Research and analysis

Thames water situation report: December 2024 summary

Updated 14 January 2025

Applies to England

• Publication for Northern Ireland
• Publication for Scotland
• Publication for Wales

1. December 2024 summary

In December, Thames area received 50mm of rainfall, 68% of the long term average (LTA). River flows increased at 9 of our key indicator sites. Bourne End (Hedsor) continued to have exceptionally high flows for the time of year, recording it’s second highest December flow since 1964. Bibury recorded its fourth highest December flow since 1961, and Windsor recorded its fifth highest flow since 1979. Groundwater levels of the relatively quicker responding Oolites declined following lower than average rainfall in December; whilst groundwater levels of the Chalk aquifer continued their seasonal increase. Reservoir levels increased in December, remaining above average for the time of year. There were 13 fluvial flood alerts, and 3 groundwater flood alerts issued during December.

2. Rainfall

In December, Thames area received 50mm of rainfall, 68% of the LTA. The majority of areal units towards the south and west of Thames area had below normal rainfall, while the north of Thames area, and North Downs Hampshire in the south, was normal for the time of year. Two-thirds of December’s rain occurred in the first 9 days of the month, with the wettest day being 4 December.

3. Soil moisture deficit and recharge

With lower than average rainfall, soil moisture deficits across most of Thames area did increase slightly, however they still remained well below the LTA for the time of year. In December, SMDs on average were 2mm, which was below the 16mm LTA. Effective rainfall this month was close to the December average (92%), however during winter so far (October to December), Thames area has had double the usually expected (204%).

4. River flows

Monthly mean flows increased at 9 of our key indicator sites, while 6 decreased. However, all were above normal or higher, apart from 3 which were normal:

• Swallowfield (Blackwater)
• Weybridge (Wey)
• Sheepbridge (Loddon)

Bourne End Hedsor (Wye), recorded exceptionally high flow for the time of year and its second highest December flow since 1964. Bibury recorded its fourth highest December flow since 1961 and Windsor recorded its fifth highest December flow since 1979.

5. Groundwater levels   

Groundwater levels decreased at 7 of our indicator sites in December, while 4, all found in the Chalk, continued to rise. Groundwater levels of the relatively quicker responding Oolites declined following lower than average rainfall in December; whilst groundwater levels of the Chalk aquifer continued their seasonal increase. Most of the indicator sites were above normal or higher, apart from Ampney Crucis, and Jackaments Bottom, in the Great Oolites, and Inferior Oolites respectively, which were normal. Three sites were exceptionally high for the time of year:

• Gibbet Cottages
• Stonor Estate
• The Flashes in the Lower Greensand

6. Reservoir stocks

Reservoir stocks rose in the Lower Thames reservoir reservoirs and ended the month at 95%, compared to 82.1% at the end of November. Stocks in Farmoor reservoir also increased from 90.9% to 97% during December. Both are above average for the time of year.

7. Environmental impact 

During November, there were 13 fluvial and 3 groundwater flood alerts issued in Thames area. At the end of the month, 1 abstraction licence was being constrained in the area to protect water resources and the environment.

Author: Thames Area Groundwater Resources and Hydrology, enquiriesWT@environment-agency.gov.uk

Contact Details: 03708 506 506

All data are provisional and may be subject to revision. The views expressed in this document are not necessarily those of the Environment Agency. Its officers, servants or agents accept no liability for any loss or damage arising from the interpretation or use of the information, or reliance upon views contained in this report.