Research and analysis

Hertfordshire and North London water situation: December 2024 summary

Updated 14 January 2025

Applies to England

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

1. Summary

During December, the Hertfordshire and North London area received 49mm of rain, which was 78% of the long term average (LTA). Monthly river flows varied according to catchment type – several chalk river indicator sites recorded flows in the exceptionally high band, while in clay and urban catchments all indicator sites recorded in the normal band. Groundwater levels remained high, with most sites in the exceptionally high band and some sites recording their highest end of month levels on record.

2. Rainfall

December was a fairly average month for rainfall across the Hertfordshire and North London area (“the Area”). The area received 49mm of rain, which was 78% of the LTA. All 5 areal rainfall units ended December below the LTA but remained in the normal band. Due to Storm Darragh, all 5 of December’s rainiest days occurred in the first half of the month, with significantly less rainfall in the second half. The wettest day of the month was 7 December, with 19.6mm recorded at Braughing (Lee Chalk). On this day, the highest rainfall totals were recorded in the Lee Chalk and Lower Lee units with rain gauges like Whitwell, Widford, Darnicle Hill and Nazeing all recording over 14mm of rain. During December, the Area had a total of 14 dry days (less than 0.2mm of rain). In both the Lee Chalk and Chilterns East Colne units, rainfall totals for the calendar year were the third highest on record, only exceeded in 2000 and 1903 (records for both begin in 1871).

3. Soil moisture deficit and recharge

Due to the slightly below average rainfall received during December, soil moisture deficits (SMDs) saw a small increase in all areal units other than the Roding, however, all SMDs remained well below the LTA. All rainfall units in the Area received effective rainfall above the LTA for December. The Chilterns East Colne unit received the most effective rainfall for the month but the Lee Chalk recorded by far the highest percentage in relation to its December LTA.

4. River flows

River base flows saw a slight increase during December, particularly in the chalk catchments, where five of eight indicator sites recorded monthly flows in the exceptionally high band. Panshanger on the River Mimram and Colney Street on the River Ver recorded their highest mean December flows on record (records begin in 1952 and 1956 respectively). All indicator sites in the more runoff-dominated clay and urban catchments recorded monthly flows in the normal band.

A total of 7 fluvial flood alerts were issued during December, however, no flood warnings were issued. The flood alerts were mostly issued on 6 December in response to the rainfall received across the Area in the preceding days, which included:

• Tidal River Crane
• Upper River Colne and Radlett Brook
• Lower River Colne and Fray’s River
• Middle River Roding

5. Groundwater levels   

December saw a continuation of high groundwater levels across the Area, with all indicator sites increasing during the month. Four sites recorded end of month levels in the notably high band, while five sites recorded in the exceptionally high band. In the Mid-Chilterns Chalk, Ballingdon Farm recorded its highest ever level for the end of December (records begin in 1975). In the Upper Lee Chalk, Cave Gate, Crescent Cottages and Lilley Bottom also recorded their highest ever groundwater levels for the end of December (records begin in 1966, 1968 and 1979 respectively). Therfield Rectory (Upper Lee Chalk), which has been recording groundwater levels since 1883 was at its highest level for the end of December since 1917.

Two groundwater flood alerts were issued during the month, both on the 19 December at:

• Kimpton and Lilley Bottom (Upper Lee Chalk)
• Flamstead (Mid-Chilterns Chalk)

6. Reservoir stocks

Both reservoir groups in the Area ended December with their levels well above the LTA. In the Lee Valley Reservoirs, stocks went from 94% to 93% of live capacity, while the Lower Thames Reservoirs increased from 82% to 95% of live capacity.

7. Environmental impact 

In the Colne catchment, all of the chalk river sources were in virtually the same location as November, the:

• River Ver started flowing above Markyate
• River Gade started flowing at Hudnall Corner
• River Bulbourne was flowing upstream of Dudswell village
• source of the River Chess remained upstream of Chesham
• River Misbourne flowed continuously from Mobwell pond

The chalk river sources in the Upper Lee remained mostly the same as in November, the:

• River Mimram started flowing at Lilley Bottom Road
• River Beane started flowing above Cromer
• source of the River Rib remained upstream of Hay Green, where it flowed intermittently before gaining a steadier flow at Buntingford
• River Ash (Herts) was flowing upstream of Brent Pelham, although it temporarily lost flow at Little Hadham
• source of the River Stort was still above Langley Lower Green

To protect the environment, during December a number of abstraction licence flow constraints were in force. This ranged between 1 and 9 per week, out of a winter maximum of 48.

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.