Historical and future climate change is likely to be one of the key drivers of alterations seen in catchments globally. The implications of changing precipitation patterns in particular range from increased seasonality of flows, to changing frequency, duration, and severity of extreme events, to variations in pollutant sourcing and transport. These changes display regional differences, but it is also reasonable to expect catchment level variation. Factors such as topography, land use/land cover and soil type all cause variation of regional trends. Such alterations could have consequences for all water users, but flow regime changes in particular will directly impact those abstracting water from rivers. In this work, hydrological regime change is investigated for five catchments in Wales (Clwyd, Conwy, Dyfi, Teifi, Tywi), both historically and under a future worst-case scenario of global greenhouse gas emissions. For the future period water quality is also considered. Subsequently, impacts on abstraction for public water supply (PWS) and hydroelectric power (HEP) are also quantified.
The first study investigates historical trends in average and extreme precipitation, air temperature and streamflow, before correlating these factors with actual total abstraction data for PWS (as a proxy for overall water demand). A strong warming trend in autumn average temperatures in all catchments is observed (Sen’s slope range: 0.38-0.41, p <0.05), with a north-south divide detected in streamflow and extreme temperature trends. A positive correlation between abstraction volume and temperature in four catchments is established (Spearman’s ρ range: 0.094-0.403, p <0.01; Pearson’s r range 0.073-0.369, p <0.01). The study provides new insight into the relationship between hydroclimatic factors and total PWS demand and highlights local variation in broader regional changes.
The second study uses the Soil and Water Assessment Tool (SWAT) to model future average and extreme streamflow and water quality. The 12-model ensemble UK Climate Projections 2018 dataset, based on Representative Concentration Pathway 8.5 (RCP8.5) conditions, is used for future daily weather input (2021-2080). A small decline in mean annual flow is observed (-4% to -13%), with much larger variations seasonally. Spring and autumn streamflows are most affected, increasing and decreasing by up to 41% and 52%, respectively; these changes corresponded with increased frequency of high flow events in spring, and low flow events in autumn. Water quality generally declines in all catchments and seasons, with increased concentrations of suspended sediment, nitrogen, and phosphorous, and decreased dissolved oxygen. Results suggest a double threat for water abstractors, especially for PWS, of both declining flows and water quality, in summer and autumn in particular.
The implications of projected hydrological regime changes are investigated in the third study, with a focus on PWS and HEP. A key PWS abstraction in the Tywi is studied using the Water Evaluation And Planning (WEAP) system. The temperature-demand relationship established in the first study is used to project future total water demand under RCP8.5 conditions; static and reducing demand scenarios are also considered. Under all scenarios there is an increase in occurrences of insufficient streamflow to satisfy demand. Alterations in HEP operations are analysed for sixteen sites in the Conwy, and nine in the Tywi. Results suggest a decrease in total annual abstraction volume, resulting in a loss of generation potential, this is especially pronounced up to the mid-2050s. Further work is needed to quantify the financial and environmental implications of observed trends; additional work on water quality changes would also strengthen PWS planning.