Research and analysis

Hertfordshire and North London water situation: September 2025 summary

Updated 14 October 2025

Applies to England

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

1. Summary

September was generally near average across the Hertfordshire and North London area in terms of rainfall, receiving 96% of the long term average (LTA). All areal units recorded rainfall in the normal band. Despite this, soil moisture deficits (SMDs) remained elevated with very little effective rainfall. River baseflows were mixed – flows in chalk catchments were mostly in the normal band, while clay and urban rivers were generally below normal. Groundwater levels across the area continued to be stable overall, with the Mid-Chilterns Chalk in the normal band and most Upper Lee Chalk sites remaining above normal, including some notably high levels.

2. Rainfall

September was a generally near-average month across the Hertfordshire and North London area (the Area), receiving 51mm of rainfall, 96% of the LTA. All 5 areal rainfall units finished the month in the normal band. Additionally, over the past 6 months, rainfall across the Area was in the notably low band, with the Roding unit in the exceptionally low band. Over the past 3 months, rainfall across the Area was below normal. Over the last 12 months, rainfall was mostly notably low, with Roding remaining in the exceptionally low band. Despite the generally near-average month, a fair amount of rain fell early in the month, most notably on 2 September. On this day, the highest rainfall total was 20.2mm recorded at Weston STW (Lee Chalk unit). The highest monthly total was 95.2mm recorded at Prestwood Reservoir (Chilterns East Colne unit). In total, there were 16 dry days during September (under 0.2mm of rain recorded).

3. Soil moisture deficit and recharge

SMDs remained above their LTAs during September. As was the case with August, very little effective rainfall was received in the Area during September. A small amount of effective rainfall was recorded in the chalk catchments, while none was received in clay and urban catchments.

4. River flows

During September, river baseflows showed a mixed picture. In chalk catchments, indicator sites were mostly in the normal band. The Mimram at Panshanger recorded a notably high flow for the time of year, the sixth highest September on record and the second highest since 2001. In contrast, the clay and urban rivers generally recorded monthly flows in the below normal band. Only minor rainfall-related flow responses were observed and no flood alerts or warnings were issued during the month.

5. Groundwater levels   

Groundwater levels across the Area were stable during September, with indicator sites ranging from the normal to notably high bands. In the Mid-Chilterns Chalk, all indicator sites recorded end of month levels in the normal band. Meanwhile, the Upper Lee Chalk still had most sites in the above normal band, with some sites in the notably high band.

6. Reservoir stocks

The Lee Valley reservoir group began September at 91% of live capacity and finished the month at 90%. The lower Thames group started September at 66% of live capacity and finished at 64%.

7. Environmental impact 

In the Colne catchment, the chalk river sources remained similar to their locations in August, the:

• River Gade started flowing downstream of Hoo Lodge
• River Bulbourne was flowing at Mandalyns before drying and gaining a steadier flow downstream of Stag’s Lane, Berkhamstead
• source of the River Chess was still just upstream of Chesham
• River Misbourne started flowing at Deep Mill Lane

The chalk river sources in the Upper Lee also remained similar to their location in August, the:

• River Mimram started flowing above Whitwell Gas Compound
• source of the River Beane was just above Walkern
• River Rib started flowing in Buntingford
• source of the River Ash (Herts) was just upstream of Much Hadham
• River Stort flowed briefly between Clavering and Manuden but ran dry before gaining a steadier flow at Stansted Springs

To protect the environment, during September a number of abstraction licence flow constraints were in force. This ranged between 5 and 11 per week, out of a maximum of 32.

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.