Research and analysis

Hertfordshire and North London water situation: August 2026 summary

Updated 14 September 2026

Applies to England

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

1. Summary

High pressure dominated the start of the August, much needed widespread rain developed during later in the month. However, the Hertfordshire and North London area (the area) recorded just 42% of the month’s long term average rainfall (LTA), receiving the least rainfall across England. Soil moisture deficits (SMDs) continued to increase during the month in all hydrological areas (with the exception of the Lee Chalk), recording their second highest August month-end SMD, only exceeded in August 1976. The Crane at Cranford, Brent at Monks Park and the Roding at Redbridge all recorded their lowest August monthly mean flows on record. Despite the dry weather, groundwater levels remained normal for August, with many chalk rivers continuing to flow along much of their length.

2. Rainfall

High pressure dominated the start of the August, continuing the dry conditions which had become established since June. Mid-month, unsettled weather with localised heavy downpours occurred, which developed into widespread heavy rainfall for the last week of the month. Despite the much need rainfall, the Hertfordshire and North London area only recorded an areal average of 26.7mm rainfall. This was just 42% of the August LTA rainfall, the lowest rainfall for any Environment Agency area in England during August.

Across the area the rainfall was not evenly distributed, due to the localised nature of the storms. The Lee Chalk hydrological area received 41.9mm or 66% of the August LTA, while in the North London hydrological area only 14.9mm or 26% of the LTA rainfall was seen, making it the driest hydrological area anywhere in England. The heaviest daily rainfall total of 30.2mm was recorded on the 26 August at Braughing rain gauge in the Lee Chalk hydrological area. Over the last 6 months the Hertfordshire and North London area has recorded exceptionally low rainfall, while the total rainfall recorded in July and August was the lowest on record (since 1871).

3. Soil moisture deficit and recharge

With the continued high temperatures and lower than average rainfall during August, SMDs continued to increase. All hydrological areas ended the month with near record SMDs, with all but the Lee Chalk recording their second highest August month-end SMD, only exceeded in 1976. With large SMDs there was negligible effective rainfall or recharge to groundwater.

4. River flows

The widespread heavy rainfall during the second half of August lead to an increase in flow at all indicator sites, with rivers peaking following the wettest weather on the 26 and 27 August. However, the rivers of north London and Essex experienced some of the lowest August monthly mean flows on record. The Ingrebourne at Upminster recorded (since 1971) its third lowest August flows (only lower in August 2025 and 1976). Meanwhile the Crane at Cranford (since 1978), Brent at Monks Park (since 1979) and the Roding at Redbridge (since 1950) all recorded their lowest August monthly mean flows on record. In contrast, the rivers in chalk catchments continued to benefit from robust groundwater levels. River flow indicator sites across the Chilterns and Upper Lee catchments had normal flows for August. The exceptions included the Colne at Denham and Lee at Waterhall, which recorded month mean flows in the below normal band. This could be attributed to reduced discharge from each of the large sewage treatment works upstream of these sites.

5. Groundwater levels   

All groundwater indicator sites across the Mid-Chilterns and the Upper Lee Chalk ended the month with groundwater levels in the normal band for August. Groundwater levels continued to decline during the month but remained resilient to the hot and dry weather.

6. Reservoir stocks

Both of Thames Waters’ reservoir groups in the Hertfordshire and North London area were significantly lower than average at the end of August. The Lower Thames group in west London were at 60% live capacity, while the Lee group in east London were at 70% live capacity.

7. Environmental impact 

In the Colne catchment, August saw most chalk river sources move slightly downstream and flows decline in the headwaters, the:

• River Ver flowed temporarily downstream of Markyate, but did not start flowing again until Redbourn
• source of the River Gade remained located just above Great Gaddesden
• River Bulbourne source moved downstream and started flowing downstream of Northchurch
• source of the River Chess remained above the tennis courts upstream of Chesham but flows were lower than July
• River Misbourne continued to flow for its entire length from downstream of Great Missenden

In the Upper Lee, the River Mimram moved slightly downstream since July where it began flowing above Nine Wells Spring, Whitwell. No survey was undertaken in August for the River Beane, River Rib, River Ash (Herts) and River Stort.

To protect the environment during August, 13 abstraction licence flow constraints were in force out of a monthly maximum of 31.

Author: Groundwater and Hydrology, 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 within.