Research and analysis

Hertfordshire and North London water situation: September 2026 summary

Updated 14 October 2026

Applies to England

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

1. Summary

September was another month of below average rainfall in the Hertfordshire and North London area (the area), receiving about 52% of the long term average (LTA). Soil moisture deficits (SMDs) increased further during September, ending the month at their highest levels on record for the time of year. On the whole, river baseflows declined in the area’s chalk rivers, however, most indicator sites recorded monthly mean flows in the normal band. In the area’s clay and urban catchments, most indicator sites recorded monthly mean flows in the notably low or exceptionally low bands, with the River Roding at Redbridge recording its lowest September flows on record. Groundwater levels declined at all indicator sites, with most remaining in the normal band at the end of the month.

2. Rainfall

Overall, September was another dry month in the Hertfordshire and North London area. All hydrological units ended the month in the below normal band, receiving an average of 27mm of rainfall (52% of the LTA). The highest rainfall totals of the month were recorded on 8 September in the Roding unit, at Nag’s Head Lane (13mm) and Havering Bower (11.9mm). However, the day with the most widespread rainfall was 7 September, with 5.3mm recorded on average. Together, the 7 and 8 September accounted for about 40% of the area’s monthly rainfall total. On the whole there were only seven “wet days” during the month (days with 1mm or more of rain).

The end of September is significant as it marks the end of the hydrological summer period (which started in April) and of the whole water year. For this summer period, all hydrological units received their lowest ever rainfall totals (records starting in 1871), other than the Lee Chalk, which received its second lowest rainfall total (after 1921). Rainfall for the water year as a whole was helped by the fairly average winter, with the area receiving about 570mm of rainfall (85% of the LTA). For the water year, therefore, all hydrological units recorded rainfall in the below normal band.

3. Soil moisture deficit and recharge

Throughout the area, SMDs increased during September, ending the month at almost double their LTA levels – the highest on record. This meant that almost zero effective rainfall was received during September; consequently, there will be no aquifer recharge as a result of the month’s rain.

4. River flows

During September, river baseflows generally declined in the area’s chalk catchments, however, most indicator sites still recorded month average flows in the normal band. Exceptions to this were Croxley Green (River Gade) and Howe Green (River Lee), which both recorded monthly mean flows for September in the below normal band. In the area’s clay and urban catchments, most indicator sites recorded monthly average flows in the notably low or exceptionally low bands. Redbridge gauging station (River Roding) recorded its lowest ever mean flow for September (records start in 1950). Despite this, Upminster gauging station (River Ingrebourne) nearby recorded month average flows in the normal band – likely helped by the localised rainfall around 8 September.

5. Groundwater levels   

Groundwater levels continued their seasonal decline throughout the area during September. In the Mid-Chilterns Chalk aquifer, all indicator sites recorded end of month levels in the normal band, apart from Ashley Green, which had just entered the below normal band. In the Lee Chalk aquifer, all indicator sites recorded end of month levels in the normal band.

6. Reservoir stocks

Reservoir levels in the Lee Valley group both started and ended September at 69% of live capacity, remaining below its average level throughout the month. Stocks in the Lower Thames group fell, however, from 59% to 53% of live capacity, well below its average level for the time of year.

7. Environmental impact 

In the Colne catchment, during September, chalk river sources generally moved slightly downstream or experienced declining flows in their headwaters, the:

• River Ver started flowing downstream of Markyate but ran dry around Redbourn Golf Club, before flowing again just downstream of Redbourn village
• source of the River Gade was still just above Great Gaddesden
• River Bulbourne’s source remained in the same location, just downstream of Northchurch
• River Chess was still flowing at the tennis courts upstream of Chesham, albeit with lower flows at its headwaters
• source of the River Misbourne remained just downstream of The Chilterns Hospital, however, flows were weaker around the Chalfonts and the river dried up briefly around Gerrards Cross Golf Club

In the Upper Lee catchment, most chalk river sources moved downstream compared to their locations the last time they were surveyed, the:

• River Mimram started flowing upstream of Whitwell, just above Nine Wells Spring
• River Beane flowed just above Walkern before running dry around Aston. The river then gained a steadier flow at Frogmore Park
• source of the River Rib was located at Buntingford
• River Ash started flowing at Much Hadham
• River Stort flowed briefly downstream of Clavering, before gaining its permanent flow below Manuden

To protect the environment, during September a number of abstraction licence flow constraints were in force. This ranged between 8 and 13 per week, out of a month maximum of 31.

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.