Research and analysis

Hertfordshire and North London water situation: June 2024 summary

Updated 12 July 2024

Applies to England

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

1. Summary

The prolonged period of wet weather over the last 8 months ended in June with the Hertfordshire and North London area (“the Area”) receiving only 32% of the long term average (LTA) rainfall. Soil moisture deficits (SMD) increased to levels well above the LTA. River flows varied according to geology, with chalk river levels high and clay river levels low. Groundwater levels remained high with sites recording their highest and second highest levels on record.

2. Rainfall

The prolonged period of wet weather over the last 8 months ended in June with the Area receiving only 32% of the LTA rainfall. The Area’s 5 areal rainfall units ended June in the notably low band except Lee Chalk which was in the below normal band. Chilterns East Colne and Lee Chalk, the areal rainfall units in the chalk, received more rainfall than the 3 that are in the clay. On average there was a total of 21 dry days (with less than 0.2mm of rainfall recorded). The wettest day of the month was 14 June, with 13.4mm of rain recorded at Lilley Manor (Lee Chalk) and 13.2mm at Runley Wood (Lee Chalk). Over the summer period (April to June), the Area recorded 164mm of rainfall, 106% of the LTA. 

3. Soil moisture deficit and recharge

With the lower rainfall in June, there was a large increase in SMD across the Area from 16mm at the end of May to 80mm at the end of June, reaching well above the end of June LTA (68mm). The dry weather meant that there was no recharge (0mm of effective rainfall) on average across the Area. Despite a dry June, the Area’s effective rainfall for the summer period from April to June remained well above average at 135% of the LTA.

4. River flows

This month, there was a clear difference between the river flows in the chalk and clay catchments. The rivers of the Upper Lee and Colne are predominately fed by chalk groundwater, while the north London and Essex rivers run over urban and clay areas and are more responsive to rainfall. The 8 sites in the chalk catchments experienced monthly mean flows that ranged from the above normal to exceptionally high bands whereas the clay rivers were in the normal, below normal, and exceptionally low bands. The Ver at Colney Street which flows over a chalk catchment recorded the highest river flows on record (records start 1956). Also flowing over the chalk, the River Lee at Howe Green and the River Misbourne at Denham Lodge recorded the second highest flows since 2001 (records start 1959 and 1984 respectively). In the clay, the River Brent at Brent (Monks Park) recorded the second lowest flows since 2015 (records start 1979) and the River Crane at Cranford (Cranford Park) recorded the third lowest flows after 2015 and 2018 (records start 1979 and 1978 respectively). There were no flood alerts issued across the Area during June.

5. Groundwater levels   

Groundwater levels were making a slow decline for the time of year and remained high at the end of the month. All sites recorded levels in the notably high and exceptionally high bands. Lilley Bottom (Upper Lee Chalk) recorded the highest June levels on record (records start 1979) with Wapseys Wood (Mid Chilterns Chalk) and Hixham Hall (Upper Lee Chalk) recording their second highest level since 2001 (records start 1988 and 1964 respectively). 

6. Reservoir stocks

The Lower Thames and Lee Valley reservoir stocks both reduced from 98% to 95% but remained above the LTA for the end of June.

7. Environmental impact 

The sources of the Chalk Rivers in the Colne catchment remained in similar locations to May, the:

• River Ver started flowing at Markyate Cell
• River Gade started flowing at Hudnall Corner
• River Bulbourne started flowing upstream of Dudswell village
• River Chess started flowing a good distance above Chesham
• River Misbourne started flowing at Mobwell pond

Some of the sources of the Chalk Rivers in the Upper Lee catchment were at different locations to May, the:

• River Mimram started flowing at the lakes upstream of Whitwell
• River Beane started flowing upstream of Cromer
• River Rib started flowing at Slate Hall Farm then lost flow temporarily until Buntingford
• River Ash (Herts) started flowing at Little Hadham
• River Stort started flowing around Clavering losing flow at Potash Farm and returning at Standstead Springs

To protect the environment during June a number of abstraction license flow constraints were in force. This ranged between one and two per week, out of a weekly 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.