Research and analysis

Thames water situation: April 2025 summary

Updated 14 May 2025

Applies to England

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

1. Summary

Thames area received 21mm of rainfall through April, 41% of the long term average (LTA). All our areal rainfall units were either notably low or below normal by the month’s end. Soil moisture deficits (SMD) increased significantly across the Thames area, rising from 31mm in March to 70mm by the end of April. 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 all indicator sites in April and ranged from exceptionally low (Inferior Oolite) to exceptionally high (Chalk). 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

April was another dry month across Thames area, with only 21mm of rainfall recorded. All our areal units received either notably low or below normal precipitation and this amounted to 41% of the LTA. Rainfall over the previous 3 months was notably low across the area, but was mostly above normal over the previous 12 months.

3. Soil moisture deficit and recharge

Soil moisture deficits (SMD) increased significantly across Thames area, rising from 31mm in March to 70mm by the end of April. This is significantly higher than the LTA of 20mm for this time of year, indicating that soils are much drier than usual. This sharp increase reflects a sustained lack of effective rainfall. Only 7% 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 of our key indicator sites compared to last month. Across the total number of indicator sites, 7 recorded normal flow, whilst 6 sites were below normal. Cassington (Evenlode) measured as notably low, in part due to declining groundwater levels in the Oolitic limestones that support it, whilst Bourne End Hedsor (Wye) remained the only site in above normal banding.

5. Groundwater levels

Groundwater levels decreased at all of our indicator sites in April and ranged from exceptionally low (Jackaments Bottom, Inferior Oolite) to exceptionally high (Stonor Estate, Chalk). Groundwater levels of the Chalk at Rockley and Tile Barn Farm dropped into the normal banding, whilst Stonor Estate remained in the exceptionally high banding for the fifteenth month in a row, despite the decline. Groundwater levels of the slower responding Lower Greensands remained notably high for the time of year.

6. Reservoir stocks

Reservoir stocks rose in the Lower Thames reservoirs and ended the month at 95.3%, compared to 94.8% at the end of March. Stocks in Farmoor reservoir decreased from 99.3% to 97.2% during April. 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, 2 abstraction licences were being constrained in the area to protect water resources and the environment. There was one groundwater flood alert in force by the end of April.

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.