Research and analysis

Hertfordshire and North London water situation: 2025 May summary

Updated 12 June 2025

Applies to England

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

1. Summary

May was another dry month across the Hertfordshire and North London area (the “Area”), with just 42% of the long term average (LTA) rainfall and most areal units in the notably low band. As a result, soil moisture deficits (SMDs) increased further and effective rainfall was minimal. Baseflows declined across the Area’s chalk rivers but the dry weather particularly affected the urban and clay rivers, where some sites recorded their second or third lowest May flows on record. Groundwater levels declined during May but remained generally high for the time of year.

2. Rainfall

May was another dry month in the Area, receiving only 42% of the LTA rainfall. All 5 areal rainfall units finished the month in the notably low band, except for North London, which ended in the below normal band. Over the last 3 months, all units have received exceptionally low rainfall. There were 19 dry days in May (less than 0.2mm rainfall recorded in a day). The wettest day of the month, on average, was 27 May, when 12.0mm of rain was recorded at Radlett STW (Chilterns East Colne unit). Over 10mm was also recorded at Northolt and Denham (both North London). However, the highest individual daily rainfall was recorded on 12 May, when 15.4mm fell at Prestwood Reservoir and 14.2mm at Chenies, both in Chilterns East Colne This rainfall appeared to be from a storm event largely isolated to parts of the Chilterns.

3. Soil moisture deficit and recharge

The effective rainfall across the Area was well below the LTA during May, with no effective rainfall in the clay and urban catchments. Soils continued to dry throughout the month, and by the end of May, all 5 areal rainfall units had SMDs well above the LTA for this time of year.

4. River flows

River baseflows across the Area declined during May. As in previous months, there was a clear north-south divide, with most chalk catchments remaining in the normal band, while the clay and urban rivers experienced significantly reduced flows. Cranford Park (River Crane) recorded its second lowest May flow on record (records begin in 1978). Redbridge (River Roding) and Upminster Gaynes Park (River Ingrebourne) recorded their third lowest May flows on record (records begin in 1950 and 1970 respectively). The most notable flow peaks occurred around 27 and 28 May, corresponding to the late-month rainfall, which was most evident in the clay and urban catchments. No fluvial flood warnings or alerts were issued during May.

5. Groundwater levels   

Groundwater levels across the Area declined during May, as expected for the time of year. However, levels remained relatively high overall. In the Mid-Chilterns Chalk, three indicator sites ended the month in the normal band and Wapseys Wood in the above normal band. In the Upper Lee Chalk, 2 indicator sites were in the above normal band and 3 in the notably high band including Lilly Bottom.

6. Reservoir stocks

Reservoir stocks fell slightly during May but remain stable. The Lee Valley group started May at 96% of live capacity and ended the month at 93%. In the Lower Thames group, reservoir stocks started May at 95% and ended at 94% of live capacity.

7. Environmental impact 

In the Colne catchment, chalk river sources moved slightly downstream compared with April, the:

• River Ver started flowing at Markyate STW
• River Gade started flowing at Four Ways Farm
• River Bulbourne was flowing slightly further down but was still above Dudswell village
• source of the River Chess stayed upstream of Chesham
• River Misbourne flowed continuously from Mobwell pond

The chalk river sources in the Upper Lee catchment showed minor changes since April, the:

• River Mimram started flowing at Whitwell Gas Compound
• River Beane started flowing below Cromer
• River Rib was flowing intermittently from Reed End, before gaining a steadier flow upstream of Buntingford
• River Ash (Herts) was flowing intermittently from Brent Pelham, before gaining a steadier flow below Little Hadham
• River Stort was flowing above Langley Lower Green but ran dry temporarily between Clavering and Manuden

To protect the environment, during May a number of abstraction licence flow constraints were in force. This ranged between 2 and 4 per week, out of a summer maximum of 30.

Author: Groundwater and Hydrology, groundwaterhydrology@environment-agency.gov.uk

Contact Details: 03708 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 within.