Research and analysis

Hertfordshire and North London water situation: 2025 March summary

Updated 14 April 2025

Applies to England

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

1. Summary

The Hertfordshire and North London area received just 6mm of rain in March (12% of the long term average), making it an exceptionally dry end to the winter period. Soil moisture deficits rose sharply across all hydrological areas. River flows varied widely, with some sites in the exceptionally low band, and River Brent at Monks Park recording its second lowest March flow on record. Groundwater levels remained high overall, with Lilley Bottom in the exceptionally high band. Chalk river source locations remained largely unchanged, and no new flood alerts or warnings were issued.

2. Rainfall

The Hertfordshire and North London area (the Area) received exceptionally low rainfall during March, with just 6mm of rainfall (12% of the long term average LTA) recorded. All 5 areal units recorded low rainfall totals which fell within the exceptionally low band. The wettest day of the month was 23 March, with 9mm recorded at Birch Green (west of the North London unit) and 5.6mm recorded at Rye Meads STW (north of the Lower Lee unit). With the exceptionally low rainfall, there were 24 dry days (days with less than 0.2mm of rain recorded). March marked the end of the winter period, which began in October, with the Area receiving 305mm of rain during this period, 91% of the LTA.

3. Soil moisture deficit and recharge

During March, soil moisture deficits (SMDs) increased significantly across the Area following an exceptionally dry month due to the low rainfall and the start to the growing season. All areal rainfall units ended the month with SMDs well above their long term averages, with the area wide SMD’s at 3 times the month end average. There was no effective rainfall due to the low rainfall and growing SMDs. Despite the dry end to winter, the Area received 175mm of effective rainfall during the winter period (October to March), equating to 131% of the LTA.

4. River flows

Monthly mean river flows varied widely across the Area, with sites ranging from exceptionally high to exceptionally low bandings. River Brent at Monks Park recorded its second lowest monthly mean flow on record (records begin in 1979), following 2021. There were no significant peaks in river flow during the month, aside from a small rise around 22 to 23 March observed at some sites, including Monks Park. One flood alert for the River Mimram, issued on 29 January, remained active, but no new flood alerts or warnings were issued in March.

5. Groundwater levels   

Groundwater levels remained high during March, many sites saw a stabilisation in levels toward the end of the winter period. In the Upper Lee Chalk, 3 sites ended the month in the notably high band, while Lilley Bottom and Therfield Rectory in the exceptionally high band. Lilley Bottom recorded its second highest end of March level on record (records begin in 1979, highest in 1994). In the Mid-Chilterns Chalk, 3 sites were in the above normal band, while Ashley Green remained in the normal band. Two groundwater flood alerts issued on 19 December remained active during March: one for Flamstead (Mid-Chilterns Chalk and River Ver catchment), and one for Kimpton and Lilley Bottom (Lee Chalk and River Mimram catchment). No new groundwater flood alerts or warnings were issued.

6. Reservoir stocks

In the Lee Valley group, reservoir levels decreased from 99% to 91% of live capacity, ending March just at the average level. Reservoir stocks in the Lower Thames group started March at 93% of live capacity and ended the month at 95%, which was slightly below the average level.

7. Environmental impact 

In the Colne catchment, the locations of chalk river sources showed little to no change since February for the:

• River Ver which started flowing close to Kensworth Lynch
• River Gade which started flowing at Hudnall Corner
• River Bulbourne which flowed upstream of Dudswell village
• source of the River Chess which remained upstream of Chesham
• River Misbourne which flowed continuously from Mobwell pond, above Great Missenden

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

• River Mimram started flowing above Whitwell Gas Compound
• River Beane started flowing above Cromer
• River Rib was flowing intermittently from Reed End, before gaining a steadier flow upstream of Buntingford
• River Ash (Herts) was flowing intermittently from Meesden, before gaining a steadier flow at Little Hadham
• River Stort was still flowing from its source above Langley Lower Green

To protect the environment, during March a number of abstraction licence flow constraints were in force. This ranged between 6 and 14 per week, out of a March maximum of 51.

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.