Research and analysis

South-east England water situation report: May 2025 summary

Updated 12 June 2025

Applies to England

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

1. Summary

May was the third consecutive month with below average rainfall, with only 42% of the long term average (LTA) 1991 to 2020 recorded across the south-east of England. There was an average of 23 ‘dry’ days when there was less than 0.2mm rainfall recorded. After a warm, sunny and dry few months, the weather became more changeable, comprising slightly lower temperatures and variable rainfall patterns. Rainfall for the last 3 month totals has been in the exceptionally low category across the south-east of England. The soil moisture deficits (SMDs) generally rose steadily during the month and there has been no significant recharge. Flows at the indicator sites have continued the expected seasonal decline which started at the beginning of March and ranged from exceptionally low to notably high across the south-east. The expected steady decline of groundwater levels for this time of year continued during the month. No fluvial or groundwater flood alerts were issued during May.

2. Rainfall

May was the third consecutive month with below average rainfall, with only 42% of the LTA (1991 to 2020) recorded across the south-east of England. There was an average of 23 ‘dry’ days when there was less than 0.2mm rainfall recorded. After a warm, sunny and dry few months, the weather became more changeable, comprising slightly lower temperatures and variable rainfall patterns. For example, on 11 May there was a band of heavy rainfall from the Cotswolds in the west to the East Sussex coast which resulted in the highest daily rainfall, 20mm, being recorded at Swindon, Thames (THM). On 2 May, there was an isolated rainfall cell from the Wey Greensand areal unit (THM) to the North London areal unit, Hertfordshire and North London (HNL) which resulted in the second highest daily rainfall total for the month, 19.5mm, being recorded at Leatherhead, Kent and South London (KSL). By contrast, on 27 May, there was widespread light rainfall across the south-east of England which resulted in daily totals of around 10mm being recorded at Radlett, HNL and Knockholt, KSL, both within the top 5 rainfall totals for their respective areas.

On the 23 and 27 May the widespread rainfall contributed 50%, on average, of the monthly total rainfall across the south-east for the month. However, the heavy isolated showers on 11 and 12 May only contributed, on average, 12% of the monthly total for the south-east.

With reference to the rainfall map for the last 3 month totals below, the rainfall has been in the exceptionally low category across the south-east of England. It was the driest period on record for both Cotswolds East and THM and the driest since 1893 for a number of areal units, particularly Berkshire Downs and Ock, both THM and the Test, SSD.

3. Soil moisture deficit and recharge

The SMDs generally rose steadily to end the month around 150% of the LTA for May. Owing to the high SMDs and low rainfall, there was no significant recharge during the month.

4. River flows

Flows at the indicator sites have continued the expected seasonal decline which started at the beginning of March. Those rivers supported by groundwater baseflow show a less steep decline than those draining impermeable catchments. Flows range from exceptionally low to notably high across the south-east. The Lymington River at Brockenhurst (SSD) recorded the lowest May flow on record, with flows in the exceptionally low category. Another example of an exceptionally low flow site was the Arun at Alfoldean, also in SSD, with the lowest flows since 2011. The Coln at Bibury and the Thames at Farmoor, both THM, recorded flows in the notably low range, possibly reflecting the low groundwater levels in the Cotswolds which provides the groundwater baseflow. However, the Mimram at Panshanger (HNL) fed by groundwater from the Lee Chalk, remained in the notably high range during May and was the 10th highest on record. No fluvial or groundwater flood alerts were issued during the month. 

5. Groundwater levels   

The expected steady decline of groundwater levels for this time of year continued during the month. Ten of the 16 indicator sites recorded levels in the normal range for May. Jackaments (THM) and Carisbrooke Castle (SSD) both had levels in the exceptionally low category and both recorded their second lowest May levels on record after 1976 and 2005 respectively. The levels for the Chilterns and Lee Chalk sites, Stonor (THM) and Lilley Bottom (HNL) respectively, remained relatively high in the notably high range.

6. Reservoir stocks

The dry weather has resulted in continued declines in some of the reservoir stocks across the south-east of England. Stocks at all reservoirs across the south-east are at or just below the LTA for May, with the exception of Weir Wood reservoir (KSL), which is above the monthly 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.