Research and analysis

Hertfordshire and North London water situation: May 2024 summary

Updated 21 June 2024

Applies to England

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

1. Summary

Every month since September 2023, rainfall totals have exceeded the monthly long term average (LTA) rainfall and the pattern continued in May. The Hertfordshire and North London area received 79mm of rain during May, 153% of the LTA. Soil moisture deficits (SMD) increased but remained below the LTA. River flows and groundwater levels remained high for the time of year with many sites recording the second highest monthly flows and levels on record. The majority of river flow indicator sites were in the exceptionally high bands while nearly all groundwater indicator sites were also in the exceptionally high band.  

2. Rainfall

Every month since September 2023, rainfall totals have exceeded the monthly LTA rainfall and the pattern continued in May across the Hertfordshire and North London area (“the Area”). During May, a total of 79mm fell across the Area, 153% of the LTA rainfall. All 5 of the Area’s areal rainfall units ended the month in the notably high and above normal bands. It was a wet start to May recording rainfall totals of:

• 13.8mm at Epping Forest (Lower Lee) and Luxborough Lane (Roding) on 1 May
• 24.8mm at Rye Meads (Lee Chalk) on 2 May
• 11.2mm in Nazeing Golf Course (Lower Lee) on 3 May

However, the largest rainfall totals were recorded on 21 May, with 53mm in St Albans (Chilterns) and over 48mm in Wheathampstead (Lee Chalk). Despite these individual wet days, there were an average of 12 dry days (with less than 0.2mm of rainfall recorded) during May. Across the Area, it was the wettest start to Summer (April and May) since 2012. Over the last 12 months (June 2023 to May 2024) rainfall totals were the third highest for that period on record (which started in 1871), with only the 12 months to May in 1904 and 2001 wetter.     

3. Soil moisture deficit and recharge

Throughout May, soil moisture deficits (SMD) increased across the Area from 5mm at the start of the month to 16mm at the end of the month but remained well below the LTA of 43mm. Despite the soils beginning to dry, the Area continued to see notable effective rainfall of 182% of LTA. For the summer period (April to May) the Area received 154% of the LTA effective rainfall.

4. River flows

River flows remained very high for the time of year with many peaking in response to the heavy rainfall around 21 May. By the end of the month monthly mean river flows at indicator sites were recorded as:

• 7 in the exceptionally high band
• 2 in the notably high band
• one in the above normal band
• 2 indicator sites recorded flows in the normal band

Many of the sites experienced their second highest May monthly mean flows on record, exceeded only by 2001. These included the:

• River Ver at Colney Street (records start 1956)
• River Gade at Croxley Green (records start 1970)
• River Misbourne at Denham Lodge (records start 1984)
• River Colne at Denham Colne (records start 1952)
• River Lee at Howe Green (records start 1959)
• River Mimram at Panshanger (records start 1952)

A total of 11 flood alerts were issued across the Area in response to heavy rainfall at the start of the month and on 21 May. 

5. Groundwater levels   

During May, groundwater levels declined slightly but remained high with all sites (except for Ballingdon Farm - Mid Chilterns Chalk) recording levels in the exceptionally high band. Four sites recorded their second highest level on record after 2001. These were:

• Ashley Green (Mid Chilterns Chalk) (record began 1987)
• Amersham Road (Mid Chilterns Chalk) (record began 1991)
• Wapseys Wood (Mid Chilterns Chalk) (record began 1988)
• Hixham Hall (Upper Lee Chalk) (record began 1964)

6. Reservoir stocks

Lower Thames and Lee Valley reservoir stocks remained above the LTA throughout the month. The Lower Thames reservoir levels increased from 96% to 98% of live capacity, while the Lee Valley reservoir increased from 94% to 98%.

7. Environmental impact 

The sources of the Chalk rivers remained at similar locations to April 2024. In the Colne catchment, the:

• River Chess flowed above Chesham
• River Ver continued to flow upstream of Markyate cell
• River Gade was still flowing from Hudnall Corner
• River Bulbourne remained upstream of Dudswell village near Cowroast
• Misbourne continued to flow from Mobwell pond

In the Upper Lee catchment, the:

• River Mimram started flowing at the lakes upstream of Whitwell
• River Beane started flowing upstream of Cromer
• River Rib started flowing upstream of Hay Green
• River Ash (Herts) started flowing at Meesden, losing flow temporarily around Clapgate
• River Stort started flowing above Langley Lower Green

To protect the environment during May a number of abstraction license flow constraints were in force. This ranged between zero and one 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.