Research and analysis

Hertfordshire and North London water situation: July 2024 summary

Updated 14 August 2024

Applies to England

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

1. Summary

After a relatively dry June, July resumed the trend of high rainfall in the Hertfordshire and North London area over the last year. July recorded 180% of the long term average rainfall over the month and in response, soil moisture deficits were again below the long term average. As a result, river flows were high for the time of year, with a few sites experiencing month mean flows at their highest levels on record. Groundwater levels mostly decreased but also remained high for the time of year, with most sites recording end of month values in the exceptionally high band.

2. Rainfall

Overall, July was a much wetter month in the Hertfordshire and North London area (“the Area”) than June, having received 180% of the long term average (LTA) rainfall. Rainfall units further to the west of the Area received the most rain on the whole, with Chilterns East Colne, North London and the Lower Lee ending July in the notably high band. Meanwhile, the Upper Lee and Roding areal rainfall units ended the month in the above normal band. Despite high rainfall totals, there were 16 dry days during July (less than 0.2mm of rain recorded). The wettest day of the month was 5 July, with 46.2mm of rain recorded at Epping Forest (Roding) and 44mm of rain recorded at Prestwood Reservoir (Chilterns East Colne). Over the summer period (April to July), the Area has received 255mm of rainfall (124% of the LTA).

3. Soil moisture deficit and recharge

Due to higher than average rainfall this month, soil moisture deficits ended July below the LTA. Across the clay and urban catchments in the Area, there was no effective rainfall during July. In contrast, the effective rainfall received in the 2 chalk units (Lee Chalk and Chilterns East Colne) were far above the LTA.

4. River flows

July saw a general decrease in chalk river baseflows in the Area, despite the high rainfall total. In response to the biggest storms during the month, notable flow peaks occurred around 5 and 15 July, particularly in urban catchments. Nine indicator sites experienced monthly mean flows in the exceptionally high band. These included the majority of the sites in the chalk catchments, in addition to both sites in the North London unit. Two other sites recorded monthly flows in the notably high band, while Upminster (River Ingrebourne) was the only site recording in the above normal band. This month, Colney Street (River Ver), Monks Park (River Brent) and Howe Green (River Lee) experienced their highest July flows on record (records start in 1956, 1979 and 1959 respectively). During July, 9 flood alerts and 2 flood warnings were issued – 8 of these were on the 15 July in response to the heavy rainfall, mostly in urbanised catchments within Greater London.

5. Groundwater levels   

Groundwater levels generally declined over July, which is expected for this time of year. Seven indicator sites in the Area recorded an end of month groundwater level in the exceptionally high band, while 2 other sites recorded levels in the notably high band. In the Mid-Chilterns Chalk aquifer, Amersham Road and Wapseys Wood recorded their second highest end of month groundwater levels on record, both only exceeded in 2001 (records start in 1991 and 1988 respectively). In the Upper Lee Chalk, Cave Gate and Hixham Hall recorded their second highest groundwater levels on record, exceeded only in 1979 and 2001 respectively (records began in 1966 for Cave Gate and in 1964 for Hixham Hall). Cave Gate was the only indicator site to record a higher end of month groundwater level than June.

6. Reservoir stocks

The Lee Valley reservoir stocks increased slightly during July, from 95% to 96% of live capacity, while in the Lower Thames reservoirs, the water level decreased from 95% to 90% of live capacity. Water levels in both reservoir groups finished the month above the LTA.

7. Environmental impact 

The sources of chalk rivers in the Colne catchment remained in similar locations to June, the:

• River Ver started flowing at Markyate Cell before drying and flowing again at Markyate STW

• River Gade was still flowing upstream of Hudnall Corner

• source of the River Bulbourne moved slightly further down but was still upstream of Dudswell village

• source of the River Chess also moved slightly further down but was still a good distance above Chesham

• River Misbourne was still flowing continuously from Mobwell pond

All of the sources of the chalk rivers in the Upper Lee catchment were at slightly different locations to June, the:

• River Mimram started flowing just above Whitwell Gas Compound

• source of the River Beane moved slightly downstream to just above Walkern village

• River Rib flowed intermittently from Reed End, before gaining a steadier flow at Buntingford

• River Ash (Herts) started flowing at Blackhall before drying and regaining flow upstream of Hadham Ford

• River Stort started flowing at its source above Langley Lower Green, although, flows were very low at Manuden

To protect the environment during July a number of abstraction license flow constraints were in force. This ranged between 1 and 2 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.