Eutrophication In Lake Rotorua. 2. Using ROTAN and OVERSEER to model historic, present and future nitrogen loads

OVERSEER is used in New Zealand to estimate nutrient losses from farmland, but does not quantify subsequent movement through the catchment, or attenuation. This paper uses the ROTAN model, based on the Scandinavian HBV-N model, to route nitrogen losses from 1900–2015 to Lake Rotorua where groundwater age ranges from 14 to 170 years. ROTAN conceptualises three delivery pathways (quickflow, groundwater and streamflow) with different attenuation. When calibrated to measured stream and groundwater concentrations, several combinations of attenuation gave equally good fits largely because of sparse and uncertain input and calibration data. Nevertheless, lake N loads were predicted for current land use (754 ± 39 t y−1) and with proposed N loss reductions (431 ± 26 t y−1). Probabilities were also calculated that the reductions are more (12%–18%) or less (82%–88%) than required to meet the target lake N load (405 t y−1). ROTAN shows promise for calculating nitrogen movement in catchments dominated by groundwater where there is limited data.