Research and analysis

Hertfordshire and North London water situation: March 2024 summary

Updated 15 April 2024

Applies to England

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

1. Summary

It was the second wettest winter on record for the Hertfordshire and North London area, receiving 72mm of rainfall during March, 138% of the long term average. The soil moisture deficits remained well below the long term average. Many rivers recorded their second largest March monthly mean river flows with many in the exceptionally high band. Groundwater levels recorded across the Mid Chilterns Chalk had only been higher in March 2001 with seven out of ten monthly indicator sites in the exceptionally high band.

2. Rainfall

With heavy downpours during March, the succession of winter months with higher than average rainfall continued. The Hertfordshire and North London area (“the Area”) received 138% of the long term average (LTA) rainfall (72mm) during March. Rainfall totals were slightly lower than February, but all 5 areal units across the Area ended the month in the above normal and notably high bands. The first day of March was the wettest of the month, with over 17mm widely recorded across the Chilterns and North London (rain gauge totals - Pinner (North London): 18mm, Heathrow (North London): 17.8mm, Northolt (Chilterns): 17.6mm, and Radlett (Chilterns): 17.3mm). The rest of March saw frequent showers, with notable totals recorded between 10 to 12 March, 14 to 17 March, and 26 to 28 March. Despite the downpours, there were warm and dry interludes with an average of 12 dry days (less than 0.2mm of rain). It was the second wettest winter (October to March) on record for the Area (540mm, 161% LTA winter rainfall) with only the winter of 2001 seeing more rainfall (642mm) (period of record 1872 to present).

3. Soil moisture deficit and recharge

The warm and dry interludes during March, along with an early start to spring for many plants, allowed a small increase in the Area’s soil moisture deficits (SMD) from 0mm at the start of the month to 2mm at the end of the month. However, the wet weather ensured the SMDs remained well below the month end average (LTA 10mm). The saturated soils meant that the heavy rainfall continued to contribute to runoff into the Area’s rivers and recharge the groundwater levels. The Area received 175% of the LTA effective rainfall for March. For the winter period (October to March), the Area recorded 241% of the LTA effective rainfall.

4. River flows

River flows remained high throughout March with all of our river flow indicator sites recording monthly mean flows in either the notably high and exceptionally high bands (with the exception of flows in above normal band on the River Roding at Redbridge). By the end of the month, the second highest March monthly mean flows had been recorded at many locations across the Area (only March 2001 were higher) including at Croxley Green (River Gade), Denham Colne (River Colne), Denham Lodge (River Misbourne), Cranford Park (River Crane), Howe Green (River Lee), and Colney Street (River Ver). (Period of record 1970, 1952, 1984, 1978, 1959, and 1956 respectively to present). Notable peak flows were seen across all rivers on 2 March, 12 March, and 28 March in response to the heavy rainfall. This resulted in a March total of 23 flood alerts and 2 flood warnings, with the flood warnings both being issued on the 2 March for the River Pinn at Yiewsley and the Colne Brook at Iver.

5. Groundwater levels   

Groundwater levels continued to rise across the Area in March at all of our groundwater level indicator sites. Three indicator sites had end of March groundwater levels in the notably high band, while the other 7 recorded levels in the exceptionally high band. With only March 2001 experiencing higher levels, Amersham Road and Ashley Green (both Mid Chliterns Chalk) recorded the locations second highest March groundwater levels on record (period of record 1991 and 1987 respectively to present). In the Upper Lee Chalk, Cave Gate, Hixham Hall, and Lilley Bottom recorded their third highest March groundwater levels on record after 2014 and 2001 (period of record 1966 and 1979 respectively to present).

6. Reservoir stocks

Lee Valley Reservoir stocks were below the LTA at the start of the month but increased from 87% to 93%, ending the month above the LTA. The Lower Thames reservoir levels remained the same throughout March starting and ending at 97%, above the LTA.

7. Environmental impact 

The sources of Chalk rivers in the Colne catchment remained in similar locations to February with the exception of the River Ver which migrated upstream to Markyate Cell. The River Gade continued to flow from Hudnall Corner, while the source of the River Bulbourne remained upstream of Dudswell village near Cowroast. With the River Chess flowing well above Chesham, the River Misbourne continued to flow from its source at Mobwell Pond. There was little change from February in the Chalk river sources in the Upper Lee catchment. The River Mimram continued to flow upstream of Whitwell, above the lakes by Lilley Bottom Road, while the source of the River Beane remained the same, flowing upstream of Cromer. The River Rib flowed upstream of Hay Green and the River Stort flowed from its source above the village of Langley Lower Green. The River Ash (Herts) started to flow at Meesden, although it temporarily lost flow near Furneaux Pelham, with flows returning by Little Hadham.

To protect the environment during March a number of abstraction licence flow constraints were in force. This ranged between 2 and 9 per week, out of a maximum of 57.

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.