Assessing the Yield and Load of Contaminants with Stream Order: Would Policy Requiring Livestock to Be Fenced Out of High‐Order Streams Decrease Catchment Contaminant Loads?

Catchment contaminant loads vary with stream order as catchment characteristics influence inputs and in‐stream processing. However, the relative influence and policy significance of these characteristics across a number of contaminants and at a national scale is unclear. We modeled the significance of catchment characteristics (e.g., climate, topography, geology, land cover), as captured by a national‐scale River Environment Classification (REC) system, and stream order in the estimation of contaminant yields. We used this model to test if potential regulation in New Zealand requiring livestock to be fenced off from large (high)‐order streams would substantially decrease catchment contaminant loads. Concentration and flow data for 1998 to 2009 were used to calculate catchment load and yields of nitrogen (N) and phosphorus (P) species, suspended sediment, and Escherichia coli at 728 water quality monitoring sites. 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 low‐order small streams (<1 m wide, 30 cm deep, and in flat catchments dominated by pasture) exempt from potential fencing regulations accounted for an average of 77% of the national load (varying from 73% for total N to 84% for dissolved reactive P). This means that to substantially reduce contaminant losses, other mitigations should be investigated in small streams, particularly where fencing of larger streams has low efficacy. Core Ideas Contaminant yields increased with increasing stream order in catchments dominated by agriculture. Fencing off high‐order streams from stock misses 77% of national contaminant load from small‐order streams. Hence, to reduce contaminant losses to small streams, other mitigations are needed.