Research and analysis

Hertfordshire and North London water situation: October 2024 summary

Updated 13 November 2024

Applies to England

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

1. Summary

In contrast to the previous month, October was neither particularly wet nor dry in the Hertfordshire and North London area, with 100% of the long term average (LTA) rainfall recorded. The first half of October was significantly wetter than the rest of the month. Despite remaining high, river base flows decreased during October because of the lower rainfall. Following the recharge that occurred during September’s rainfall, groundwater levels for October were mostly in the exceptionally high band, with some sites recording their highest end of October levels on record.

2. Rainfall

Despite the exceptionally wet September in the Hertfordshire and North London area (“the Area”), October’s rainfall was very normal compared to the LTA. The Area received 60mm of rainfall, or 100% of the LTA, with all 5 rainfall units ending October in the normal band. The wettest day of the month was 1 October, with 31.2mm of rain recorded at Weston sewage treatment works (STW) and 27.2mm at Chipping, both in the Lee Chalk unit. Other Lee Chalk rain gauges recording over 20mm on 1 October included Stevenage, Whitwell STW, Dane End STW and Braughing. During October, there were a total of 13 dry days (less than 0.2mm of rain), with the first half of the month being wetter than the second half.

3. Soil moisture deficit and recharge

Thanks to the heavy rainfall in September and the first half of October, soil moisture deficits (SMDs) across the Area remained well below the LTA at the end of the month. Despite this, the clay and urban rainfall units in the Area received no effective rainfall during October. Meanwhile, the Lee Chalk and Chilterns East Colne rainfall units both received effective rainfall far above their LTA for the month.

4. River flows

October generally saw a decline in river base flows across the Area from their September levels, although still remained high for the time of year, particularly in chalk catchments. Both Panshanger (River Mimram) and Colney Street (River Ver) recorded their highest monthly mean flows for October on record (records start in 1952 and 1956 respectively). Apart from 3 gauging stations, all chalk indicator sites recorded flows in the exceptionally high band for October. The 3 outliers in chalk catchments were Wareside (River Ash), which recorded flows in the above normal band, and Denham Lodge (River Misbourne) and Feildes Weir (River Lee) which were both in the notably high band. Despite the high flows in chalk catchments, all of the clay and urban indicator sites recorded mean flows in the normal band for October. Overall, the most notable flow peak during the month was seen around 1 and 2 October in response to the heavy rainfall.

During October, there were 11 flood alerts, most of which were issued on the 1 October across the area, including, the:

• Tidal River Crane
• River Beane catchment
• River Stort and Stansted Brook catchment
• Coln Brook at Iver and Colnbrook
• Upper Colne and Radlett Brook
• River Rib and Quin catchment
• River Lee at Luton, Harpenden and Wheathampstead

Despite the various flood alerts, no flood warnings were issued during October.

5. Groundwater levels   

Groundwater levels in the Area remained high during October, supported by the recharge seen during September. For October, the majority of indicator sites showed end of month groundwater levels higher than in September, or a slowed decline. With the exception of Therfield Rectory, all groundwater sites in the Upper Lee Chalk showed an increase in groundwater level. All indicator sites recorded groundwater levels in the exceptionally high band, other than Ashley Green and Amersham Road (both in the Mid-Chilterns Chalk) which were in the notably high band.

Some sites recorded their highest end of month groundwater levels on record for October, including:

• Lilley Bottom (Upper Lee Chalk) – records began in 1979
• Crescent Cottages (Upper Lee Chalk) – records began in 1968
• Cave Gate (Upper Lee Chalk) – records began in 1966
• Ballingdon Farm (Mid-Chilterns Chalk) – records began in 1975
• Wapseys Wood (Mid-Chilterns Chalk) – records began in 1988

Therfield Rectory, which has a much longer record than the other abovementioned sites was at its highest end of October level since 1918 (records start in 1883).

6. Reservoir stocks

October saw very little change in reservoir levels, with the Lower Thames reservoirs starting the month at 80% of live capacity and ending at 79%. The Lee Valley reservoirs both started and ended October at 94% of live capacity. Both reservoir groups remained above the LTA for October.

7. Environmental impact 

In the Colne catchment, all chalk river sources moved upstream from their locations in September, the:

• River Ver started flowing at Markyate STW
• Source of the River Gade moved slightly upstream but still below Hudnall
• River Bulbourne was flowing upstream of Dudswell village
• Source of the River Chess moved further upstream from Chesham
• River Misbourne flowed continuously from Mobwell pond

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

• River Mimram started flowing at the lakes above Whitwell
• Source of the River Beane was around the village of Walkern
• River Rib started flowing at Reed End but temporarily lost flow around Chipping Bridge
• River Ash (Herts) started flowing upstream of Brent Pelham, although it temporarily lost flow around Clapgate
• Source of the River Stort moved upstream of Langley Lower Green

To protect the environment, during September a number of abstraction licence flow constraints were in force. This ranged between 1 and 2 per week, out of a maximum of 35.

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.