Research and analysis

Hertfordshire and North London water situation: November 2024 summary

Updated 13 December 2024

Applies to England

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

1. Summary

The Hertfordshire and North London area recorded 61mm of rainfall in November, which was 99% of the long term average (LTA). River base flows varied from normal to exceptionally high, with some sites reaching record November levels. Groundwater levels remained in the exceptionally high band at most indicator sites, with Lilley Bottom and Crescent Cottages recording their highest November levels since records began.

2. Rainfall

The Hertfordshire and North London area “the Area” recorded 61mm of rainfall in November, which was 99% of the LTA. All 5 areal rainfall units remained in the normal band, consistent with October’s conditions. The month was characterised by dry conditions in the first half, with significantly more rainfall in the second half. The highest rainfall occurred on 26 November, with 26mm falling at Wanstead (Roding). During November, there were a total of 14 dry days (less than 0.2mm of rain). For the water year to the end of November (October to November), rainfall totals were normal with 100% of the LTA.

3. Soil moisture deficit and recharge

Soil moisture deficits (SMDs) decreased from October across the Area and remained well below the LTA for November. All areal rainfall units, except the Roding, received effective rainfall far greater than the LTA. The Roding experienced no effective rainfall both during October and November.

4. River flows

River base flows decreased during November and monitoring sites displayed a varied picture of flow conditions. Five sites recorded monthly mean flows in the normal band, one site was in the above normal band, 3 were in the notably high band, and 3 were in the exceptionally high band. Notably, Colney Street (River Ver) and Panshanger (River Mimram) reached their highest recorded November monthly mean flows since monitoring began in 1956 and 1952 respectively.

After the heavy rainfall on 26 November, the heightened river conditions led to 8 flood alerts being issued across the region on 27 November, including:

• the Lower Lee tributaries
• the Lower River Lee from Hoddeson to Canning Town
• the River Ingrebourne at Harold Park and Hornchurch
• the Middle River Roding
• 2 on the Upper River Colne and Radlett Brook
• 2 on the Colne Brook at Iver and Colnbrook

A single flood warning was issued for the Ching Brook in Waltham Forest.

5. Groundwater levels   

Groundwater levels in November remained high across the Area with most sites seeing little change or increasing slightly. Of the 9 indicator boreholes monitored, seven recorded exceptionally high levels, while two were in the notably high band.

Two boreholes reached their highest November levels since records began; Lilley Bottom with records beginning in 1979 and Crescent Cottages with records beginning in 1968.

Several other boreholes approached historic peaks:

• Cave Gate was at its second highest level since 2001 (records began 1966)
• Ballingdon Farm was at its second highest level since 1987 (records began 1975)
• Amersham Road was at its second highest since 2001(records began 1991)
• Wapseys Wood was at its third highest level since 2001 (records began 1988)

6. Reservoir stocks

The Lower Thames reservoir increased from 79% to 82%, matching the LTA. The Lee reservoir remained stable at 94%, which was higher than the LTA.

7. Environmental impact 

In the Colne catchment, most of the chalk river sources moved upstream from their locations in October, including the:

• River Ver started flowing above Markyate
• River Gade started flowing at Hudnall Corner
• River Bulbourne was flowing upstream of Dudswell village
• source of the River Chess stayed upstream of Chesham
• River Misbourne flowed continuously from Mobwell pond

Most of the chalk river sources in the Upper Lee moved upstream compared to October, including the:

• River Mimram started flowing at Lilley Bottom Road
• River Beane started flowing above Cromer
• source of the River Rib flowed continuously from upstream of Hay Green
• River Ash (Herts) flowed continuously from upstream of Brent Pelham
• River Stort started flowing above Langley Lower Green

To protect the environment, during November a number of abstraction license flow constraints were in force. This ranged between 11 and 22, out of a maximum of 49.

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.