Research and analysis

Hertfordshire and North London water situation: August 2024 summary

Updated 13 September 2024

Applies to England

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

1. Summary

Overall, August was considerably drier than most months within the last year in the Hertfordshire and North London area (“the Area”). August recorded 40% of the long term average (LTA) rainfall, sending soil moisture deficits (SMD) above the LTA. As a result, river flows in urban catchments were low for the time of year, with 3 indicator sites in the exceptionally low band. Despite this, flows in chalk catchments remained high for the time of year, still being supported by relatively high groundwater. August saw a decline in groundwater levels but 6 indicator sites still recorded end of month levels in the exceptionally high band.

2. Rainfall

Compared to most other months in the past year, August was dry for the Area, receiving just 40% of the monthly LTA rainfall for the month. In the North London areal unit, rainfall was in the below normal band, while the 4 other units were in the notably low band. The wettest day of the month was 24 August, with 14.5mm of rainfall recorded at Hatfield Heath (Roding) and 12.9mm recorded at Mill Green (Lee Chalk). In total, there were 20 dry days during August (under 0.2mm of rain recorded). Over the summer period (April to August), the Area has received 277mm of rainfall (107% of the LTA).

3. Soil moisture deficit and recharge

The lower than average rainfall during August meant that SMD in the Area ended the month above the LTA. Across the clay and urban-dominated rainfall units, there was no effective rainfall during the month, while in the 2 chalk-dominated units (Chilterns East Colne and Lee Chalk), a small amount of effective rainfall was received. The effective rainfall received in chalk catchments in the Area was below the LTA for August.

4. River flows

August saw a continued decline of chalk river baseflows in the Area. There were also no storms large enough to cause any significant flow peaks during the month. The most notable flow peak occurred on 24 August, in response to the rainfall on that day – this flow peak was mostly recorded in urban catchments. Despite this small peak, the low rainfall received during August meant 3 of the 4 indicator sites in Greater London recorded monthly flows in the exceptionally low band. Flows at Monks Park (River Brent) and Cranford Park (River Crane) were at their lowest August levels on record (records start in 1979 and 1978 respectively). Contrastingly, river flows in chalk catchments remained high for the time of year, still supported by a delayed decline in groundwater levels. In the Colne catchment, Denham Lodge (River Misbourne) and Colney Street (River Ver) recorded their second highest August flows on record (records start in 1984 and 1956 respectively). In the Upper Lee catchment, flows at Howe Green (River Lee) were at its third highest August level on record, while Panshanger (River Mimram) was at its second highest August level on record (records start in 1959 and 1952 respectively). Five flood alerts were issued during the month, all of which were in urban catchments on 1 August in response to heavy rainfall on 31 July. No flood warnings were issued in August.

5. Groundwater levels   

During August, groundwater levels declined across the Area but remained at a high level for the time of year. Six indicator sites recorded end of month groundwater levels in the exceptionally high band, while 3 other sites were in the notably high band. In the Mid-Chilterns Chalk, Amersham Road and Wapseys Wood were at their second highest monthly levels on record for August (records start in 1991 and 1988 respectively). In the Upper Lee Chalk, Hixham Hall recorded its second highest groundwater level on record for August (records start in 1964).

6. Reservoir stocks

During August, reservoir stocks in the Lee Valley reservoirs decreased slightly from 98% to 96% of live capacity. In the Lower Thames reservoirs, the water level decreased from 91% to 80% of live capacity. Reservoir stocks in both groups ended the month above the LTA.

7. Environmental impact 

Most of the sources of chalk rivers in the Colne catchment moved downstream of their locations in July, including the:

• River Ver started flowing just below Markyate
• source of the River Gade moved downstream of Hudnall
• source of the River Bulbourne moved slightly further down but was still just above Dudswell village
• source of the River Chess moved to just upstream of Chesham
• River Misbourne was flowing continuosly from Mobwell pond

Most of the chalk river sources in the Upper Lee catchment also moved downstream compared to July, including the:

• River Mimram started flowing upstream of Whitwell
• River Beane started flowing downstream of Cromer
• source of the River Rib moved to just above Buntingford
• source of the River Ash (Herts) was upstream of Hadham Ford
• River Stort started flowing at Stansted Springs, a considerable distance downstream from where it was last month

To protect the environment, during August a number of abstraction licence flow constraints were in force. This ranged between 2 and 8 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.