Assessing the yield and load of contaminants with stream scale: would policy to decrease contaminant losses from first order streams benefit downstream water quality?

Contaminant loads vary with catchment characteristics. Concentration and flow data for 1998-2009 were used to calculate catchment load and yields (load divided by catchment area) of nitrogen and phosphorus species, suspended sediment and Escherichia coli at 728 water quality monitoring sites across New Zealand. The New Zealand River Environment Classification (REC) and stream order were used as predictors in statistical models of mean annual yields. We wished to know if modelled catchment contaminant yields varied as a function of stream order and if excluding small streams from a requirement to fence-off stock access would impair our ability to decrease catchment contaminant loads. On average, the yields of all contaminants increased with increasing stream order in catchments dominated by agriculture (generally lowland and pastoral REC land cover classes). Loads from small and shallow streams in relatively flat catchments dominated by pasture contributed an average of 26% of the national contaminant load, but contributed much greater proportions of the total load for some contaminants in some regions (e.g. 52% of total nitrogen in Otago). This suggests that excluding small stream from stock fencing requirements could miss the opportunity to reduce a substantial proportion of contaminant losses. If small streams are excluded from stock fencing requirements, other mitigations should be investigated, particularly where fencing of larger streams has low efficacy.