Research and analysis

Hertfordshire and North London water situation: December 2025 summary

Updated 14 January 2026

Applies to England

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

1. Summary

The Hertfordshire and North London area (the Area) received 99% of the long term average (LTA) rainfall during December, recording 64mm of rain. Soil moisture deficits (SMD) fell considerably during the month but still remained above the LTA, with only a small amount of effective rainfall recorded. River baseflows increased during December in chalk catchments, with almost all indicator sites recording monthly flows in the normal band. Clay and urban indicator sites also recorded monthly flows in the normal band due to spikes in river flows caused by days of heavy rainfall. During December, groundwater levels mostly ended the month in the normal band, with some indicator sites showing signs of winter recharge.

2. Rainfall

Much like November, December was a fairly average month in the Hertfordshire and the Area for rainfall. A monthly total of 64mm was recorded in the area, equivalent to 99% of the LTA for December. All 5 areal units finished December in the normal band; rainfall totals also fell into the normal band in the Area for the last 3 months, and the last 6 months (excluding the Roding unit, which fell into the below normal band). Despite the near average rainfall total, the overall distribution of rain across the month was very inconsistent, as over half of the rain fell between 1 and 8 December, around the time of Storm Bram. However, the wettest day of the month was 18 December, with the highest daily rainfall totals recorded in the Chilterns East Colne unit, including:

• 24mm at Chenies rain gauge
• 22.8mm at Prestwood Reservoir rain gauge
• 21.4mm at Markyate STW rain gauge

Elsewhere in the Area, high daily rainfall totals included:

• 18mm at Birch Green rain gauge (North London unit)
• 17.4mm at Lilley Manor rain gauge (Lee Chalk unit)
• 17.2mm at North Mymms Pumping Station rain gauge (North London unit)

Despite the abovementioned rainfall events, there were 13 dry days during December (days with under 0.2mm of rain recorded).

3. Soil moisture deficit and recharge

In the Area, SMDs fell considerably due to December’s rainfall, but still remained above the end of month LTA. This allowed for continued recharge in chalk catchments, however, due to the scattered nature of the month’s rainfall, very little effective rainfall was received. Effective rainfall for the winter period so far (since October) has still been well below average.

4. River flows

December saw a general increase in river baseflows in the Area’s chalk catchments, where most indicator sites recorded month mean flows in the normal band. Only 2 sites, Colney Street (River Ver) and Panshanger (River Mimram) recorded monthly flows in the above normal band. The main flow peak recorded during the month was on 18 December due to the heavy rainfall, seen most notably in the Area’s clay and urban catchments. This was enough to ensure that all clay and urban indicator sites recorded month mean flows in the normal band. Three flood alerts were issued during December, including:

• 5 December – Upper River Colne and Radlett Brook
• 18 December – Upper River Colne and Radlett Brook
• 19 December – Lower River Colne and Frays River

No flood warnings were issued during December.

5. Groundwater levels   

During December, groundwater levels in the area showed some signs of winter recharge, with several indicator sites showing an increase compared to November’s end of month levels. In the Mid-Chilterns Chalk aquifer, all indicator sites recorded groundwater levels in the normal band, with Amersham Road and Ashley Green beginning to record an increase in groundwater levels. In the Upper Lee Chalk aquifer, all indicator sites recorded groundwater levels in the normal band apart from Therfield Rectory, which recorded its end of month level at the bottom of the above normal band. Groundwater recharge was also recorded at Crescent Cottages, Hixham Hall and Cave Gate.

6. Reservoir stocks

The Lee Valley reservoir group started December at 86% of live capacity and ended the month at 87%, remaining around its average level for the time of year. At the Lower Thames reservoirs, the water level increased during December from 71% to 85% of live capacity, ending the month just below its average level.

7. Environmental impact 

In the Colne catchment, December saw chalk river sources moving slightly upstream of their locations in November overall. The:

• River Ver was still flowing downstream of Markyate
• River Gade started flowing downstream of Hoo Lodge
• source of the River Bulbourne moved upstream to Northchurch
• source of the River Chess was still just upstream of Chesham, with slightly stronger flows in its headwaters
• River Misbourne was flowing downstream of Great Missenden, slightly upstream from the previous month

The chalk river sources in the Upper Lee showed slight changes from their November locations overall. The:

• River Mimram was flowing slightly downstream, at the top of Whitwell
• source of the River Beane remained just above Walkern
• River Rib started flowing above Reed End, although, flows were not consistent until Buntingford
• River Ash (Herts) flowed intermittently upstream of Brent Pelham, before gaining a steadier flow at Much Hadham
• River Stort flowed from its source above Langley Lower Green, despite running dry briefly around Manuden

To protect the environment, during December a number of abstraction licence flow constraints were in force. This ranged between 8 and 21 per week, out of a monthly maximum of 49.

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.