Risk-based targeting of diffuse contaminant sources at variable spatial scales in a New Zealand high country catchment

Management of agricultural diffuse pollution requires targeting or prioriti sing critical source areas at various spatial scales within watersheds. Thi s study develops, evaluates and illustrates a risk-based approach for asses sment and targeting of source areas at catchment, subarea and individual fa rm scales. Catchment water quality data are used in conjunction with inform ation on watershed characteristics from the New Zealand Land Resources Inve ntory at the subarea scale and land use information at the farm scale to as sess risk and target source areas. Total phosphorus in the Lake Hayes Catch ment, a high country pastoral catchment in the South Island of New Zealand, is used as a case study. Use, comparison and evaluation of several differe nt methodologies for subareas and individual properties showed that a subar ea in the upper catchment and one immediately upstream from the lake were t he worst source areas. Targeting of other subareas varied dependent on the method used. The worst individual properties were targeted based on the com bination of intensity of cattle and sheep grazing, fertilizer usage, bank e rosion and location in the worst subareas. Water quality results are critic al to successful targeting, particularly for convincing landowners that str eams will benefit from best management practices on their properties. In ad dition to concentrations, average and extreme loadings are important. Data on catchment characteristics, particularly land use, are needed for targeti ng, but are not always readily available at small scales. This study demons trated simple but useful methods for application of assessment information for quantitative targeting of contaminant source areas at different spatial scales. (C) 2001 Academic Press.