 Most of the current nutrient flows to the Lake come from non-point rural sources - measuring these emissions is challenging. We find that it is possible to monitor/model nutrient loss from a wide range of activities in the Rotorua catchment. The model OVERSEER combined with ROTAN and some other models for forestry, urban and geothermal activities and horticulture already exist. They are currently in a process of enhancement - a particular area of current weakness is knowledge of the groundwater lags from specific locations in the catchment. The land-based models need to be used in a specific form that relies on initialisation with verifiable data and uses easily collated and verified data on an annual basis. The form of the model should be fixed for each regulatory year to minimise uncertainty for landowners and regulators. The models need to be updated to reflect new science. The process for doing this needs to be strategic and credible (this will be discussed in a later paper on governance processes). Once changes are recommended they need to be implemented in a way that is perceived to be fair.