By Richard Dallison
Water service providers in the UK face a vast array of challenges when it comes to planning their future operations and services; the impact of climate change on the quality of water arriving at drinking water treatment plants (DWTPs) is just one of these. Changes in water quality are induced by a variety of factors, when considering non-point source inputs. Large or prolonged rainfall events, for example, can lead to greater washing of pollutants from the land in to rivers. Additionally, large streamflow events provide rivers with greater stream-power, enabling more erosion of river banks and the movement of larger particles. Very low streamflow events can also cause spikes in water quality issues, as there is less water available to dilute any pollutants. Therefore, any changes in future climate, and subsequent alterations in streamflow regime could have far reaching consequences in terms of water quality, this in turn could cause problems at DWTPs if they are unprepared for any such changes.
Using the Soil and Water Assessment Tool (SWAT) hydrological model, we have projected and analysed future (2021-2080) streamflows and water quality under a worst case scenario of future emissions, that being representative concentration pathway 8.5, as laid out by the Intergovernmental Panel on Climate Change. To account for uncertainty in future modelling, an ensemble of 12 regionally downscaled models derived from the Met Office Hadley Centre Global Environmental model (HadGEM3), and supplied by UK Climate Projections 2018, have been used as the future climate inputs to SWAT. Five catchments have been studied, the Clwyd and Conwy in north Wales; the Dyfi in mid Wales; and the Teifi and Tywi in south Wales; these systems represent a variety of catchment characteristics in terms of land use, soil types, underlying geology and topography. We have investigated annual and seasonal average changes for four water quality aspects, suspended sediment (SS); total phosphorous (TP); total nitrogen (TN); and dissolved oxygen (DO). Mann Kendall trend analysis was performed on the model outputs, in order to detect any statistically significant trends.
Figure 1. Study area and catchments showing streams larger than third order as defined by Strahler method.
Previous work by the Dŵr Uisce project has shown that extreme streamflow events will become more frequent in the future, with winter and spring seeing more very large events, and summer and autumn seeing much lower streamflows. This is reflected in the water quality outputs, with increases in SS loads seen in winter and spring in four of the five catchments each. At annual average level, SS loads are increasing significantly (p <0.01) in all catchments except the Tywi, were a non-significant increase is seen. TP and TN display broadly similar trends, with concentrations increasing in all seasons, in all catchments except the Dyfi, where a decline is shown in winter, spring and annually on average. Summer concentrations of TP and TN in particular show a statistically significant increase in all catchments (except TP in the Tywi). DO levels show the most variation, although a decline is seen in all catchments in summer. The Teifi catchment is the most divergent, with a statistically significant increase in winter and spring DO levels, compared to declines seen in all other catchments for winter.
Figure 2. Overview of the direction and significance of annual and seasonal average trends in projected future (2021 – 2080) water quality concentrations, as detected by Mann Kendall trend analysis, based on the average of all 12 RCM model outputs.
Given the trends seen in river water quality, it is clear that for the vast majority of the time in the future, water quality will be worse than it is currently. This is an issue that needs to be managed at DWTPs in particular, to ensure that current operating systems, procedures, and technologies will be able to cope with more contaminated water in the future. This adaptation need comes hand-in-hand with the need to also ensure future water security, especially in summer and autumn, when lower precipitation and streamflows, as well as higher temperatures, are projected for Wales. It is clear that the projected changes in climate will have a large impact on catchments in Wales, this will have a significant knock-on impact on a variety of aspects of drinking water supply. Work is therefore needed now to plan and mitigate against these potential changes in order to maintain continued high-quality supplies in to the future.