Research and analysis

Hertfordshire and North London water situation: April 2024 summary

Updated 15 May 2024

Applies to England

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

1. Summary

The Hertfordshire and North London area recorded 69mm of rain during April, 139% of the long term average (LTA) rainfall for the month. Soil moisture deficits increased but remained well below the month end long term average. River flows started to decline, but many baseflow-dominated rivers had flows that were exceptionally high for the time of year. Groundwater levels also remained high, with most sites in the exceptionally high band and recording their highest April level since 2001.

2. Rainfall

During April, the Hertfordshire and North London area (“the Area”) continued to receive rainfall above the LTA. In total, 69mm of rain fell across the Area, 139% of the April LTA rainfall. All 5 areal rainfall units ended the month in the above normal band. Despite the large monthly total, nearly half the month’s rainfall fell on 27 April, particularly in the Roding, Lee Chalk and Lower Lee catchments. This was the wettest day with rain gauges recording totals of:

• 35.4mm at Moreton sewage treatment works (STW, Roding)
• 33.2mm at Weston STW (Lee Chalk)
• 32.5mm at Runley Wood Pumping Station (Chilterns East)
• 32.4mm at Nazeing Golf Course (Lower Lee)
• 32.2mm at Lilley Manor (Lee Chalk).

The rest of the month saw much smaller but relatively consistent rainfall totals. There was an average of 11 dry days (less than 0.2mm of rain) during April.

3. Soil moisture deficit and recharge

April marked the start of the summer period, which usually sees soils beginning to dry out and a decline in recharge and runoff. Soil moisture deficits (SMD) across the Area did increase as soils dried from 2mm to 5mm but remained well below the month end LTA of 22mm. As a result, the Area received 142% of the LTA effective rainfall for April and the summer period so far.

4. River flows

River flows remained much higher than normal during April despite beginning their seasonal decline. Six river flow indicator sites recorded monthly flows in the exceptionally high band, all being Chalk rivers or those with a high baseflow component. Each of these sites recorded their second highest flows on record for April (exceeded only by 2001). These sites were the:

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

The River Ash at Wareside was the only Chalk river to record flows in the above normal band, while the River Crane at Cranford Park experienced above normal flows and the River Lee at Feildes Weir had monthly mean flows in the notably high band. A total of 8 flood alerts were issued during this month across the Area, all on the 27 and 28 April in response to high rainfall around this time.

5. Groundwater levels   

Groundwater remained high in April despite many sites plateauing or starting their seasonal decline. All indicator sites recording end of month levels in the exceptionally high band other than Therfield Rectory (Upper Lee Chalk), which recorded in the notably high band. The majority of groundwater indicator sites recorded their second highest end of April level on record, exceeded only by 2001. Exceptions were Lilley Bottom, which recorded its third highest April level behind 2001 and 1994 (records start in 1980) and Therfield Rectory, which typically has a delayed response compared to other indicator sites.

6. Reservoir stocks

Lower Thames Reservoir levels started April at 96% of live capacity, while the Lower Lee Reservoirs started April at 94% of live capacity. Reservoir stocks in both the Lower Thames and Lee Valley ended April around the LTA, remaining at 96% and 94% of live capacity respectively.

7. Environmental impact 

The sources of Chalk rivers in the Colne catchment remained in similar locations to March. The River Ver continued to flow upstream of Markyate Cell, while the River Gade was still flowing at Hudnall Corner. Sources of the River Chess and River Misbourne also remained at the same locations, flowing far above Chesham and at Mobwell Pond respectively. The source of the River Bulbourne moved slightly downstream but still remained in the fields above Dudswell village. Most of the sources of Chalk rivers in the Upper Lee catchment remained in similar locations to March. The River Mimram started flowing at the lakes upstream of Whitwell, while the River Beane started flowing at its source upstream of Cromer. The River Stort also continued to flow above the village of Langley Lower Green. The River Rib started flowing upstream of Hay Green, temporarily losing flow around Chipping, while the River Ash (Herts) started flowing at Meesden, temporarily losing flow around Clapgate.

To protect the environment during March a number of abstraction licence flow constraints were in force. This ranged between 0 and 2 per week, out of a maximum of 32.

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.