Research and analysis

Thames water situation report: September 2024 summary

Updated 11 October 2024

Applies to England

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

1. Summary

Thames area received 193mm of rainfall in September, 317% of the long term average (LTA); making it the wettest month since records began in 1871. Following the high rainfall, soil moisture deficits (SMDs) were reduced to zero and this resulted in high effective rainfall of 50mm (LTA is 5mm). Monthly mean flows increased at all our sites and 4 measured their highest ever September flows since site records began. Due to the heavy rainfall, the seasonal declining trend of groundwater levels reversed at the majority of our indicator sites in September, which is earlier than normal. Fifty-one flood alerts and 15 flood warnings were issued on rivers during September.

2. Rainfall

Thames area experienced its wettest month since records began in 1871. Overall, Thames area received 193mm of rainfall during the month (317% of the LTA) and over a third of this fell on just 2 days, 22 and 23 September. The highest daily total of 93.2mm was recorded at Stokenchurch (Chilterns West) on 22 September. Over the past 12 months, Thames area recorded exceptionally high rainfall in every areal unit and during the summer period (April to September inclusive), the area received 146% of LTA rainfall.

3. Soil moisture deficit and recharge

High rainfall during the month cleared the above average SMDs recorded in August and resulted in SMDs of 0mm or 1mm being recorded in every areal unit except for one. This was considerably below the LTA of 88mm, which means that soils were wetter than usual for the time of year. The saturated soils enabled high effective rainfall of 50mm, compared to the September LTA of 5mm.

4. River flows

Monthly mean river flows increased at all our indicator sites in September following the high rainfall. Exceptionally high flows were recorded at all sites except 2, both of which recorded notably high flows. Four of our sites measured their highest ever September flows since site records began:

• Abingdon, River Ock (since 1963)

• Banbury, River Cherwell (since 1967)

• Bourne End (Hedsor), River Wye (since 1965)

• Wheatley, River Thame (since 1989)

5. Groundwater levels 

Following the above average rainfall, groundwater levels increased at the majority of our sites, although it would be expected that most sites would continue their seasonal decline during September. Groundwater levels at sites located in the slower responding Chalk aquifers started to show early signs of recovery towards the end of the month. Exceptionally high groundwater levels were recorded at 5 sites, with the rest recording notably high levels for the time of year due to the exceptionally high rainfall recorded over the past 12 months.

6. Reservoir stocks

Reservoir stocks decreased at both Farmoor and Lower Thames. At the end of September, Farmoor reservoir was 87.5% full, slightly below the LTA. The Lower Thames reservoir was 79.8% full, which was slightly above the LTA.

7. Environmental impact 

During September, there were 51 flood alerts and 15 flood warnings issued on rivers in Thames area. At the end of the month, no abstraction licences were 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: 030708 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.