Research and analysis

Wessex water situation: May 2025 summary

Updated 12 June 2025

Applies to England

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

1. Summary

May was a third consecutive dry month in Wessex. An average of 26mm, 44% of the long term average (LTA) fell across Wessex, largely during the latter half of the month. When considered alongside a preceding dry March and dry April, this has been the fourth driest 3 month period (March to May) since records began in 1871. Soil moisture deficit (SMD) continued to increase during May. SMD spent the majority of the month above the historic maximum but it was back below the historic record by the end of May due to rainfall between 20 and 27 May. Monthly mean flows recorded across the north of Wessex were largely notably or exceptionally low in May. In the south of Wessex, monthly mean flows were either below normal or notably low due to the supporting baseflow from the Chalk aquifer. The majority of groundwater sites, including all sites monitoring the Chalk aquifer, reported below normal or notably low levels at the end of May. The only exception was Didmarton in the north of Wessex monitoring the Inferior Oolite which recorded normal levels. Reservoir levels for Wessex Water were approximately 83% capacity at the end of May while for Bristol Water, levels were at approximately 79% capacity.

2. Rainfall

An average of 26mm of rain fell across Wessex in May (44% of the LTA). This is the third consecutive dry month, resulting in the driest 3 month period (March to May) since 1990 and the fourth driest 3 month period (March to May) since records began in 1871. For the Little Avon hydrological area in the north of Wessex, this was the driest 3 month period on record. Most rain in May fell during the latter half of the month. In total, 89% of rain during May fell between 20 May and 27 May. The lowest relative rainfall was recorded in the middle and lower reaches of the Dorset Stour and Hampshire Avon in the south east of Wessex (31 to 32% of the LTA) and the Little Avon in the north of Wessex (30% of the LTA). The highest relative rainfall was recorded in the Mendips and River Chew (56% of the LTA). All hydrological areas in Wessex received either below normal or notably low rainfall during May. Most hydrological areas which received below normal rainfall were either to the south west of Wessex or to the north of Wessex.

Over the past 3 months, almost all hydrological areas have received exceptionally low rainfall. The 2 exceptions are the West Somerset Streams and Tone to the west of Wessex which have both received notably low rainfall. Over the past 6 months, most hydrological areas received below normal rainfall except for three areas to the north of Wessex which received notably low rainfall and the River Piddle to the south which received normal rainfall. Over the past 12 months, only the Upper Hampshire Avon received above normal rainfall while all other hydrological areas received normal rainfall.

3. Soil moisture

SMD in Wessex continued to increase throughout most of May. For the first couple of weeks, SMD increased substantially due to the lack of rainfall. This resulted in a SMD greater than the historic maximum for most of the month. The rain between 20 and 27 May slowed the rate of SMD increase, causing it to fall below the historic maximum SMD by the end of May. Across most hydrological areas, SMD was in the range of 101mm to 130mm at the end of the month and between 51mm and 75mm greater than the LTA. The 2 exceptions were the West Somerset Streams and the Mendips and River Chew areas which both recorded SMD values between 71mm to 100mm at the end of May. This was between 26mm and 50mm greater than the LTA.

4. River flows

In May, the majority of flow sites in the north of Wessex reported either notably low or exceptionally low monthly mean flows. The Bristol Frome at Frenchay reported its lowest May monthly mean flow on record. Most sites monitoring flows on or downstream of the Chalk aquifer recorded below normal monthly mean flows. The exceptions were the Dorset Frome at East Stoke Combined and the Dorset Stour at Throop which both recorded notably low monthly mean flows. Relative flows were higher across most sites in the south of Wessex as flows were supported by baseflow from the Chalk aquifer. The Somerset Yeo at Pen Mill was the only site which recorded higher flows at the end of May than at the beginning of the month. For most sites in Wessex, daily mean flows fell throughout most of May except for small peaks in flow following rain on 20 and 21 May.

5. Groundwater levels   

Following 3 consecutive dry months, the majority of groundwater sites in Wessex, including all sites monitoring the Chalk aquifer, ended May reporting either below normal or notably low levels. The only site which reported normal levels at the end of May was Didmarton (monitoring the inferior Oolite) in the north of Wessex. For all sites, groundwater levels decreased throughout May in response to the largely dry weather.

6. Reservoir stocks

Wessex Water reservoir levels began to decrease again during May, having steadied towards the end of April. Bristol Water reservoir levels continued to decrease throughout May. The combined levels at the end of May for Wessex Water were approximately 83% capacity while for Bristol Water, combined levels at the end of the month were approximately 79% capacity. For both water companies this is notably lower than reservoir levels recorded this time last year. For Wessex Water, reservoir levels at the end of May are similar to 1995 while for Bristol Water, levels are slightly lower.

Author: Wessex Hydrology, hydrologywessex@environment-agency.gov.uk

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 in this report.