Research and analysis

Hertfordshire and North London water situation: September 2024 summary

Updated 11 October 2024

Applies to England

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September saw exceptionally high rainfall (243% of long term average (LTA)) in Hertfordshire and North London, making it the second wettest September on record. The month was fairly dry until 22 September, when intense rainfall caused rapid drops in soil moisture deficits and significant river flow increases. Groundwater level trends varied but all observation boreholes remained in the exceptionally high band. Several sites recorded their highest September levels on record for rainfall, river flows, and groundwater. Not only was it the second wettest September, but this water year (October 2023 to September 2024) has been the second wettest on record, with rainfall above the long term average in most months.

1. Rainfall

September was characterised by exceptionally high rainfall across the Hertfordshire and North London area (“the Area”). The Area received 133mm of rainfall, which was 43% of the LTA. This made September 2024 the second wettest September on record for the Area (records start in 1871), surpassed only by September 1896, which had 158mm of rainfall. It was also the Area’s ninth wettest month ever recorded dating back to 1871, out of a total of 1,845 months. During the same period, the Lee Chalk experienced the third wettest month on record.

In addition, the Lee Chalk and the Chilterns East Colne had their wettest Septembers on record, following the records set in 1896. These areas and North London were in the exceptionally high rainfall band while Lower Lee and Roding were in the notably high band.

The month started relatively dry with only 44mm recorded between 1 and 21 September. The dry spell was dramatically broken on 22 September with the month’s heaviest rainfall. On that day, Markyate (Chilterns East Colne) recorded the highest rainfall of 87mm and Whitwell (Lee Chalk) recorded a total of 86mm with the highest hourly intensity reaching 30.66mm. During this extreme rainfall, nearly half of the daily total fell within a 90 minute period.

The exceptional rainfall in September contributed to broader trends observed throughout the water year (October 2023 to September 2024). This water year was the second wettest year on record for the Area, following 2001 (based on records dating back to 1871). Over the last 12 months, the Area consistently received greater than the LTA rainfall every month except June and August.

2. Soil moisture deficit and recharge

Soil moisture deficits (SMD) dropped rapidly after the rainfall on 22 September, leading to unusually early soil saturation in some areal rainfall units. Chilterns East Colne began the month with an SMD of 95mm which dropped to zero by 26 September. Similarly, Lee Chalk started September with an SMD of 99mm and ended the month with a deficit of just 3mm. These chalk units received significant amounts of effective rainfall, well above the LTA. In contrast, North London, Lower Lee, and Roding experienced zero effective rainfall, despite the heavy precipitation. SMD levels dropped significantly but a meaningful level remained at the end of the month across these areas.

3. River flows

There were significant increases in river base flows across the Area in September, driven by the exceptionally high rainfall totals and high groundwater levels. The majority of sites recorded flows in the exceptionally high band for September with the exception of Brent Monks Park and Wareside Mardock which recorded flows in the notably high band, and Redbridge which was in the above normal band.

The monthly mean flows were the highest on record at a number of stations, including the:

• River Ver at Colney Street (records began 1956)
• River Mimram at Panshanger (records began 1952)
• River Lee at Howe Green (records began 1959
• River Crane at Cranford (records began 1978)
• River Colne at Denham Colne (records began 1952)

Notable peaks were seen following the intense rainfall on 22 September at all sites which led to 13 flood alerts on the 22 September, 8 on 23 September, and a further 15 towards the end of that week.

Three flood warnings were also issued, the:

• River Lee at Luton on 22 September
• River Lee at Harpenden on 23 September
• Yeading Brook East at South Ruislip on 23 September

4. Groundwater levels   

Groundwater levels increased at Amersham Road, Ashley Green, and Lilley Bottom. In contrast, the remaining groundwater indicator sites responded to the heavy rainfall with increases in groundwater level but overall they ended the month with levels lower than they started. Despite the variation in trends, all groundwater sites recorded levels in the exceptionally high band for September.

Wapseys Wood, Lilley Bottom, and Amersham Road all recorded their highest September levels since the start of their records in 1988, 1979, and 1991 respectively. Hixham Hall and Ashley Green were not far behind, measuring their second highest September levels since the start of their records in 1964 and 1987 respectively.

5. Reservoir stocks

September saw minor changes in reservoir levels. Lower Thames increased slightly from 79% to 80% of live capacity, while Lee Valley decreased from 96% to 94%. Both remained above their LTA for September.

6. Environmental impact 

Most of the sources of chalk rivers in the Colne catchment moved downstream of their locations in September, the:

• River Ver started flowing below Markyate.
• River Gade started flowing downstream of Hudnall Corner.
• source of the River Bulbourne started flowing downstream of Dudswell village.
• source of the River Chess remained just upstream of Chesham.
• River Misbourne was flowing along its full length from Great Missenden.

Most of the chalk river sources in the Upper Lee catchment stayed in similar locations compared to August, the:

• River Mimram started flowing just above Whitwell
• River Beane started flowing around Walkern
• source of the River Rib remained just above Buntingford
• River Ash (Herts) started flowing upstream of Hadham Ford
• River Stort started flowing at Stansted Springs

To protect the environment, during September a number of abstraction licence flow constraints were in force. This ranged between 1 and 9 per week, out of a winter 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.