Nitrogen yields from New Zealand coastal catchments to receiving estuaries

A model to estimate nitrogen loads from coastal catchments to New Zealand estuaries is presented. The Sub-Catchment to Estuary Nitrogen Yield (SCENY) model estimates total nitrogen inputs from atmospheric deposition, fertiliser application, and biological nitrogen fixation for catchments with different land use practices. Nitrogen losses in the vadose zone and aquifers were assessed, and the net nitrogen yield that enters estuaries was quantified. The model was applied to 13 sub-catchments in the South Island, which encompass 0-91% agricultural land use. Nitrogen yields to the study estuaries ranged from 0.6 kg N ha(-1) yr(-1) to 17.0 kg N ha(-1) yr(-1), with nitrogen flux increasing directly with the percentage of agriculture in the catchment. Moreover, fertiliser contribution to nutrient loading increased proportionately with increased nitrogen yields. The model was in close agreement with other New Zealand nitrogen yield models and confirms that management of water quality for estuaries surrounded by agricultural catchments should be targeted at the local level, with improved measures for controlling fertiliser run-off.