The potential benefits of on-farm mitigation scenarios for reducing multiple pollutant loadings in prioritised agri-environment areas across England

Mitigation of diffuse water pollution from agriculture is a key national environmental policy objective in England. With the recent introduction of the new agri-environment scheme, Countryside Stewardship, there is an increased emphasis on the macro-spatial targeting of on-farm mitigation measures to reduce pollutant pressures, and a concomitant need to forecast the technically feasible impacts of on-farm measures detailed in current policy and their associated costs and benefits. This paper reports the results of a modelling application to test these limits in the context of the associated costs and benefits for the reduction of diffuse water pollution from agriculture for each Water Framework Directive (WFD) water management catchment (WMC) and nationally. Four mitigation scenarios were modelled, including pollutant source control measures only (SC), mobilisation control measures only (MC), delivery control measures only (DC) and measures for source, mobilisation and delivery control (SMDC) combined. Projected impacts on nitrate, phosphorus and sediment export to water, ammonia, methane and nitrous oxide emissions to the atmosphere, together with the associated costs to the agricultural sector were estimated for each WFD WMC and nationally. Median WMC-scale reductions (with uncertainty ranges represented by 5th-95th percentiles) in current agricultural emissions, were predicted to be highest for the SMDC scenario; nitrate (18%, 11-23%), phosphorus (28%, 22-37%), sediment (25%, 18-43%), ammonia (26%, 17-32%), methane (13%, 7-18%) and nitrous oxide (18%, 16-20%). The median benefit-to-cost ratios (with uncertainty ranges represented by 5th-95th percentiles) were predicted to be in the following order; DC (0.15, 0.09-0.65), MC (0.19, 0.09-0.95), SMDC (0.31, 0.20-1.39) and SC (0.44, 0.19-2.48). Of the four scenarios simulated, the SC and SMDC suites of measures have the greatest potential to deliver reductions in BAU emissions from agriculture, and the best benefit:cost ratio.