Research and analysis

Hertfordshire and North London water situation: July 2026 summary

Updated 12 August 2026

Applies to England

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

1. Summary

July was a very hot and dry month across the Hertfordshire and North London area. Only a negligible amount of rainfall was received during the month and all hydrological units experienced their lowest rainfall totals on record. This brought a large increase in soil moisture deficits (SMDs), with all hydrological units finishing July well above the long term average (LTA). As a result, a flash drought was declared this month. River flows varied considerably according to catchment type, with chalk indicator sites mostly recording month means in the normal band, while some clay and urban sites recorded their lowest ever month means for July. Groundwater levels continued declining at all indicator sites, however, all recorded end of month values in the normal band, remaining stable for the time of year.

2. Rainfall

For the Hertfordshire and North London area (the area), July 2026 was an extraordinary month due to its hot and dry weather throughout, with England as a whole experiencing two heatwaves. With negligible rainfall, the area experienced its driest ever July since records began in 1871, therefore meaning all five hydrological units received month rainfall totals in the exceptionally low band. This led to a flash drought being declared across the area mid-month. The highest rainfall total recorded in the area (and in fact the whole of South East England) was just 3.0mm at Clavering STW (Lee Chalk) on 25 July. Elsewhere on this day, only small amounts of rainfall were recorded at several other raingauges including:

• 1.6mm at Mill Green STW (Lee Chalk)
• 1.6mm at North Mymms PS (North London)
• 1.4mm at St Albans Oaklands College (Chilterns East Colne)
• 1.4mm at Wheathampstead STW (Lee Chalk)

Aside from the short-lived showers on 25 July, almost zero measurable rainfall was recorded for the other days of the month.

3. Soil moisture deficit and recharge

With there being no significant rainfall in the area during July, SMDs increased considerably, with all hydrological units ending the month far above their LTAs. In fact, SMDs in the area were at their highest level on record for the end of July. This further drying of soils meant no effective rainfall was seen during July.

4. River flows

During July, river flows in the area varied significantly depending on catchment type. Chalk river flows across the north of the area continued to be supported by resilient groundwater baseflow, with almost all indicator sites recording month mean flows in the normal band. Howe Green gauging station on the River Lee was the only chalk river site to record its month mean flow in the below normal band. In clay and urban catchments, where flows are reliant on recent rainfall, all indicator sites recorded month means in the exceptionally low band. The following sites recorded their lowest month mean flows on record for July:

• River Crane at Cranford Park (records start in 1978)
• River Brent at Monks Park (records start in 1979)
• River Roding at Redbridge (records start in 1950)
• River Ingrebourne at Upminster (records start in 1970)

The lack of significant rainfall in July meant that there were no notable river flow peaks during the month. Consequently, no flood alerts or warnings were issued.

5. Groundwater levels   

During July, groundwater levels in the area continued their seasonal decline, with all indicator sites showing lower levels compared to June. Despite this, across the Upper Lee Chalk and Mid-Chilterns Chalk aquifers, all indicator sites recorded groundwater levels in the normal band for the end of July.

6. Reservoir stocks

The volume of water stored in the Lee Valley reservoir group fell from 95% to 82% of live capacity during July. Therefore, stocks fell below their average level for the time of year. In the Lower Thames group, the volume of water stored fell from 88% to 72% of live capacity, meaning stocks remained below their average level for July.

7. Environmental impact 

In the Colne catchment, July saw chalk river sources generally move downstream from their locations in June, the:

• River Ver flowed temporarily downstream of Markyate but did not gain a meaningful flow until just above Redbourn
• source of the River Gade moved downstream and was located just above Great Gaddesden
• River Bulbourne started flowing just downstream of Northchurch
• source of the River Chess remained above the tennis courts upstream of Chesham
• River Misbourne still flowed for its entire length but its source moved to just downstream of The Chiltern Hospital

The chalk river sources in the Upper Lee stayed largely the same as in June, with some moving slightly downstream, the:

• River Mimram flowed briefly from Whitwell Gas Compound, before gaining a more meaningful flow just above Nine Wells Spring
• source of the River Beane remained just above Walkern
• River Rib still flowed from Buntingford
• source of the River Ash (Herts) moved further downstream to Much Hadham
• River Stort started flowing at Common Mead Bridge, although, flows were very low until Stansted Springs

To protect the environment, during July a number of abstraction licence flow constraints were in force. This ranged between 10 and 14 per week, out of a monthly maximum of 33. With drought declared in the area, an increase in the number of dry weather-related environmental incidents was reported during July, including numerous reports of distressed or dead fish. This has been noted particularly around ponds and slow-moving water bodies in London and Essex.

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.