Predicting land-based nitrogen loads and attenuation in the rangitikei river catchment – the model development

Under the National Policy Statement for Freshwater Management 2014 (NPSFM), Regional Councils are required to produce a set of ‘freshwater resource accounts’ for defined individual Freshwater Management Units (FMUs) in their regions. This requires establishment of a freshwater quality accounting system which is based on measured, modelled or estimated loads of relevant water contaminants in groundwater and surface water bodies. Nutrient budgeting tools (such as Overseer NB) have been developed to account for nutrient flows and losses from farming systems. However, these models mostly account for nutrient flows within the farm boundary and predict nutrients losses from the root zone. Catchment characteristics like topography, rainfall, the nature of the vadose zone, underlying geology, and subsurface geochemistry may further affect the transport and transformation of nutrients such as nitrate-nitrogen (NO3-N) along flow pathways from farms to rivers and lakes. We investigated and developed a simple model to account for the influence of different soil types and underlying geology on the transformation of nitrogen (N) in the Rangitikei River catchment. The main soil and rock types of the catchment were classified into low, moderate and high N attenuation capacities, depending on their texture, drainage rate and carbon content. These N attenuation capacity classes were assigned nitrogen attenuation values in order to predict soluble inorganic nitrogen (SIN) loads to the river. The river SIN loads predicted in this manner were compared with the SIN loads measured in the river. We found that the SIN loads measured in the river were significantly smaller than the estimates of the quantities of nitrogen leached from the root zone. The prediction of SIN loads in the river was improved by incorporating the spatial effects of both of the different soil types and underlying geologies on N attenuation in the subsurface environment of the Rangitikei River catchment.