Research and analysis

Wessex water situation: September 2024 summary

Updated 11 October 2024

Applies to England

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

1. Summary

It was the wettest September since records began in 1871 for Wessex, it was the most rainfall received in any month since January 2014 and it was the wettest water year (October to September) on record. Nearly all areas of Wessex received exceptionally high rainfall this month but it was particularly wet in the east where 2 hydrological areas received over 300% of the long term average (LTA). Soil moisture deficit (SMD) started the month close to 100mm and declined as the month went on ending with most of Wessex having a SMD of less than 10mm. Monthly mean river flows were all above normal across Wessex, with the majority reporting exceptionally high flows. All reporting sites in Wessex reported exceptionally high daily mean flows at the end of September, with the River Tone at Bishops Hull, being the exception reporting notably high flows. At the end of the month groundwater levels across Wessex ranged from exceptionally high at Overcompton to normal at the slower responding Tilshead and Woodyates. Reservoirs were at around 63% and 65% capacity for Bristol Water and Wessex Water respectively, lower than the levels seen this time last year.

2. Rainfall

Wessex received an average of 186mm of rainfall during September (257% of the LTA). It has been the wettest September since records began in 1871 and it has been the wettest for any month since January 2014. Despite June and August being drier than average it has still been the wettest 8, 10, 12 and 24 month periods to September since records began. The majority of the total rainfall for the month fell across only 9 days, with 25% of the month’s rain falling between 5 and 7 September and 60% of the rainfall occurring on 6 days between the 20 and 30 September.

The River Bourne catchment received 339% of the LTA, and the Upper Hampshire Avon catchment received 337% of the LTA rainfall making it the second wettest of any month on record for this catchment. Rainfall was exceptionally high across all areas of Wessex bar the Poole Harbour and Purbeck area in the south of Wessex, where notably high levels of rain fell in September.

Over the longer term, the last 3 and 6 month periods have seen either exceptionally or notably high rainfall for most of Wessex with some areas recording above normal rainfall. Over the past 12 months all hydrological areas across Wessex received exceptionally high rainfall.

3. Soil moisture

The soil moisture deficit (SMD) started the month close to 100mm on average for the Wessex area. It drastically declined throughout the month and ended September with less than 10mm. SMD was slightly higher in areas to the south and west of Wessex with the value at the end of September ranging between 11mm to 40mm. At the end of the month the SMD was significantly lower than the LTA and the SMD this time last year.

4. River flows

All sites in Wessex reported monthly mean flows of above normal or higher. The majority of sites recorded exceptionally high flows, 6 sites reported notably high flows and the 2 most westerly sites, Beggearn Huish monitoring Washford and Bishops Hull monitoring the Tone, recording above normal.

By the last week of September all sites were reporting exceptionally high daily mean flows apart from the River Tone at Bishops Hull, which reported notably high flows. The majority of sites ended the month with increasing daily mean flows.

5. Groundwater levels   

Groundwater levels ended the month at normal or above across Wessex, with all sites either remaining at the same banding as the previous month or reporting increased levels.

Overcompton (monitoring the Bridport sand formation) reported the highest levels of the Wessex area at exceptionally high. Tilshead and Woodyates monitoring sites were reporting the lowest levels of the area, at normal, by the end of the month. All sites showed rising levels at the end of September, apart from Chittern Down and Tilshead (monitoring the Chalk) which continued decreasing and are yet to show a response to rainfall.

6. Reservoir stocks

Wessex Water reservoirs ended the month at around 65% capacity, with Bristol Water reporting similar levels of around 63%. All the reservoir levels increased throughout the month, due to increased rainfall after a drier few previous months. The current combined levels are below those recorded at this time last year for both Bristol Water and Wessex Water, however, they are both significantly higher than the levels seen in September of 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.