Research and analysis

Hertfordshire and North London water situation: January 2026 summary

Updated 17 February 2026

Applies to England

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

1. Summary

January was a very wet month in the Hertfordshire and North London area, recording 103mm of rain (162% of the long term average). Soil moisture deficits were reduced to zero during January, meaning that they finished the month below the long term average. Despite some large variation between catchments, overall, effective rainfall was above average for January. River baseflows continued to increase in chalk catchments, with most indicator sites recording monthly flows in the normal band for January. In the clay and urban catchments, meanwhile, there were several prominent flow peaks recorded on days with heavy rainfall, meaning most sites also recorded monthly flows in the normal band. Groundwater levels increased at nearly all indicator sites in January, with all sites ending the month in the normal band.

2. Rainfall

In contrast to December, January was a very wet month across the Hertfordshire and North London area (the Area), with 103mm of rainfall recorded, equal to 162% of the long term average (LTA). Most areal units ended January in the notably high band, apart from the North London unit, which finished the month in the exceptionally high band. Over the last 3 months rainfall totals in the chalk units fell into the notably high band, while in the clay and urban units, rainfall totals were in the above normal band. The distribution of rain across January was quite consistent overall, with the two named storms (Storm Goretti and Storm Chandra) resulting in heavy rainfall at both ends of the month. Indeed, the wettest day of the month was during Storm Goretti, on 8 January, with 23.2mm of rain recorded at North Mymms (North London unit). Other high rainfall totals on this day were mostly recorded in the north of the Area, including:

• 22.2mm at Chenies (Chilterns East Colne unit)
• 21.9mm at Mill Green STW (Lee Chalk unit)
• 21.8mm at Weston STW (Lee Chalk unit)
• 21.2mm at Stevenage (Lee Chalk unit)
• 20.7mm at Whitwell STW (Lee Chalk unit)

During January, there were only 4 dry days in the Area (days with under 0.2mm of rain recorded).

3. Soil moisture deficit and recharge

Due to January’s heavy rainfall, soil moisture deficits (SMDs) were reduced to zero at the end of the month, across the Area (which was below the LTA for the end of January). The elimination of SMDs across the Area was significant for this month as it meant soils became saturated enough to allow for a proper recharge of groundwater with any effective rainfall received. On average, the Area received effective rainfall above the LTA for January, however, the Roding and Lee Chalk units (the north-east of the Area) ended the month below the LTA.

4. River flows

During January, river baseflows continued to increase across the Area’s chalk catchments, with all indicator sites recording monthly mean flows in the normal band, other than Colney Street (River Ver) and Howe Green (River Lee), which both fell in the above normal band. There were several pronounced storm peaks recorded throughout January, which also allowed for most clay and urban indicator sites to record monthly mean flows in the normal band. Cranford Park (River Crane) and Monks Park (River Brent) recorded monthly mean flows in the above normal band.

Due to the high rainfall, there were 21 flood alerts across the Area during the month. The highest number of alerts was issued around 8 and 9 January during Storm Goretti, in catchments including:

• Tidal River Crane
• Upper River Colne and Radlett Brook
• Lower Lee tributaries
• River Stort and Stansted Brook
• River Lee at Hertford
• River Ingrebourne at Harold Park and Hornchurch
• River Beane
• Middle River Roding

No flood warnings were issued during January.

5. Groundwater levels   

January saw groundwater levels increase from December’s end of month levels almost throughout the Area, helped by the effective rainfall and lack of SMDs. The only indicator site which did not show an increase in groundwater level during the month was Therfield Rectory (Upper Lee Chalk aquifer), which is in an area where the groundwater is known to have a delayed response. Across the Mid-Chilterns Chalk and Upper Lee Chalk aquifers, all indicator sites recorded end of month groundwater levels in the normal band.

6. Reservoir stocks

At the start of January, the Lee Valley reservoir group was at 87% of live capacity and ended the month at 88%. The Lower Thames reservoir group, meanwhile, started January at 86% of live capacity, ending the month at 89%. This meant that both reservoir groups ended January slightly below their average levels for the time of year.

7. Environmental impact 

In the Colne catchment, January saw chalk river sources staying in similar locations to where they were in December. The:

• River Ver was still flowing downstream of Markyate, although, it temporarily lost flow downstream of Flamstead
• River Gade started flowing downstream of Hoo Lodge
• source of the River Bulbourne stayed at the same location in Northchurch
• source of the River Chess was still just upstream of Chesham, with slightly stronger flows in its headwaters
• River Misbourne was flowing from the same location downstream of Great Missenden

The chalk river sources in the Upper Lee all flowed upstream of their December locations. The:

• River Mimram was flowing slightly upstream, above Whitwell
• source of the River Beane moved upstream of Cromer
• River Rib flowed for its entire length, from upstream of Reed End
• River Ash (Herts) flowed for its entire length, from Meesden
• River Stort flowed from its source above Langley Lower Green and did not run dry at any point

To protect the environment, during January a number of abstraction licence flow constraints were in force. This ranged between 4 and 19 per week, out of a month 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.