Research and analysis

Hertfordshire and North London water situation: March 2026 summary

Updated 14 April 2026

Applies to England

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

1. Summary

In contrast to the preceding months, March was a drier month across the Hertfordshire and North London area, with 28mm of rain received (70% of the long term average, LTA). Soils dried out during March, with deficits rising from near zero at the end of February to above the LTA across most of the Area by the end of March. As a result, effective rainfall was below average for the time of year and the recharge season drew to a close during March. River flows fell back from the elevated levels seen in February, with chalk indicator sites recording month mean flows mostly in the above normal or normal bands, and all clay and urban indicator sites returning to the normal band. Groundwater levels remained broadly stable during March, with end of month levels unchanged from February at most indicator sites.

2. Rainfall

March was a drier month for the Hertfordshire and North London area (the Area), with a rainfall total of 28mm, equivalent to 70% of the LTA. Most rainfall units ended the month in the normal band, with the exception of the Chilterns East Colne and Roding units, which fell into the below normal band. Over the last 3 months, rainfall totals in all units fell into the notably high band, with the exception of the North London unit, which ended the period in the exceptionally high band. The winter period as a whole (October 2025 to March 2026) was above normal across all units, reaching 118% of the LTA for the Area.

March was very much a month of two halves, with approximately 81% of the monthly total falling within the first 15 days and little of note thereafter. The most significant rainfall events occurred on 6 March (approximately 20% of the monthly total), 10 March (approximately 11%), 12 March (approximately 30%, widespread across the Area), and 15 March (approximately 10%). The highest daily totals across the Area were recorded on 12 March, including:

• 13.9mm at Northolt MO (North London unit)
• 13.0mm at Radlett STW (Chilterns East Colne unit)
• 13.0mm at The Avenue North (North London unit)
• 12.0mm at North Mymms PS (North London unit)
• 12.0mm at RAF Northolt (North London unit)

Many gauges across the Area recorded between 15 and 23 dry days during March, consistent with the drier second half of the month.

3. Soil moisture deficit and recharge

Soil dried out across the Area during March as the balance shifted away from wet conditions. At the end of February, soils had been at or near saturated capacity across the area, but by the end of March they had dried to approximately 20mm across all units, above the LTA for most of the Area. The Lee Chalk unit was the exception, where the end of month soil moisture deficit (SMD) of 20mm remained slightly below its LTA of 23mm. The Area received 7mm of effective rainfall during March, equivalent to 61% of the LTA. Over the winter period as a whole, effective rainfall reached 105% of the LTA, reflecting the sustained wet conditions of the preceding months. The rise in SMDs during March indicates that the recharge season is drawing to a close.

4. River flows

River flows fell back from the elevated levels recorded during February. In the Colne catchment, the Ver (Colney Street) and Gade (Croxley Green) maintained above normal month mean flows, while the Misbourne (Denham Lodge) and the River Colne (Denham) both returned to the normal band, having been above normal and notably high respectively in February. In the Upper Lee catchment, the Lee at Howe Green and the Mimram at Panshanger continued to record above normal flows, whilst the Ash (Wareside) and the Lee at Feildes Weir both dropped back into the normal band. In the clay and urban catchments, all indicator sites recorded month mean flows in the normal band during March, a notable reduction from the notably high and exceptionally high levels seen in February. During March, one fluvial flood alert was issued in the Area. No flood warnings or severe warnings were issued.

5. Groundwater levels   

Groundwater levels remained broadly stable across indicator sites in the Area during March, consistent with the end of the recharge season and the rising SMDs noted above. In the Mid-Chilterns Chalk aquifer, all indicator sites (Ashley Green, Ballingdon Farm, Amersham Road and Wapseys Wood) ended the month in the normal band. In the Upper Lee Chalk aquifer, end of month level were similar to those recorded in February:

• Lilley Bottom and Therfield Rectory remained in the normal ban
• Crescent Cottages and Hixham Hall in the above normal band
• Cave Gate in the notably high band

6. Reservoir stocks

During March, reservoir stocks in the Lee Valley group started the month at 92% of live capacity and ended the month at 96%, above the average level for the time of year. In the Lower Thames group, reservoir stocks started March at 90% of live capacity and ended the month at 93%, also above its average level.

7. Environmental impact 

In the Colne catchment, chalk river sources broadly held their positions from February during the March survey. The:

• River Ver was still flowing from downstream of Markyate and flowed for its full length
• River Gade started flowing at Bradden Lane
• source of the River Bulbourne remained at the fields above Dudswell
• source of the River Chess remained at Missenden Road, upstream of Chesham
• River Misbourne was flowing upstream of Great Missenden

Chalk river sources in the Upper Lee catchment were also broadly in line with February positions following the March survey. The:

• source of the River Mimram was flowing from the lakes upstream of Whitwell
• River Beane was flowing from Halls Green upstream of Cromer, running dry at Cromerhill Common before resuming downstream
• source of the River Rib was upstream of Reed End, where it flowed intermittently before gaining a steadier flow downstream of Chipping
• River Ash (Herts) flowed intermittently from Blackhall, gaining a steadier flow at Little Hadham
• River Stort was still flowing from upstream of Park Lane, above Langley Lower Green

To protect the environment, during March a number of abstraction licence flow constraints were in force. This ranged between 4 and 12 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.