Research and analysis

Hertfordshire and North London water situation: February 2025 summary

Updated 12 March 2025

Applies to England

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

1. Summary

The Hertfordshire and North London area (the Area) received 51mm of rain in February (125% of the long term average, LTA), with all areal rainfall units in the normal band. River base flows continued to increase during the month, with sites in chalk catchments remaining relatively high. Groundwater levels also increased in February with most indicator sites high for the time of year and three sites ending the month in the exceptionally high band.

2. Rainfall

The Area received above average rainfall during February, with 51mm of rainfall or 125% of the LTA recorded, while all 5 areal rainfall units were at the top of the normal band. The wettest day of the month was 23 February, with 17.2mm recorded at Birch Green and 15.7mm recorded at Denham (both to the west of the North London unit). Over 14mm of rainfall was also recorded at:

• RAF Northolt (North London unit)
• Chenies (Chilterns East Colne unit)
• Moreton (Roding unit)

Despite the heavy rainfall, there were 14 dry days (days with less than 0.2mm of rain recorded).

3. Soil moisture deficit and recharge

During February, throughout the month soil moisture deficits (SMD) increased slightly across the area, although, all areal rainfall units finished the month well below their LTAs. All rainfall units received effective rainfall above their LTAs, with the highest effective rainfall recorded in Chilterns East Colne. For the winter period overall, (October 2024 to February 2025), the Area has received well above the LTA for effective rainfall.

4. River flows

In chalk catchments, base flows continued increasing during February, with 5 sites recording month mean flows in the notably high band. The most significant flow peaks during the month were recorded around 10 and 24 February – most notably in runoff dominated catchments like North London and the Lower Lee.

During February, there was a total of 5 flood alerts, mostly to the west of the Area. These included:

• Lower River Colne and Frays River (10 February)
• Upper River Colne and Radlett Brook (23 February)
• Lower River Colne and Frays River (24 February)
• Colne Brook at Iver and Colnbrook (24 February)
• Tidal River Crane (27 February)

No flood warnings were issued in February.

5. Groundwater levels   

Groundwater levels continued increasing across the Area in February, albeit at a slightly lower rate than the previous month. At the end of February, most indicator sites still had relatively high groundwater levels for the time of year. In the Upper Lee Chalk, Crescent Cottages, Lilley Bottom, and Therfield Rectory were in the exceptionally high band, with the latter achieving its highest level for the end of February since 1918 (records began in 1883). In the Mid-Chilterns Chalk, groundwater levels were slightly lower relative to their historic levels in February, with only one site (Ballingdon Farm) reaching an end of month level in the notably high band. Amersham Road and Wapseys Wood ended February in the above normal band, while Ashley Green was in the normal band.

6. Reservoir stocks

In the Lee Valley group, reservoir levels increased from 89% to 98% of live capacity, ending February well above average. Reservoir stocks in the Lower Thames group started February at 96% of live capacity and ended the month at 93%, which was slightly below the average level.

7. Environmental impact 

In the Colne catchment, there was little to no change in the locations of chalk river sources since January for the:

• source of the River Ver which remained above Markyate
• River Gade which started flowing above Hudnall Corner
• River Bulbourne which flowed upstream of Dudswell village
• source of the River Chess which was upstream of Chesham
• River Misbourne which was flowing continuously from Mobwell Pond to its confluence with the River Colne

The chalk river sources in the Upper Lee catchment remained almost the same as in January, the:

• source of the River Mimram was still above Whitwell Gas Compound
• River Beane started flowing above Cromer
• source of the River Rib remained upstream of Hay Green, but briefly ran dry downstream of Chipping.
• River Ash (Herts) was flowing upstream of Brent Pelham but temporarily lost flow near Clapgate
• River Stort was still flowing from its source above Langley Lower Green

To protect the environment, during October a number of abstraction licence flow constraints were in force. This ranged between 2 and 6 per week, out of a monthly maximum of 48.

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.