Diffuse water pollution during recent extreme wet-weather in the UK: Environmental damage costs and insight into the future?

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.