Research and analysis

Hertfordshire and North London water situation: April 2026 summary

Updated 13 May 2026

Applies to England

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

1. Summary

April was a very dry month in the Hertfordshire and North London area, receiving only 10% of the long term average (LTA) rainfall, with all areal rainfall units ending the month in the exceptionally low band. Consequently, throughout the month no effective rainfall was received, while soil moisture deficits (SMD) rose to more than double the long term month end value. The lack of rainfall meant that there were no distinctive flow peaks in clay and urban catchments, with all indicator sites ranging from the below normal to notably low bands. In chalk catchments, despite a decline in river baseflows, most indicator sites recorded month mean flows in the normal band. Groundwater levels declined at most indicator sites, however, most ended the month in the normal band.

2. Rainfall

April was a very dry month across the Hertfordshire and North London area (the Area), with a general lack of notable rainfall events and only 5mm recorded, equal to 10% of the LTA. All rainfall units ended April in the exceptionally low band. Despite this, the rainfall totals for the last 3 months were in the normal band and above normal for some rainfall units in the last 6 months. The wettest day of the month was 13 April, with the highest rainfall totals recorded in the Roding areal rainfall unit, including:

• 7.6mm recorded at Thornwood STW
• 5.8mm recorded at Takeley STW
• 5.6mm recorded at Luxborough Lane

During April there was a total of 25 dry days across the Area (days with under 0.2mm of rain recorded).

3. Soil moisture deficit and recharge

The exceptionally low rainfall recorded in the Area meant that no effective rainfall was experienced during April. As a result, SMDs increased significantly throughout the Area to more than double the end of month LTA.

4. River flows

The lack of rainfall meant that during April there were generally no distinctive flow peaks in the Area’s clay and urban indicator sites, with month mean flows ranging from the below normal to notably low bands. Of particular note, Cranford Park (River Crane) recorded its fifth lowest monthly flow for April on record (records start in 1978). With the low rainfall, river baseflows generally declined in the Area’s chalk catchments. However, due to chalk river flows being supported by resilient groundwater levels, most indicator sites still recorded month mean flows in the normal band, with Panshanger (River Mimram) and Colney Street (River Ver) recording in the above normal band.

5. Groundwater levels   

Despite remaining at a stable level, the lack of rainfall and effective rainfall during April meant most groundwater sites in the Area started their seasonal decline. In the Mid-Chilterns Chalk aquifer, all sites recorded end of month levels in the normal band, with Ballingdon Farm being the only site not to have started its seasonal decline. For the Upper Lee Chalk aquifer, most indicator sites recorded end of month levels in the normal band, with Cave Gate and Hixham Hall in the above normal band. Lilley Bottom and Therfield Rectory were the only two indicator sites not to have shown a decline in groundwater level.

6. Reservoir stocks

Reservoir levels in the Lee Valley group increased slightly during April, starting the month at 96% of live capacity and ending the month at 98% (above its average level for the time of year). In the Lower Thames group, reservoir levels both started and ended April at 93% of live capacity (below its average level for the time of year).

7. Environmental impact 

In the Colne catchment, April saw chalk river sources remain at the same locations as March but there was a decrease in flows in the upper reaches for most of the rivers, the:

• River Ver continued to flow downstream of Markyate and flowed for its entire length
• River Gade started flowing above Great Gaddesden
• source of the River Bulbourne remained in the fields above Dudswell
• source of the River Chess remained at Missenden Road, upstream of Chesham
• River Misbourne continued to flow upstream of Great Missenden at Mobwell

The chalk river sources in the Upper Lee generally moved downstream since March, the:

• River Mimram was flowing upstream of Whitwell
• source of the River Beane moved downstream, but stayed above Walkern
• River Rib flowed from Buntingford
• River Ash (Herts) flowed intermittently from Blackhall and gained a steady flow above Hadham Ford
• River Stort flowed from above Langley Lower Green but dried between Clavering and Manuden

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