Data from: Do agri-environment schemes result in improved water quality?

1. Improved water quality, through a reduction in diffuse pollution from agricultural sources, is an expected benefit of agri-environment schemes, but this has yet to be demonstrated in practice. Here we evaluate the impact of Welsh agri-environment schemes on water quality and freshwater ecosystem condition through a combined monitoring and modelling framework. 2. To determine the influence of the agri-environment schemes on ecosystem condition, spatially independent catchments dominated by a single scheme (>40% of catchment) were compared to control catchments dominated (>70%) by agricultural land that was not part of any scheme. Biological indicators of water quality were monitored at the outfall of each catchment and a spatially explicit modelling framework of diffuse pollutant emissions applied to each of the 80 catchments. 3. Direct comparison (scheme/non-scheme) was unable to identify any significant effect of agri-environment scheme participation. However, derived biological indicators that reflected organic pollution, eutrophication and pesticide runoff were strongly correlated with modelled concentrations of corresponding diffuse pollutants, thus providing a ground-truth for the models. Scenarios that assessed the correct counterfactuals (i.e. the influence of scheme entry on pollutant output) were developed for the whole of Wales. The models indicated an important effect of scheme entry on water quality, but this effect was not evenly distributed across the landscape. 4. Synthesis and applications. Our results indicate that agri-environment schemes can deliver improvements in water quality, through a reduction in diffuse pollution from agricultural sources. However, it is not easy to demonstrate scheme effectiveness; the combination of field survey and modelling used here provides a framework for addressing these difficulties. A spatially targeted approach for agri-environment scheme options to protect water resources from diffuse pollution is likely to be most effective at delivering water quality improvements.1. 通过削减农业源面源污染以改善水质，是农业环境计划（agri-environment schemes）的预期效益之一，但该效益尚未在实践中得到证实。本研究采用监测与建模相结合的综合框架，评估威尔士农业环境计划对水质及淡水生态系统状态的影响。 2. 为明确农业环境计划对生态系统状态的影响，研究将单一计划覆盖占比超集水区总面积40%的空间独立集水区，与未参与任何农业环境计划、农业用地占比超70%的对照集水区进行对比。研究人员在每个集水区的出水口监测水质生物指标，并为全部80个集水区分别构建空间显式面源污染物排放建模框架。 3. 直接对比参与计划与未参与计划的集水区，未能识别出农业环境计划参与带来的显著效应。不过，反映有机污染、富营养化及农药径流的衍生生物指标，与模型模拟的对应面源污染物浓度呈显著相关，从而为模型提供了实测真值基准。本研究为威尔士全境构建了评估合理反事实情景（即参与农业环境计划对污染物输出的影响）的分析框架。模型结果显示，参与计划对水质存在重要影响，但该影响在区域空间分布并不均匀。 4. 综合与应用：本研究结果表明，农业环境计划可通过削减农业源面源污染实现水质改善。然而，验证计划的实施效果并非易事；本研究采用的野外调查与建模结合的方法，为解决此类难题提供了可行框架。若采用空间靶向策略，在农业环境计划中设置针对面源污染以保护水资源的方案，或能最有效地达成水质改善目标。