Abstract
Agricultural diffuse water pollution remains a notable global pressure on water quality, posing risks to aquatic ecosystems, human health and water resources and as a result legislation has been introduced in many parts of the world to protect water bodies. Due to their efficiency and cost-effectiveness, water quality models have been increasingly applied to catchments as Decision Support Tools (DSTs) to identify mitigation options that can be introduced to reduce agricultural diffuse water pollution and improve water quality. In this study, the Soil and Water Assessment Tool (SWAT) was applied to the River Wensum catchment in eastern England with the aim of quantifying the long-term impacts of potential changes to agricultural management practices on river water quality. Calibration and validation were successfully performed at a daily time-step against observations of discharge, nitrate and total phosphorus obtained from high-frequency water quality monitoring within the Blackwater sub-catchment, covering an area of 19.6 km2. A variety of mitigation options were identified and modelled, both singly and in combination, and their long-term effects on nitrate and total phosphorus losses were quantified together with the 95% uncertainty range of model predictions. Results showed that introducing a red clover cover crop to the crop rotation scheme applied within the catchment reduced nitrate losses by 19.6%. Buffer strips of 2 m and 6 m width represented the most effective options to reduce total phosphorus losses, achieving reductions of 12.2% and 16.9%, respectively. This is one of the first studies to quantify the impacts of agricultural mitigation options on long-term water quality for nitrate and total phosphorus at a daily resolution, in addition to providing an estimate of the uncertainties of those impacts. The results highlighted the need to consider multiple pollutants, the degree of uncertainty associated with model predictions and the risk of unintended pollutant impacts when evaluating the effectiveness of mitigation options, and showed that high-frequency water quality datasets can be applied to robustly calibrate water quality models, creating DSTs that are more effective and reliable.
Highlights
Agricultural diffuse water pollution remains a notable global pressure on surface water and groundwater quality (Carpenter et al, 1998; Vo€ro€smarty et al, 2010; European Environment Agency, 2012), and trends suggest that agricultural expansion will continue to exacerbate those pressures well into the 21st Century (Tilman et al, 2001)
In the United Kingdom (UK) agricultural diffuse water pollution is estimated to be responsible for 61% of the total nitrogen load and 28% of the total phosphorus load experienced within surface water bodies (Hunt et al, 2004; White and Hammond, 2007)
Water quality models are cost-effective Decision Support Tools (DSTs) which can be applied to assess the quantitative impacts of a variety of mitigation options on water quality
Summary
Agricultural diffuse water pollution remains a notable global pressure on surface water and groundwater quality (Carpenter et al, 1998; Vo€ro€smarty et al, 2010; European Environment Agency, 2012), and trends suggest that agricultural expansion will continue to exacerbate those pressures well into the 21st Century (Tilman et al, 2001). Agricultural diffuse water pollution is cited as a significant pressure in 40% of rivers and coastal water bodies and one-third of lakes and transitional water bodies. Such poor water quality has consequences for the health of aquatic ecosystems, biodiversity, human health, the use of water in industry and agriculture and as a resource for public water supply and recreation (Carr and Neary, 2008). In Europe, agricultural diffuse water pollution contributes 50e80% of the total nitrogen load and approximately 50% of the total phosphorus load in surface water bodies (European Environment Agency, 2005; Kronvang et al, 2009). Water is considered to be unfit for human consumption under the Drinking Water Directive applied within Europe if it contains a nitrate concentration above 50 mg LÀ1 (equivalent to 11.3 mg NO3-N LÀ1) (Council of the European Union, 1998), but many surface water and groundwater bodies within the UK contain concentrations of nitrate that approach or exceed this limit (European Environment Agency, 2012)
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