Research and analysis

Hertfordshire and North London water situation: May 2026 summary

Updated 11 June 2026

Applies to England

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May was a very dry month in the Hertfordshire and North London area, receiving just 15mm of rain in total, or 31% of the long term average (LTA). This meant most rainfall units finished May in the notably low band. As a result, soil moisture deficits (SMDs) increased, meaning there was virtually zero effective rainfall during May. River flows varied according to catchment type, with chalk catchments recording month mean flows in the normal band or higher despite a decline in baseflows. In the clay and urban catchments, all indicator sites recorded month mean flows in the exceptionally low band with an absence of significant flow peaks – some sites recorded their lowest month mean flows on record. Groundwater levels remained at a good level for the time of year despite mostly declining, with all indicator sites ending May in the normal band.

1. Rainfall

Although slightly wetter than April, May was another dry month in the Hertfordshire and North London area (the Area). A total of 15mm of rain was recorded during the month, equivalent to just 31% of the LTA. All areal rainfall units ended May in the notably low band, apart from the Roding unit, which was in the exceptionally low band. The rainfall total over the last 3 months was also low and ended the period in the exceptionally low band across all 5 areal units. The last 6 months, however, had near average rainfall (benefiting from robust winter rainfall in January and February) – over this period, all units were in the normal band. The wettest day of the month was 13 May, which accounted for over 30% of the month’s recorded rainfall. On this day, the highest rainfall totals were recorded in the North London unit, including 11.5mm at Allenby and 10.4mm at RAF Northolt. During May, there was a total of 20 dry days across the Area (days with under 0.2mm of rain recorded).

2. Soil moisture deficit and recharge

The low rainfall received during May meant that SMDs increased throughout the Area, resulting in there being virtually no effective rainfall during the month. Although the increase during May was not massive, SMDs still ended the month well above the LTA. Consequently, going into the summer, soils will already be unsaturated and groundwater levels across the Area will continue their seasonal decline.

3. River flows

During May, there was a diverse range of river flows recorded across the Area, which varied according to catchment type. In chalk catchments, despite a decrease in baseflows, most indicator sites recorded month mean flows in the normal band – the only exception being the River Mimram at Panshanger which recorded flows in the above normal band. However, owing to the low rainfall in May, all clay and urban indicator sites recorded month mean flows in the exceptionally low band. In North London, Cranford Park (River Crane) and Monks Park (River Brent) recorded small flow peaks on 13 May, due to that day’s localised rainfall. Despite this, both Cranford Park and Monks Park recorded their lowest ever month mean flows for May (records start in 1978 and 1979 respectively).

4. Groundwater levels   

Despite groundwater levels declining at most of the Area’s indicator sites during May, levels remained strong for the time of year generally recording end of month values at the top of the normal band. Therfield Rectory was the only indicator site not to have recorded a decrease in groundwater level in May, although levels stabilised during the month.

5. Reservoir stocks

In the Lee Valley group, reservoir stocks decreased during May from 98% of live capacity to 95%, meaning it ended the month above its average level for the time of year. In the Lower Thames group, the reservoir levels also decreased from 93% of live capacity to 90%, ending May below its average level for the time of year.

6. Environmental impact 

In the Colne catchment, chalk river sources moved slightly downstream from their locations in April, the:

• River Ver continued to flow downstream of Markyate but briefly ran dry downstream of Flamstead
• source of the River Gade moved slightly downstream but remained above Great Gaddesden
• River Bulbourne started flowing below Dudswell Village
• source of the River Chess remained at Missenden Road, upstream of Chesham
• River Misbourne was flowing downstream of Great Missenden, just above The Chiltern Hospital

The locations of chalk river sources in the Upper Lee stayed largely the same as in April, with some moving slightly downstream, the:

• River Mimram was still flowing above Whitwell
• source of the River Beane remained just above Walkern.
• River Rib flowed from Buntingford
• source of the River Ash (Herts) moved downstream of Little Hadham
• River Stort flowed briefly at Clavering but did not gain a steady flow until Stansted Springs

To protect the environment, during May a number of abstraction licence flow constraints were in force. This ranged between 3 and 4 per week, out of a month maximum of 33.

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.