Research and analysis

River water temperature projections for English Chalk streams: summary

Published 25 November 2022

Applies to England

1. Chief Scientist’s Group report summary

This project developed water temperature projections to 2080 for English chalk streams. The projections will help the Environment Agency and others better understand the potential impacts of climate change and inform where adaptation measures may help.

1.1 Background

Exactly how freshwater ecosystems and water quality will be affected by ongoing climate change is uncertain but increasing river temperatures are likely to be of critical importance. Understanding how much and where change is likely will help plan and target management activity.

Chalk streams are globally rare, with most found in England, and support rich biodiverse ecosystems, including important salmonid fish species such as Atlantic salmon (Salmo salar) and brown trout (Salmo trutta). Understanding future change is important because of the scarcity and significance of chalk streams and their importance in water supply. This study also served as a pilot for projections in all English rivers.

1.2 Approach

The project modelled future projections of chalk stream water temperature to 2080 based on a single high greenhouse gas future emission scenario (UKCP18 ‘high emissions scenario’). The average water temperature was modelled at a monthly resolution for 100 years 1981-2080. From this time series we were able to calculate the warmest month of each decade to explore the warming trend for 893 sites covering 57 of the 59 chalk river catchments in England.

1.3 Results

Under the high emissions scenario the average temperature of the warmest month each decade is projected to rise by about 0.6°C per decade. Sites in the north-east of England (Lincolnshire and Yorkshire Wolds) may experience the lowest increases whereas those around London (rivers Colne, Lee, Hogsmill, Mole and Wandle) may experience the highest. Adult brown trout will be under threat from high summer temperatures.  Salmonid eggs need winter temperatures below 12°C during the winter period to survive. This threshold could be exceeded at over 85% of sites by 2080.

Baseline historical water temperature records allowed monthly average water temperatures to be projected. These monthly average projections may miss local changes and the influence of short-term heatwaves, which may have important ecological effects. More detailed projections (e.g., daily or along river channels) would be possible with higher sampling frequency or a specifically designed temperature monitoring network.

1.4 Conclusions

Average monthly chalk stream water temperatures are expected to increase as the climate changes, reaching levels that will affect the species that they support, including salmon and trout. The effects will be different across the country. More detailed projections (in space and time) could be produced with more and higher resolution baseline river temperature data.

Quantifying the amount and timing of future warming in rivers provides important data to understand and prepare for climate change impacts. The evidence can inform the targeting of adaptation measures by the Environment Agency and others, to help reduce adverse changes to important river ecosystems and water quality.

1.5 Project details

This summary relates to information from the following project:

• Report: SC210011/R
• Title: River water temperature projections for English Chalk streams
• Project manager: Judy England, Chief Scientist’s Group

This project was commissioned by the Environment Agency’s Chief Scientist’s Group, which provides scientific knowledge, tools and techniques to enable us to protect and manage the environment as effectively as possible.

Enquiries: research@environment-agency.gov.uk.

© Environment Agency