Management of critical source areas (CSAs) in pasture grazed by deer to reduce contaminant losses to water

Agriculture is important for producing food but can have environmental impacts on water quality. There is a need to develop mitigation strategies for pastoral farming systems to find a better balance between food production and environmental footprint. This study used a paired catchment experimental design to test the effectiveness of three mitigation strategies targeting the critical source areas (CSAs) that were ephemeral streams in deer grazed pastures. The runoff from four small catchments on the same hill slope (2.2–3.6 ha) was monitored for 2 years, under typical farm management. The mitigation options of: full fencing, partial fencing and temporary fencing was then applied to the CSA areas in three of the catchments. The fourth catchment was left unchanged as a control to account for different weather conditions in the pre- and post-treatment phases. Post-treatment monitoring was conducted for a further 2 years. The effectiveness of the mitigation options was calculated for both reducing contaminant concentrations (nitrogen, phosphorus, sediment and E. coli) under low-flow conditions and contaminant loads during storm events. The effectiveness of the three mitigation options for reducing low-flow concentrations ranged from not effective for filterable reactive phosphorus (FRP), total phosphorus (TP), and total suspended solids (TSS) to 83 % effective for reducing E. coli. The effectiveness of the three mitigation options for reducing storm-flow event loads ranged from not effective for FRP to 93 % effective for reducing E. coli. The CSA managements all mitigate multiple contaminants and hence will have multiple water quality benefits downstream.