Mapping the potential for Payments for Ecosystem Services schemes to improve water quality in agricultural catchments: A multi-criteria approach based on the supply and demand concept.

Payments for Ecosystem Services (PES) schemes are an increasingly popular form of catchment management for improving surface water and groundwater quality. In these schemes, downstream water users who are impacted by agricultural diffuse pollution incentivise upstream farmers to adopt better practices. However, this type of scheme will not be successful in all situations, in part, due to a lack of potential for agriculture to improve the suuply of good water quality and/or a lack in demand from downstream users for good water quality. As such, this study aims to present a flexible approach to mapping the potential for PES schemes to improve water quality in agricultural catchments. The approach is based on multi-criteria analysis, with supply and demand as key criteria. It uses expert judgement or current guidance on PES to select supply and demand sub-criteria, expert judgement to weight all criteria through pairwise comparisons and readily available, national datasets to indicate criteria. Once indicator data are normalized, it combines them in a weighted sums analysis and presents results spatially at the national scale, all within a geographical information system. The approach can easily be applied to the country or region of interest by using locally relevant criteria, expert judgement and data. For example, when applied to the situation for river waterbodies in England, supply sub-criteria were the contribution of agriculture to loads of the major pollutants (nitrogen, phosphorus and sediments) and demand sub-criteria were the different downstream water users present (water companies and, tourist and local recreational users). Experts assigned equal weight to supply and demand criteria and the highest weights to sediments and water companies for sub-criteria, respectively. When national scale datasets to indicate these criteria were combined in a weighted sums analysis, it was possible to identify areas of high potential for PES. This would hopefully motivate more detailed research at the individual catchment level into the constraints in linking supply and demand. Three case-study schemes were also examined to show how some of these constraints are being identified and overcome. As such, the approach forms the first tier in a two-tier framework for establishing PES schemes to improve water quality in agricultural catchments.

Determining riverine sediment storage mechanisms of biologically reactive phosphorus in situ using DGT.

Phosphorus (P) is the main reason many surficial water bodies in the UK are currently failing to meet the chemical standards set by the Water Framework Directive (WFD). This work focuses on the role of sediments in the upper reaches of the River Taw in the South West of the UK. Point and diffuse sources of P have been identified as well as a number of mitigation measures applied or planned to address the issues. However, it is unknown what effect these sources have had on the river's sediments and how they will react to diminishing inputs of P into the water column in the future. The diffusive gradient in thin-films (DGT) method is utilised in situ to quantify and identify labile, potentially bioavailable P fluxes and potential storage mechanisms at sites of known diffuse and point P inputs. In the vicinity of a heavily contaminated point source, data present here shows that sediments are still acting as a sink. The proposed mechanism for this is the formation of a 'calcium cap' which provides a geochemical barrier between the sediment and overlying water to prevent loss of labile P. The strong correlation between calcium and total P under most circumstances supports this hypothesis. This conclusion provides some confidence that even though P concentrations in some sediments are significantly elevated, mobility between the sediment and overlying water is restricted. In the context of routine monitoring against WFD targets, the molybdenum blue method generally employed to determine soluble reactive phosphorus was shown to not be equivalent to the DGT labile P pool, especially at pristine or moderately point/diffuse influenced sites. This is likely due to desorption of weakly bound P from colloids, which is unavailable to DGT devices. These results have the potential to be scaled up to the full catchment or other catchments which exhibit similar physical and chemical sediment composition and provide a stronger foundation for management and target setting than current monitoring approaches.