Research and analysis

Thames water situation: May 2025 summary

Updated 12 June 2025

Applies to England

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

1. Summary

Thames area received 23mm of rainfall through May, 41% of the long term average (LTA). Over the last 3 months’ rainfall in Thames area was the second lowest since records began in 1871. All our areal rainfall units were either notably low or below normal by the month’s end, with the exception of Cotswolds West which received exceptionally low levels of precipitation. Soil moisture deficits (SMD) increased significantly across the Thames area, rising from 70mm in April to 92mm by the end of May. River flows responded to the lack of rainfall by decreasing at all our key indicator sites compared with last month. Likewise, groundwater levels declined at the majority of indicator sites in May and ranged from exceptionally low (Inferior Oolite) to notably high (Chalk), with the only exception being Frith Cottage (Lower Greensand). The Lower Thames reservoirs ended the month remaining below average for the time of year however Farmoor ended the month with a value which exceeded the LTA.

2. Rainfall

May was another dry month across Thames area, with only 23mm of rainfall recorded, 41% of the LTA. All our areal units received either notably low or below normal precipitation, with only Cotswolds West receiving exceptionally low precipitation, making it the seventh driest May since records began in 1871. Although normal rainfall has occurred across Thames area over the last 12 months, it was exceptionally low in all but one unit over the last 3 months. The previous 3 months has been the second driest period since 1871, with it even being the driest recorded for Cotswolds East and Upper Thames. 

3. Soil moisture deficit and recharge

Soil moisture deficits (SMD) increased significantly across Thames area, rising from 70mm in April to 92mm by the end of May. This was significantly higher than the LTA of 49mm for the time of year, indicating that soils are much drier than usual. The sharp increase reflected a sustained lack of effective rainfall, only 9% of the LTA fell during April, and this was due to a combination of low rainfall and increased sunshine hours.

4. River flows

Monthly mean flows decreased at all key indicator sites compared to last month, largely due to declining groundwater levels and low rainfall. The majority of sites are recording flows that are below or notably below the LTA for this time of year. Cassington (Evenlode) was particularly affected, reflecting the influence of low groundwater levels in the underlying Oolitic limestone geology. Other sites showing below normal flows included Windsor (River Thames), Bibury (River Coln), and Farmoor (River Thames). The only site recording above-normal flows was Bourne End Hedsor (Wye).

5. Groundwater levels

Groundwater levels decreased at the majority of our indicator sites in May and ranged from exceptionally low (Jackaments Bottom, Inferior Oolite) to notably high (Stonor Estate, Chalk). Groundwater levels of all of sites remained in the same banding as last month, with the exception of Stonor Estate dropping into the notably high banding for first time in 15 months. Overall, groundwater levels of the Chalk remained normal or above, whilst levels of the slower responding Lower Greensands remained notably high for the time of year with Frith Cottage (Lower Greensand) being the only increase in levels.

6. Reservoir stocks

Reservoir stocks decreased in the Lower Thames reservoirs and ended the month at 93.6%, compared to 95.3% at the end of April. Stocks in Farmoor reservoir increased from 97.2% to 99.1% during May. The Lower Thames reservoirs ended the month remaining below average for the time of year however Farmoor ended the month above the LTA.

7. Environmental impact 

At the end of the month, 23 abstraction licences were being constrained in the area to protect water resources and the environment. There were no groundwater flood alerts in force by the end of May.

Author: Thames Area Groundwater Resources and Hydrology, enquiriesWT@environment-agency.gov.uk

Contact Details: 030708 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 in this report.