Research and analysis

Hertfordshire and North London water situation: June 2025 summary

Updated 15 July 2025

Applies to England

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

1. Summary

June was another dry month across the Hertfordshire and North London area (the “Area”), with only 48% of the long term average (LTA) rainfall. All areal units ended the month in the below normal band, continuing a period of notably low rainfall across the Area. As a result, soil moisture deficits (SMDs) increased further, with record or near-record levels in some catchments, and effective rainfall remained minimal. River baseflows declined, with chalk rivers generally within normal to notably high bands, while clay and urban rivers remained significantly depressed. Groundwater levels continued to fall through June but remained stable overall, with many sites still above normal or notably high for the time of year.

2. Rainfall

June was another dry month in the Area, receiving only 48% of the LTA rainfall. All 5 areal rainfall units finished the month in the below normal band. All units have been in the notably low band for the past 3 months. Over the past 6 months, all were in the notably low band except for North London, which was below normal. There were 18 dry days during June (less than 0.2mm rainfall recorded in a day). The wettest day of the month, on average, was 7 June, when 16.0mm was recorded at Clavering STW (Lee Chalk unit). Over 10mm was also recorded at Heathrow, Allenby, and Northolt gauges, indicating that rainfall was associated with a single mid-month event that was most intense in North London and Lee Chalk.

3. Soil moisture deficit and recharge

The effective rainfall across the Area remained well below the LTA during June, resulting in no effective rainfall across the clay and urban catchments. Soils continued to dry throughout the month. By the end of June, all five areal rainfall units had SMDs well above the LTA. The Chilterns East Colne unit recorded the highest end of June SMD on record. In the Lee Chalk unit, the SMD was the second highest for June, exceeded only in 1976.

4. River flows

River baseflows across the Area declined further during June. Chalk rivers generally remained within the normal to notably high flow bands. Notably high flows were observed in the Mimram at Panshanger, and above normal flows were recorded for the Misbourne at Denham and the Ver at Colney Street. In contrast, flows in the clay and urban rivers were significantly reduced. The Roding at Redbridge and Ingrebourne at Upminster remained in the notably low band, while the Brent at Monks Park and Crane at Cranford were in the exceptionally low band. There were 8 flood alerts issued during the month, corresponding to localised rainfall events.

5. Groundwater levels   

Groundwater levels across the Area generally declined through June, as expected for the time of year, but remained stable overall. Indicator sites across the Area ranged from normal to notably high. The Mid-Chilterns Chalk, Therfield Rectory and Lilley Bottom ended the month in the notably high band, with Lilley Bottom recording its second wettest June after 2014. In the Upper Lee Chalk, levels remained above normal or notably high at several sites.

6. Reservoir stocks

The Lee Valley group began the month at 93% of live capacity and ended at 94%. The Lower Thames group began June at 94% and ended the month at 90%.

7. Environmental impact 

In the Colne catchment, the following chalk river sources moved slightly upstream from their locations in May, the:

• River Ver started flowing at Markyate STW
• River Gade started flowing at Bradden Lane
• River Bulbourne was flowing upstream of Dudswell village
• source of the River Chess stayed upstream of Chesham
• River Misbourne moved upstream and started flowing from Deep Mill Lane

The chalk river sources in the Upper Lee showed some slight changes since May, the:

• River Mimram started flowing at Whitwell Gas Compound
• River Beane started flowing below Church End Lane
• River Rib was flowing intermittently from Reed End, before gaining a steadier flow upstream of Vicarage Road
• River Ash (Herts) was flowing intermittently from Furneaux Pelham, before gaining a steadier flow below Hadham Ford
• River Stort was flowing above Poor Bridge

To protect the environment, between 4 and 10 flow constraints were in force during June, within the expected summer range of up to 29.

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.