Scottish Water (SW) is responsible for water supply for the whole of Scotland, managing supplies from over 450 raw water sources across the country. The varied geography and range of source types and sizes across Scotland means that there are significant challenges in being able to provide reliable hydrological yield assessments for all water supply sources.
Fundamental to this is the availability of gauged river flow data. The vast majority of SW sources are located in ungauged catchments, or in catchments where long term flow records upstream of SW abstractions are not available. Therefore, data from analogous neighbouring catchments are frequently used to inform water resource assessment. As with any hydrometric network, there are limitations in terms of the availability and suitability of data contained within the existing Scottish hydrometric archive. This has significant implications for SW, particularly in terms of the lack of suitable long-term river flow records in small remote, upland catchments which are typical of many SW raw water sources. In turn, any assessment of hydrological yield in these catchments is likely to have a high degree of associated uncertainty, making it difficult to quantify the resilience of water supply to SW customers.
In response to this, SW has established a program of long-term strategic flow gauging in source catchments in order to address the limitations of the existing hydrometric network. The development of long-term flow gauging sites will contribute to improved rainfall-runoff modelling and flow estimation capability used to inform future water resource planning and climate change modelling.
This paper will discuss the design of the site-selection methodology; the identification of suitable SW catchments which could provide a suitable gauge location and had no analogue gauge as part of the existing hydrometric network; finally, this paper considers water supply climate change assessment as a case study illustrating the benefit of continued development and maintenance of hydrometric networks.
Results from initial climate change vulnerability assessments have identified that approximately 25% of SW's supply systems may be at risk due to potential river flow variation under climate change scenarios. These systems are currently subject to a program of detailed climate change modelling assessments against the full range of UKCP09 climate change scenarios to gain an improved understanding of climate change impacts on water supply. A credible and robust assessment of climate change impacts on all water supply systems in Scotland is only possible with the expansion of the existing hydrometric network.