Identifying critical source areas for water quality: 2. Validating the approach for phosphorus and sediment losses in grazed headwater catchments

Phosphorus (P) and suspended sediment (SS) can impair surface water quality. Recent work purports that the majority of loss comes from a small part of the catchment where areas of high potential for supply (source) and transport (e.g., surface runoff) overlap. However, these areas, termed critical source areas (CSAs) may also be small enough to enable the targeting of mitigation practices so that the approach is cheaper and more efficient than managing whole catchments. We aimed to determine CSAs in two headwater catchments. Firstly, we measured P fractions and SS in baseflow and stormflow in sub-catchments on the premise that CSAs were most active during stormflow. Using stormflow data we then aimed to determine the contribution to streamflow P and SS load from saturation- and infiltration-excess areas and the utility of five hydrological models to isolate transport areas which would also help define CSAs. While concentrations were greater in summer–autumn, loads were greater in winter–spring. Stormflow loads accounted for on average >60% of P and SS loss, with the largest storms accounting for >75% of stormflow load. Data collected from samplers located in infiltration-excess areas indicated that stream P and SS load during small events were dominated by infiltration-excess surface runoff, whereas larger events included more saturation-excess surface runoff. Utilising estimates of flow from five hydrological approaches for modelled storms and empirical equations for P loss from various sources (e.g., surface runoff from soil and dung), the topographic index combined with infiltration-excess areas gave the best estimate of P loads in the stream. While large events accounted for most P loss, we may not be able to manage for them since they involve most of the catchment. Furthermore, since they occurred in winter, they may have little effect on surface water quality parameters such as periphyton growth in summer. Since during small storms most saturation-excess surface runoff occurred within a short distance either side of the stream channel, the simplest approach to mitigating P and SS losses would be to target mitigation strategies to infiltration-excess areas like gateways and water troughs and fence-off animals from near-stream areas.