Research and analysis

Water situation: May 2023 summary

Updated 11 January 2024

Applies to England

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

May was a dry month, with almost all catchments across England receiving below average rainfall. Soil moisture deficits increased across England, with soils in many places drier than would be expected for the time of year. River flows remain normal or higher at the majority of reported sites, although flows decreased at the nearly all sites. Groundwater levels decreased at many sites however almost all sites remain classed as normal or higher for the time of year. Reservoir stocks at the end of May decreased at almost two thirds of the reservoirs and reservoir groups we report on, three quarters of sites were classed as normal or higher for the time of year.

1. Rainfall

The May rainfall total for England was 39mm which represents 65% of the 1961 to 1990 long term average (LTA) for the time of year (68% of the 1991 to 2020 LTA). With the exception of thirteen catchments in the south west and eastern England, all catchments received below average rainfall during May. The wettest hydrological area was West Dorset Streams in south west England which received 131% of LTA rainfall during May. The driest hydrological area (relative to its LTA) was the Romney Marsh catchment in south east England receiving 32% of LTA rainfall for the time of year.

May rainfall totals were classed as normal for the time of year in the majority of catchments across England. Nearly a third of the catchments were classed as below normal and almost a quarter were notably low for the time of year. At the regional scale, south east, south west and east England all received normal rainfall during May. North east and north west England both received notably low rainfall for the time of year while central England received below normal rainfall. May rainfall across England as a whole was below normal for the time of year.

The 3 month cumulative rainfall totals show that all but one catchment, the Tweed, were classed as normal or higher, with a nearly three quarters notably or exceptionally high. The 6 month cumulative rainfall totals show the majority of catchments are either normal or above normal for the period. Twelve month cumulative rainfall totals were normal for more than half of catchments in England, more than a third of catchments, predominantly across southern England were above normal or higher.

2. Soil moisture deficit

Soil moisture deficits (SMD) have increased across England at the end of May following the below average rainfall and higher temperatures across most of the country. Soil moisture deficits across England increased significantly particularly during the second half of the month.

May SMD values across the majority of England are now higher than average for the time of year, meaning soils are drier than would be expected. At a regional scale, the end of May SMDs are higher than average for the time of year particularly across west areas of England.

3. River flows

May monthly mean river flows decreased at all but one of indicator sites we report on however the majority of sites were remain above normal or higher for the time of year. Nearly a third of sites were normal for the time of year, while the 10 sites primarily in the north of the country are classed as below normal.

Monthly mean river flows declined at all regional index sites during May. May monthly mean flows at Offord on the Bedford Ouse, Horton on the Great Stour and the naturalised flows on the River Thames at Kingston were classed as above normal. Flows at the River Dove in central England, the River Exe in the south west, the South Tyne in the north east were classed as normal and River Lune in the north west was below normal for the time of year.

4. Groundwater levels

At the end of May, groundwater levels decreased at more than half of reported indicator sites. Almost all end of month groundwater levels were classed as normal or higher for the time of year. Two sites were below normal for the time of year.

The major aquifer index sites reflected a varied picture at the end of May, ranging from normal to exceptionally high levels. Normal groundwater levels for the time of year were reported at Dalton Estate Well in the Hull and East Riding Chalk, at Skirwith in the Carlisle Basin and Eden Valley Sandstone, at Jackaments Bottom in the Burford Jurassic Limestone, at Stonor Park in the South West Chilterns Chalk and at Weir Farm in the Bridgnorth Sandstone. Little Bucket in the East Kent Stour Chalk and Redlands Hall in the Cam and Ely Ouse Chalk were both above normal for the time of year. Chilgrove in the Chichester Chalk remains exceptionally high at the end of May after groundwater levels increased rapidly in the previous month.

5. Reservoir storage

Reservoir stocks at the end of May had decreased at almost two thirds of the reservoirs and reservoir groups we report on. The remaining reservoirs and reservoir groups seen an increase or no change in stocks. The largest stock increase was at Grafham Reservoir in east England which saw a 7% change. In contrast, Haweswater and Thirlmere in north west England and the Teesdale Group in north east England both saw a decrease of 13% in reservoir stocks. A fifth of reservoirs or reservoir groups were classed as below normal or notably low at the end of May. All other reservoirs or reservoir groups were classed as normal or higher for the time of year.

At a regional scale, total reservoir stocks ranged from 82% in north west England to 96% in south east England. Total reservoir stocks for England were 89% of total capacity at the end of May.

6. Forward look

June began with fine and settled weather and these conditions are forecast to prevail throughout for the remainder of the month. There is an increased chance that the month will be warmer than average however there remains a good likelihood of usual temperatures.

Rainfall in June is expected to be near average with an increased chance of localised thundery downpours.

For the 3 month period for the UK from June to August there is an increased chance of a hot summer and heatwaves. However, near average temperatures and rainfall remain the most likely outcomes. Notable differences in rainfall distribution are likely due to the showery and potential thundery nature of summer precipitation.

7. Projections for river flows at key sites

By the end of September 2023, river flows have a greater likelihood of being above normal or high in south east and south west England. In north west, north east and central England there is a slightly higher chance of river flows being below normal or lower. By the end of March 2024, river flows in the south east and south west of England have a greater likelihood of being above normal or higher. In north west, north east and central England there is a slightly higher chance of river flows being below normal or lower.

8. Projections for groundwater levels in key aquifers

By the end of September 2023 groundwater levels in south west and east England have a greater likelihood of being above normal of higher. Groundwater levels are most likely to be in their expected range across the rest of England. By the end of March 2024 groundwater levels have a greater likelihood of being above normal or higher in south east, east, and central England. In south west and north west England groundwater levels are most likely to be in their expected range. In north east England there is a slightly higher chance of groundwater levels being below normal or lower.

Author: National Water Resources Hydrology Team, [email protected].

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.