Research and analysis

Kent and South London water situation: September 2025 summary

Updated 14 October 2025

Applies to England

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

1. Summary

During September, the Kent and South London (KSL) area received 116% of the long term average (LTA) rainfall for the time of year, based on a whole-area average. Rainfall varied considerably between catchments, ranging from 80% of the LTA in North West Grain to 148% in Eastern Rother. All catchments recorded rainfall within the normal range, except Dover Chalk and Eastern Rother, which were above normal. At month-end, the average soil moisture deficit (SMD) across KSL was 155mm. Largely due to the relatively dry conditions between March and August 2025, this is still 29% higher than the LTA for this time of year. Compared to August, most catchments saw either an increase or decrease in SMD, with North Kent Chalk remaining unchanged. Effective rainfall averaged 3mm across the area (96% of the LTA), though this varied considerably between catchments. Monthly mean flows (MMFs) at key indicator sites were predominantly normal, however these ranged from below normal to above normal. Daily mean flows (DMFs) showed considerable variability, reflecting the distribution of rainfall and effective rainfall throughout the month. Groundwater levels in the Chalk and Greensand aquifers declined at all key sites, with rates and magnitudes of decline varying. By the end of September, groundwater levels were generally within the normal range, though Chalk sites ranged from below normal to above normal. Water company reservoir levels declined across the board, except at Weir Wood, where levels rose during the first half of the month. At month-end, all reservoirs were classified as below normal, apart from Weir Wood, which was notably high.

2. Rainfall

During September, the KSL area received 116% of the LTA rainfall for the time of year, based on a whole-area average. Rainfall received by individual catchments varied considerably, ranging from 80% of the LTA in North West Grain, to 148% in Eastern Rother. Rainfall totals were within the normal range for all catchments except Eastern Rother and Dover Chalk, which recorded above normal rainfall for September. The highest daily rainfall total of 28.8mm was observed on the second day of the month at Hartfield Rain Gauge in the Medway catchment.

Rainfall was particularly notable and widespread across the patch during 2 key periods – 1 to 4 and 10 to 16 of September. These wetter spells were driven by a succession of Atlantic low-pressure systems that brought frontal rainbands across the UK. These systems, typical of early autumn, were steered by a southward-shifted and strengthened jet stream, which acted as a conveyor belt for unsettled weather.

September featured only one dry day across the KSL area, defined as a day with 0.2 mm or less of recorded rainfall. This occurred on 6 September, and coincided with a brief interlude of high pressure, which became more notable in the final week of the month as conditions temporarily settled.

3. Soil moisture deficit and recharge

At the end of September, the KSL area had an average SMD of 155mm, which is 29% higher than the LTA for this time of year. Despite September’s rainfall being within the normal range, the SMD remains relatively high due to the relatively dry conditions that persisted between March and August 2025. Across the area, SMDs in individual catchments were also generally above their respective LTAs on 30 September, based on catchment-scale averages. Compared to the end of August, each catchment either saw an increase or a decrease in SMD, except for the North Kent Chalk catchment, where SMD remained unchanged. Among the remaining catchment, the changes were evenly split, with approximately half showing an increase and half showing a decrease.

While the KSL area received an average of 3mm of effective rainfall during September, equivalent to 96% of the LTA, this varied considerably between catchments. For example, the South London catchment received no effective rainfall during the month, contributing to a sharp increase in its SMD. In contrast, the Dover Chalk catchment received 130% (10mm) of its LTA effective rainfall for September, resulting in a notable decrease in its SMD over the course of the month.

4. River flows

MMFs at key indicator sites for September were predominantly normal across the area. Below normal MMFs were recorded at the Wandle at Connolly’s Mill and the Ravensbourne at Catford in the northwest, while above normal flows were observed at the Mole at Dorking in the far west of the patch.

DMFs showed considerable variability across most sites, reflecting the distribution of rainfall and effective rainfall throughout the month. Flow peaks were less pronounced, but still evident, at sites supported by groundwater baseflow, such as the Dour at Crabble Mill. In contrast, flashy sites influenced by clay-rich catchments, including the Ravensbourne at Catford and the Mole at Dorking, exhibited more marked fluctuations in DMFs.

5. Groundwater levels   

Groundwater levels in the Chalk and Greensand aquifers at all key indicator sites declined as expected for the time of year. The effective rainfall received, resulting in groundwater recharge, has the potential to slow the rate of decline of groundwater levels, as shown by the varied rate of decline across the Chalk sites. Chipstead in the west recorded a sharp and continuous drop throughout the month, partly due to the very low effective rainfall in the western part of the KSL area. While sites such as Fleete Reservoir in the east of the area showed a slower and more gradual decline, which is likely to be attributed to higher rates of effective rainfall observed in the eastern part of the KSL area. The only key indicator site monitoring the Greensand aquifer, Riverhead, followed a similar pattern, with a gradual decrease in groundwater levels.

By the end of the month, groundwater levels across the KSL area were generally within the normal range for the time of year. Nonetheless, variations were observed across the Chalk aquifer sites, with levels ranging from below normal at Chipstead in the central west to above normal at Riddles Lane in the north east.

6. Reservoir stocks

Of the 5 water company reservoirs in KSL, Bewl, Bough Beech and Powdermill experienced steady declines in water levels throughout September. Levels at Darwell reservoir remained relatively stable, with a slight increase compared to the end of August. In contrast, Weir Wood reservoir saw a more notable rise in levels, primarily during the first half of the month, before stabilising later.

By the end of September, the reservoirs held by the following live storage capacities. All were classified as below normal relative to their LTA levels – except Weir Wood, which was notably high for the time of year:

• Darwell – 41%
• Bewl – 48%
• Bough Beech – 49%
• Powdermill – 50%
• Weir Wood – 91%

7. Environmental impact 

Widespread hands off flow (HoF) restrictions remained in place across the Medway catchment throughout September, persisting from early April. In the Stour catchment, some of the widespread HoF restrictions introduced in early August also persisted throughout September.

Hands-off groundwater level restrictions affected a small number of abstractors in the Stour, South London and Darent catchments during September.  

Author: Groundwater and Hydrology Team, ksl.gwh@environment-agency.gov.uk 

Contact Details: 03708506506 

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.