Research and analysis

Hertfordshire and North London water situation: July 2025 summary

Updated 12 August 2025

Applies to England

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

1. Summary

July was wetter than recent months across the Hertfordshire and North London area (the “Area”), receiving only 103% of the long term average (LTA) rainfall. The rainfall for all areal units ended the month in the normal band, supported by several intense rainfall events during the middle and latter part of the month. Despite this, soil moisture deficits (SMDs) increased further, with all units ending the month significantly above the LTA and effective rainfall remained insignificant outside of the Chilterns East Colne and Lee Chalk units. River baseflows responded to rainfall, with chalk rivers ranging from normal to notably high, while clay and urban rivers remained more variable, from notably low to normal. Groundwater levels remained stable across the Area, with Mid-Chilterns Chalk sites in the normal band and Upper Lee Chalk sites above normal or notably high for the time of year.

2. Rainfall

July was wetter than recent months across the Area, receiving 103% of the LTA rainfall. All five areal rainfall units finished the month in the normal band. All units have been in the notably low band for total rainfall over the past three months, except for North London, which was below normal. For total rainfall over the past 6 months, all units were in the exceptionally low band, except for North London and the Lower Lee, which were notably low. The wettest day of the month was 19 July, when 28.6mm was recorded at Wheathampstead STW (Lee Chalk). On the same day, 26.2mm was measured at the Avenue North, Wembley (North London) and 26.0mm at Oaklands College, St Albans (Chilterns East Colne). There were 12 “dry” days during July (<0.2mm rainfall recorded in a day).

3. Soil moisture deficit and recharge

The effective rainfall across the Area remained limited during July. Only the Chilterns East Colne and Lee Chalk units recorded any effective rainfall, both slightly above or close to the LTA, while the clay and urban catchments received none. Soils continued to dry during the month, with month end SMDs well above the LTA across all five areal rainfall units. The SMDs for both Lee Chalk and Chilterns East Colne units increased and all units ended the month with deficits higher than their LTAs.

4. River flows

River baseflows across the Area responded to several intense rainfall events during July. The majority of the Area’s Chalk rivers were within the normal monthly mean flow band for July. The exceptions to this were the Ver at Colney Street, which recorded above normal flows and the Mimram at Panshanger where notably high flows were observed. In contrast, flows in the clay and urban rivers generally fell in the lower flow bands. The Brent at Monks Park was in the normal band, while the Crane at Cranford and the Ingrebourne at Upminster were both below normal. The Roding at Redbridge remained in the notably low band. There were nine flood alerts issued during the month, majority of which were issued on 18 and 19 July, along with two flood warnings in response to high rainfall.

5. Groundwater levels   

Groundwater levels across the Area remained stable during July. Indicator sites across the Area ranged from the normal to the notably high. In the Mid-Chilterns Chalk, all sites ended the month in the normal band. In the Upper Lee Chalk, levels remained above normal at the majority of sites, while Therfield Rectory continuing to see groundwater levels in the notably high band for July.

6. Reservoir stocks

The Lee Valley group began the month at 94% of live capacity and ended at 95%, remaining well above LTA. The Lower Thames group began July at 90% and ended the month at 79%, falling well below LTA. The decline in the Lower Thames group reflects the ongoing dry weather and the low flow in the Thames, while the Lee Valley group has maintained more stable levels.

7. Environmental impact 

In the Colne catchment, the chalk river sources moved slightly downstream from their positions in June:

• source of the River Ver was just downstream of Markyate
• River Gade started flowing at Bradden Lane, between Great Gaddesen and Hemel Hempstead
• River Bulbourne had moved downstream to just above Northchurch
• source of the River Chess moved to just upstream of Chesham
• River Misbourne started flowing downstream of Great Missenden

In the Upper Lee catchment, there were also slight downstream shifts since June:

• River Mimram continued to flow at Whitwell Gas Compound.
• source of the River Beane was just downstream of Cromer.
• River Rib was flowing intermittently from Chapel Green STW, gaining a steadier flow in Buntingford.
• intermittent flow from Brent Pelham on the River Ash (Herts), before gaining a steadier flow above Much Hadham
• River Stort flowed intermittently from Lower Green, but flows were continuous downstream of Manuden

To protect the environment, a number of abstraction licence flow constraints were in force during July, ranging from 4 to 11 per week, with a monthly total of 32.

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.