Assessment of drought impacts on water quantity and quality: Implications for water resources and the Severn Thames Transfer
Keywords: water resource management
water quality
climate change
drought
inter-basin water transfer

Water resource management planners are increasingly aware of the effects that climate change and population growth will have on current, conventional water resources management plans. This requires an understanding of the vulnerability and sustainability of the current water supply options and the development of new water resource options to cope with more intense, longer, and more frequent droughts. Water stress in South East England is projected to increase over the next planning horizon, and water planners are continuously looking to develop drought management options with neighbouring water companies to transfer water cross basins to alleviate some of the water stress. The Severn-Thames Transfer is actively discussed as the preferred investment scheme to secure water supply in the Thames basin in the United Kingdom. In terms of inter-basin water transfers, understanding surplus water availability from the donor catchment is a vital factor. Thus thorough exploration of the wide range of extreme events in both donor and receiving catchments must be incorporated into the feasibility assessments. Together with consideration of water quantity resilience in the system, water quality risks associated with mixing two waters must be addressed prior to implementation.
The overarching aim of this research is to provide a comprehensive water quality and quantity modelling representation of the hypothetical inter-basin water transfer scheme under extreme and spatially extensive drought events. The aim is to explore the possible occurrence of trade-offs between water quality and quantity within the proposed transfer scheme. This research utilised state of the art hydro-climatic data alongside hydrological modelling and water quality modelling to achieve this research aim. A new proposed inter-basin transfer scheme, the Severn-Thames Transfer, has been chosen to demonstrate the new modelling framework.
The findings herein suggest the value of the new modelling framework, because of the potential to explore multiple aspects of risks on the new proposed water resources. The methodology used has contributed to an enhanced understanding of catchment specific water quality responses to extreme drought, and enabled the identification of critical water quality variables, which might impose risks during inter-basin water transfers. The use of this case study demonstrated that more spatially extensive and severe droughts are projected under a changing climate, and it will threaten the water supply in London and degrade the water quality in the Thames and Severn Rivers. The results also suggest that inter-basin transfers can degrade water quality although water stress in the Thames basin can be alleviated by increasing supply. Regarding water quality impacts by the suggested transfers, total phosphorus and dissolved organic carbon will be a concern and introduce an additional burden on water supply systems and ecological conditions in the Thames. Reduced nitrate and suspended sediments concentrations are expected in the Thames while transfers as the water quality in lower Severn is better. Therefore, the timing of transfers will become the most critical factor for the new proposed transfer scheme.