Research and analysis

Hertfordshire and North London water situation: October 2025 summary

Updated 12 November 2025

Applies to England

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

1. Summary

October was a slightly below average month for rainfall in the Hertfordshire and North London area, with 89% of the long term average (LTA) and all areal units in the normal band. Soil moisture deficits (SMDs) decreased during October but remained well above the long term average, with a small amount of effective rainfall recorded. River baseflows decreased but remained stable overall, with most chalk river indicator sites recording monthly flows in the normal band. Due to several rainstorms causing spikes in river flows, the clay and urban indicator sites also recorded monthly flows in the normal band. Groundwater levels fell across the area, with most indicator sites ending October in the normal band.

2. Rainfall

In the Hertfordshire and North London area (the Area), October’s rainfall was lower than average, with 64mm of rainfall recorded (89% of the LTA. Like in September, all 5 areal units finished the month in the normal band. Despite this, the total rainfall for the last 6 months (apart from the North London unit) and the last 12 months was in the notably low band for all units in the Area. The wettest day of the month was 22 October, with high rainfall recorded in all areal units. The largest daily rainfall totals were seen in the Roding unit, with 19.7mm at Nags Head Lane, 17.3mm at Havering Bower and 16.7mm at Gascoigne Road. Notable rainfall totals were also recorded on 31 October, with 17.6mm recorded at Luxborough Lane (Roding unit) and 16.8mm at Holland Park (North London unit). Despite there being days with heavy rainfall during the month, there were 14 dry days during October, when less than 0.2mm if rain was recorded.

3. Soil moisture deficit and recharge

SMDs across the Area remained above the end of month LTA for October, but significantly reduced thanks to the rainfall the Area received. The drop in SMDs meant that the chalk catchments of the Area received around half the monthly LTA effective rainfall. However, soils in the clay and urban catchments remained too dry to generate effective rainfall.

4. River flows

Most indicator sites in the Area recorded month mean flows in the normal band for October, with the Area’s chalk rivers remaining at a consistent level, despite a drop in baseflows overall. In the clay and urban rivers, month mean flows were boosted by days with high rainfall totals, where the most notable flow peaks were recorded on 3, 20, 22 and 31 October. Only a handful of indicator sites recorded monthly flows outside of the normal band, including:

• River Ver at Colney Street (notably high band)
• River Mimram at Panshanger (notably high band)
• River Misbourne at Denham Lodge (above normal band)
• River Roding at Redbridge (below normal band)

No flood alerts or warnings were issued during October.

5. Groundwater levels   

During October, groundwater levels decreased at all indicator sites, ending the month with them ranging from the normal to above normal bands. In the mid-Chilterns Chalk aquifer, all indicator sites recorded end of month groundwater levels in the normal band. Meanwhile, the majority of indicator sites in the Upper Lee Chalk aquifer were in the above normal band, with the exceptions of Cave Gate and Crescent Cottages, recording end of month levels in the normal band.

6. Reservoir stocks

Reservoir stocks in the Lee Valley group started October at 89% of live capacity and finished the month at 83%, remaining above its average level for the time of year. In the Lower Thames group, reservoir levels fell slightly from 63% to 62% during October, with levels lower than average, but similar to September.

7. Environmental impact 

In the Colne catchment, the chalk river sources remained at similar locations to where they were in September.

• the River Ver started flowing just below Markyate
• the source of the River Gade was still upstream of Great Gaddesden
• the River Bulbourne started flowing downstream of Northchurch
• the source of the River Chess was still just upstream of Chesham
• the River Misbourne started flowing at Deep Mill Lane, above Little Missenden

The locations of chalk river sources in the Upper Lee also stayed fairly similar to September.

• the River Mimram continued to flow upstream of Whitwell
• the source of the River Beane remained just above Walkern
• the River Rib flowed briefly downstream of Dane End before gaining a steadier flow in Buntingford
• the River Ash (Herts) flowed intermittently from Brent Pelham, and gained a steadier flow at Much Hadham
• the River Stort also flowed intermittently from Lower Green but did not gain meaningful flow until Stansted Springs

To protect the environment, during October a number of abstraction licence flow constraints were in force. This ranged between 5 and 10 per week, out of a monthly maximum of 28.

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.