Changing water temperatures: a surface water archive for England and Wales

The temperature of water in lakes, rivers and streams has a fundamental influence on aquatic organisms, ecological processes and the potency of many pollutants. We expect water temperature in the UK to increase under projected climate change, but rates and spatial variation of change remain unclear for inland waters. It is also unclear how we might manage land and river flow regimes to modify river temperature where this may be desirable, for example, to protect fish from lethal temperatures. Although water temperature broadly correlates with air temperature, the drivers of surface water temperature involve dynamic heat (energy) exchanges at the water surface and river bed and complex hydrological fluxes. To date, published reports of water temperature change in British rivers have been based on a limited number of sites: these have provided useful monitoring data on the thermal processes of rivers but rather less about the thermal regime of rivers. Our understanding of river thermal spatio-temporal variability and processes remains incomplete. In part, this has been related to a lack of long-term data across a range of different environments. The Environment Agency has recently created a surface water temperature archive for England and Wales based on measurements at over 30 000 sites, with observations extending back to between <5 and 30 years. The archive is being used to assess post-1990 trends in water temperature across England and Wales. Here, we briefly describe the archive and its potential uses. We present preliminary observations of the variation in river water temperature across England and Wales and the changing relationship between water and air temperatures at and between sites. We also illustrate the potential for exploring ecological response to changes in water temperature.