Predicting the impact of projected change in agriculture by 2015 on annual mean fluvial suspended sediment concentrations across England and Wales

摘要

With respect to achieving "good ecological status" (GES) for freshwaters by 2015, ADAS has recently been commissioned by the Department for Environment, Food and Rural Affairs (Defra) to estimate the gap between current and target losses of suspended sediment from diffuse agricultural sources across England and Wales. The work required a model for predicting mean annual total suspended sediment loads (SSL) and time-weighted mean fluvial suspended sediment concentrations (SSC)., GES was defined in terms of the guideline annual average SSC of 25 mg L"1 cited by the EC Freshwater Fish Directive. National scale sediment source apportionment was undertaken to estimate the contributions of diffuse agricultural and urban sector sediment losses, channel bank erosion and point source discharges to the total SSL in all rivers. Landscape sediment retention was taken into account. The total SSL estimated for each Water Framework Directive (WFD) sub-catchment across England and Wales was used in conjunction with predicted flow exceedence to derive corresponding SSC time-exceedence plots. Baseline (year 2000) sediment load reductions required from the agricultural sector for meeting the target threshold SSC of 25 mg L"1 were estimated at national scale. Projected change in agriculture (structural and uptake of sediment mitigation methods) by 2015 was incorporated in the modelling exercise to predict the associated changes in SSL and mean annual SSC. The findings suggest that in addition to current agii-environment schemes, e.g.. Countryside Stewardship, Catchment Sensitive Farming (CSF), further mitigation will be necessary under the River Basin Management Plans (RBMPs) of the WFD to reduce diffuse agricultural sediment losses for achieving GES in some parts of England and Wales.