Climate change impacts on river flow in England: a comparison of the UKCP18 and euro-CORDEX regional climate projections

In England, the priority catchment project focuses on developing innovative solutions to ensuring a clean and plentiful supply of water and environmental protection. Understanding the impacts of climate change on streamflow and water availability will ensure resilient management solutions into the future. The latest 12-member dynamically downscaled perturbed parameter ensemble of regional climate model projections (PPE-RCM) is part of the country specific UK Climate Projections UKCP18. In this study it was applied to estimate future changes in streamflow in an application of a new, revised version of the Soil and Water Assessment Tool (SWAT+) to two contrasting priority catchments in England. Both catchments are influenced by high rates of freshwater withdrawals but differ in their natural hydrological regimes and geographies. One is a wet coastal catchment with steep slopes while the other is a dry lowland catchment. Modelled impacts on natural monthly flows and flow duration statistics until the 2080s under the 12 member PPE were compared to those from 18 members of the euro-CORDEX initiative. Both ensembles are available for emissions pathway RCP8.5. To cover a broad range of scenarios, we also modelled the impact of the lower emissions (RCP4.5 & RCP2.6) euro-CORDEX projections.

SWAT+ performs well in simulating natural flows during the validation period in both catchments. The PPE estimates are consistently drier than euro-CORDEX. It projects streamflow in the coastal catchment to increase in seasonality with higher winter and lower summer flows, while streamflow in the dry lowland catchment is projected to decrease across all months apart from February. In the dry lowland catchment, the euro-CORDEX under RCP8.5 predict the strongest decreases in streamflow for June at -13%, while the PPE projects beyond -20% decrease throughout June to September. The climate change signal in the coastal catchment is less clear. The PPE projects winter streamflow to increase by between 5% to 36% while the euro-CORDEX under RCP8.5 predict increases between 13% to 23%, summer streamflow is projected to decrease by -16% to -23% and -0.5% to -4% respectively. RCP2.6 and RCP4.5 represent a mixed result with rarely beyond 10% change and more months with increasing trends than under RCP8.5. The different emissions pathways largely agree on increasing high flows and decreasing low flows in the coastal catchment. For the lowland catchment both ensembles driven by RCP8.5 project decreases across the whole flow duration curve while RCP 2.6 and 4.5 project medium to high flows to increase and low flows at Q₇₀ and Q₉₅ to largely stay the same.

This study suggests the need to adapt environmental protection and water withdrawals to decreasing water availability across the whole year in the lowland catchment and to pronounced changes in streamflow timing in the coastal catchment. To understand a broader range of climate impacts the UKCP18 PPE-RCMs should be used with other projections. However, they represent high-end warming scenarios translating into strong hydrological response, in particular streamflow decreases, that other ensembles might not capture, providing further insights into the challenges that water management may face.