Research and analysis

Wessex water situation: December 2024 summary

Updated 14 January 2025

Applies to England

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

1. Wessex water situation: December 2024

December marked a shift away from the wetter weather of the previous 3 months with an average of 54mm of rainfall across Wessex, 58% of the long term average (LTA). Despite the drier end to the year, 2024 was the fifth wettest year on record since 1871. Soil moisture deficit (SMD) remained close to zero during December. In river catchments on the Chalk most monthly mean flows recorded were notably high except on the Dorset Stour which recorded normal mean flows. Monthly mean flows recorded across all catchments not on the Chalk aquifer were normal. Groundwater sites monitoring the Great and Inferior Oolite to the north of Wessex reported notably high and exceptionally high levels at the end of December while those further south including sites monitoring the Chalk reported either normal or above normal levels. Reservoirs ended December at around 99% capacity and 94% capacity for Wessex Water and Bristol Water respectively. For Wessex Water this was similar to levels this time the previous year. For Bristol Water, levels were slightly lower than they were in December 2023.

2. Rainfall

In December an average of 54mm (58% LTA) of rain fell across Wessex. All hydrological areas across Wessex received below normal rainfall. Rainfall during December was largely restricted to 2 periods: 45% of rain fell between 4 and 6 December and 28% between 17 and 20 December.

In the last 3 months, all hydrological areas have received normal rainfall on average. In the last 6 months, the majority of areas in Wessex received between above normal and notably high rainfall with 2 hydrological areas to the east receiving exceptionally high rainfall and the West Somerset Streams to the west of Wessex receiving normal rainfall. Over the past 12 months, almost all areas of Wessex received exceptionally high rainfall. Prior to December, 2024 was on course to be the wettest year on record. After a comparatively dry December, last year was instead the fifth wettest calendar year since records began in 1871. Overall, 2024 was slightly drier than 2023 which was the fourth wettest on record.

3. Soil moisture

Despite the lower rainfall, SMD in Wessex in December remained close to zero on average throughout the month. SMD at the end of December was near zero on average across all areas of Wessex. SMD at the end of December was within 5mm of the LTA across most of Wessex and between 6mm and 25mm lower than the LTA in some hydrological areas to the south and east of Wessex.

4. River flows

Amongst sites reporting rivers within Chalk catchments, the majority reported above normal to notably high mean monthly flows. The exceptions were sites monitoring the Dorset Stour which both recorded normal mean flows. All river flow reporting sites across Wessex outside of Chalk catchments recorded normal mean flows for December.

Daily mean flows decreased during December across all reporting sites following the high rainfall from November. By the end of the month, all reporting sites across Wessex recorded between normal and below normal daily mean flows. Reporting sites in Chalk catchments typically recorded higher daily mean flows than in other geologies as flows are supplemented by water from the aquifer.

5. Groundwater levels

The majority of groundwater monitoring sites across Wessex ended December reporting between normal to above normal levels. The 2 exceptions were Allington and Didmarton to the north of Wessex monitoring Oolite formations which reported notably high and exceptionally high levels respectively. Most sites monitoring the Chalk reported normal levels at the end of December except Tilshead and Chitterne Down which reported above normal levels. At the end of December, all sites except Allington showed slightly decreasing levels.

6. Reservoir stocks

Wessex Water reservoir levels remained relatively constant during December, beginning and ending the month at approximately 99% capacity. Overall, Bristol Water reservoir levels increased slightly during December, having begun the month at approximately 91% capacity and ending December at approximately 94% capacity. The current combined levels for Wessex Water are similar to those recorded this time last year while for Bristol Water they are slightly lower than in December 2023. For both Wessex Water and Bristol Water, levels are significantly higher than in December 1995.

Author: Wessex Hydrology, hydrologywessex@environment-agency.gov.uk

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.