Research and analysis

Hertfordshire and North London water situation: November 2025 summary

Updated 11 December 2025

Applies to England

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

1. Summary

The Hertfordshire and North London area received slightly above average rainfall in November, with 104% of the long term average (LTA) and areal units in the normal or above normal band. Soil moisture deficits (SMDs) decreased during November but remained well above the LTA, with a small amount of effective rainfall recorded. River baseflows increased and remained stable overall, with most chalk river indicator sites recording monthly flows in the normal or above normal band. Spikes in river flows were experienced on days with heavy rainfall, resulting in most clay and urban indicator sites recorded monthly flows in the normal band. Groundwater levels also remained stable across the area, with most indicator sites ending November in the normal band.

2. Rainfall

In the Hertfordshire and North London area (the Area), November’s rainfall was higher than average, with 74mm of rainfall recorded (104% of the LTA). The Chilterns East Colne and Lee Chalk areal units finished the month in the above normal band, while the Lower Lee, North London and Roding units were all in the normal band. Despite the above average monthly rainfall, the cumulative rainfall for the last 12 months remained below normal or notably low across all units. The wettest period of the month was associated with Storm Claudia, which brought a large band of heavy, persistent rain across the region on 14 November, with 29.2mm of rainfall was recorded at Lilley Manor (Lee Chalk unit). Despite the heavy rainfall events, there were 14 dry days during October, when less than 0.2mm if rain was recorded.

3. Soil moisture deficit and recharge

SMD across the Area remained well above the end-of-month LTA for November, although they fell markedly compared with October. The reduction in SMDs allowed some recharge to occur in the chalk catchments, however, effective rainfall was still some way below average. Soils in the clay and urban catchments of the Lower Lee, North London and Roding units remained too dry to receive any effective rainfall. Effective rainfall for the winter period so far (starting in October) has also been well below average.

4. River flows

In the Area’s chalk catchments, most indicator sites recorded monthly flows in the normal band for November, with a slight increase in baseflows. A few chalk river indicator sites recorded monthly flows in the above normal band, including Colney Street (River Ver), Croxley Green (River Gade) and Panshanger (River Mimram). Meanwhile, clay and urban indicator sites recorded several flow peaks during the month, most notably around 14 November due to Storm Claudia. The occurrence of such storm events meant most clay and urban sites also recorded monthly flows in the normal band, with the exception of Redbridge (River Roding), which recorded in the below normal band.

There was one flood alert during the month, which was issued on the 13 November for the Upper River Colne and Radlett Brook.

5. Groundwater levels   

During November, groundwater levels decreased at most indicator sites, ending the month 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. In the Upper Lee Chalk aquifer, the majority of indicator sites recorded groundwater levels in the normal band, apart from Hixham Hall and Therfield Rectory, which remained in the above normal band.

6. Reservoir stocks

Lee Valley Reservoir group started November with 83% of live capacity and ended the month with 86% of live capacity, remaining above its average level for the time of year. Lower Thames Reservoir group started November with 62% of live capacity and ended the month with 71% of live capacity, showing an increase during the month though levels remained below average.

7. Environmental impact 

In the Colne catchment, the chalk river sources in late November were broadly similar to their locations in October, the:

• River Ver was flowing downstream of Markyate, but ran dry briefly before gaining a steadier flow at Redbourn Golf Club
• River Gade was still flowing downstream of Hoo Lodge
• River Bulbourne was flowing downstream of Northchurch
• source of the River Chess was still just upstream of Chesham, with slightly weaker flows in its headwaters
• River Misbourne was still flowing from Deep Mill Lane

The locations of chalk river sources in the Upper Lee showed slight changes compared to October, the:

• River Mimram still started flowing upstream of Whitwell village
• source of the River Beane remained just above Walkern
• River Rib flowed intermittently from Reed End before gaining a steadier flow in Buntingford
• River Ash (Herts) continued to flow intermittently from Blackhall and gained a steadier flow at Much Hadham
• River Stort flowed from its source above Langley Lower Green, but ran dry between Clavering and Manuden

To protect the environment, during November a number of abstraction licence flow constraints were in force. This ranged between 12 and 32 per week, with the highest number recorded in the week of 4 to 10 November.

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.