Abstract
Periods of extreme wet-weather elevate agricultural diffuse water pollutant loads and climate projections for the UK suggest wetter winters. Within this context, we monitored nitrate and suspended sediment loss using a field and landscape scale platform in SW England during the recent extreme wet-weather of 2019–2020. We compared the recent extreme wet-weather period to both the climatic baseline (1981–2010) and projected near- (2041–2060) and far- (2071–2090) future climates, using the 95th percentiles of conventional rainfall indices generated for climate scenarios downscaled by the LARS-WG weather generator from the 19 global climate models in the CMIP5 ensemble for the RCP8.5 emission scenario. Finally, we explored relationships between pollutant loss and the rainfall indices. Grassland field-scale monthly average nitrate losses increased from 0.39-1.07 kg ha−1 (2016–2019) to 0.70–1.35 kg ha−1 (2019–2020), whereas losses from grassland ploughed up for cereals, increased from 0.63-0.83 kg ha−1 to 2.34–4.09 kg ha−1. Nitrate losses at landscape scale increased during the 2019–2020 extreme wet-weather period to 2.04–4.54 kg ha−1. Field-scale grassland monthly average sediment losses increased from 92-116 kg ha−1 (2016–2019) to 281–333 kg ha−1 (2019–2020), whereas corresponding losses from grassland converted to cereal production increased from 63-80 kg ha−1 to 2124–2146 kg ha−1. Landscape scale monthly sediment losses increased from 8-37 kg ha−1 in 2018 to between 15 and 173 kg ha−1 during the 2019–2020 wet-weather period. 2019–2020 was most representative of the forecast 95th percentiles of >1 mm rainfall for near- and far-future climates and this rainfall index was related to monitored sediment, but not nitrate, loss. The elevated suspended sediment loads generated by the extreme wet-weather of 2019–2020 therefore potentially provide some insight into the responses to the projected >1 mm rainfall extremes under future climates at the study location.
Highlights
Water quality faces threats globally from both climate change and intensive farming (Dunn et al, 2012; Michalak, 2016; Malhi et al, 2020)
These changes in autumn and winter rainfall are, in turn, elevating runoff and the water pollution externalities arising from contemporary intensive grassland and cereal agroecosystems, since current on-farm mitigation strategies, including those subsidised by agri-environment schemes, are delivering limited efficacy (Collins and Zhang, 2016; Ockenden et al, 2017; Collins et al, 2021)
The wet-weather period in 2019–2020 is only representative of projected rainfall extremes for near- (2041–2060) and far- (2071–2090) future climates in terms of >1 mm rainfall, we found a strong correlation between this particular rainfall index and monitored suspended sediment loads
Summary
Water quality faces threats globally from both climate change and intensive farming (Dunn et al, 2012; Michalak, 2016; Malhi et al, 2020). Managing land to produce food whilst ensuring clean surface and ground water for the environment and society continues to be a demanding challenge (Grafton et al, 2015) and contemporary farming remains a significant source of water pollution, including that arising from nitrate and sediment, across scales (Zhang et al, 2014). Interactions between weather patterns, climate change and agriculture impact water quality, aquatic ecosystems and water availability (Whitehead et al, 2009; Arnell et al, 2015). The climate-land-water nexus is important since river systems are among the ecosystems most sensitive to climate change (Millennium Ecosystem Assessment, 2005; Watts et al, 2015). Understanding the implications of climate change, weather extremes and land use in the future is funda mental for assessing the challenges facing productive and sustainable agriculture (Ritchie et al, 2019)
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