Research and analysis

South-east England water situation report: September 2025 summary

Updated 14 October 2025

Applies to England

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

1. Summary

September was a wetter than average month, with 128% of the long term average (LTA) recorded across the south-east of England. Hertfordshire and North London (HNL) was the driest area, recording below average rainfall (96%). All the other areas were above average, with Solent and South Downs (SSD) being the wettest with 153% rainfall. September brought the hydrological summer to a close, when around three-quarters of the LTA summer rainfall across the south-east was recorded over the last 6 months. The total effective rainfall (that contributes to recharge) across the south-east was well below the LTA for the summer period (March to September). All areas were below 50%, with Kent and South London (KSL) having 47% of the LTA and Thames (THM) only 25% of the LTA for the last 6 months.

Soil moisture deficits (SMDs) generally fell during the first half of the month in response to the rainfall but ended the month more or less where they were at the beginning of the month.  There was also some recharge during September due to the heavy rain at the beginning of the month.

River flows ranged from notably high to exceptionally low for September. Most key indicator sites were normal for the time of year. The Coln at Bibury (THM) was the only site across the south-east that was exceptionally low which was at its third lowest September flows since 1976. Only one fluvial flood alert was issued in SSD on the 9 September.

Groundwater levels continued to fall at most of the key indicator sites across the south-east. By the end of September, groundwater levels ranged from above normal to notably low. Half of the key indicator sites remain at normal levels for the time of year.

2. Rainfall

September was a wetter than average month, with 128% of the LTA recorded across the south-east of England. Several frontal systems brought bands of heavy rain at the start of the month. The highest daily total was 44.4mm recorded at Folkington RG (Pevensey Levels, SSD) on 12 September. However, the wettest day was the 2 September, closely followed by the 3 September. Both of these days combined recorded around a third of the monthly total rainfall.

HNL was the driest area with below average rainfall being recorded (96% of the LTA). All the other areas were above average, with SSD being the wettest with 153% of the LTA which was received there.

September brought the hydrological summer to a close; around three quarters of the LTA summer rainfall across the south-east was recorded between April and September. There was also a north-south divide during the summer when both THM and HNL had 66% LTA rainfall whereas KSL and SSD recorded 80% and 88% respectively. The Cherwell (THM) and Roding (HNL) both recorded their lowest summer rainfall since 1996. Most of the remaining areal units recorded their driest summer since 2022. 

The total effective rainfall (that contributes to recharge) across the south-east was well below the LTA for the summer period (April to September). All areas were below 50%, KSL had 47% LTA and THM only 25% LTA for September. 

3. Soil moisture deficit and recharge

SMDs generally fell during the first half of the month in response to the rainfall. However, deficits then began to rise again when the weather became drier later in September. SMDs ended the month more or less where they were at the beginning of the month and were above the end of month LTA, significantly so in THM, HNL and KSL, but closer to the average in SSD.  There was also some recharge during September due to the heavy rain at the beginning of the month.

4. River flows

River flows ranged from notably high to exceptionally low for September. Most key indicator sites were normal for the time of year. Most of the rivers responded to the rainfall during the beginning of the month, but flows quickly returned to levels seen at the end of August afterwards. The 2 sites that were notably high were the Mimram at Panshanger and Ver at Colney Street, both in HNL. These are both chalk fed streams where groundwater levels have been higher than normal for most of the year so far. They were also the sixth and tenth highest (respectively) September flows since 1979. At the other end of the flow scale were the Thames at Farmoor and Kennet at Marlborough (both in THM), that recorded below normal flows due to groundwater levels falling faster than elsewhere in the south-east during the summer. The Coln at Bibury (THM) was the only site across the south-east that was exceptionally low which was the third lowest September flows since 1976. Only one fluvial flood alert was issued in SSD on 9 September.

5. Groundwater levels   

Groundwater levels continued to fall at all key indicator sites across the south-east, with 2 exceptions – Carisbrooke (Isle of Wight, SSD) and Chilgrove (SSD). At Carisbrooke, levels rose in response to the heavy rainfall at the beginning of the month and at Chilgrove, levels have flattened out at notably low levels. By the end of September, groundwater levels ranged from above normal at Lilley Bottom (Lee chalk, HNL) and Stonor Park (Chilterns, THM) to notably low at Jackaments (Oolites, THM) and Chilgrove (West Sussex chalk, SSD). Half of the key indicator sites remain at normal levels for the time of year.

6. Reservoir stocks

Reservoir stocks generally continued to decline during the month. All of the reservoirs remained below the LTA for September  with just the exceptions of Weir Wood (KSL) and the Lee Valley (HNL) that ended the month above the LTA.

Author: groundwaterhydrology@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.