South-east England water situation report: December 2024 summary
Updated 14 January 2025
Applies to England
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1. Summary
There was 71% of the long term average (LTA) rainfall received in December for the south-east of England. It was the first month since August to record below average rainfall. For the 12 months ending December, Thames (THM) and half of the areal units in THM recorded the highest rainfall on record. The soil moisture deficits (SMDs) were all but wiped out by Storm Darragh in the first week of the month. Thereafter, the drier weather throughout the rest of the month allowed the SMDs the build but were still well below the LTA for the end of December. Recharge was below average for December. River flows responded to the rainfall brought by Storm Darragh and then fell for the rest of the month. The additional rainfall and recharge ensured that the Chalk groundwater fed rivers across Hertfordshire and North London (HNL), THM and Solent and South Downs (SSD) all ended December at notably high or exceptionally high flows. There were 34 fluvial flood alerts issued across the south-east during the month. Groundwater levels rose at most of the key indicator sites across the south-east as would be expected at this time of year. Levels did fall at three sites, at Jackaments (THM), Carisbrooke Castle (Isle of Wight, SSD) and Riddles Lane, Kent and South London (KSL).
2. Rainfall
There was 71% of the LTA rainfall received in December for the south-east of England. It was the first month since August to record below average rainfall. This ranged from below normal rainfall for December along the Thames Valley and western part of (SSD) to normal rainfall across THM, HNL, KSL and the remainder of SSD. The wettest day was on 4 December when the highest daily total was 28.5mm, recorded at Plumpton (SSD) and also when around 16% of the monthly total fell. Storm Darragh then followed on 6 to 8 December, that affected the highest daily totals only in HNL. Higher daily totals were recorded on 4, 5 and 9 December in THM, SSD and KSL. There was no significant rainfall during the last week of the year across the south-east of England. For the 12 months ending December, THM and half of the areal units in THM recorded the highest rainfall on record. In addition, Chilterns West and Ock areal units, both THM, were the second highest totals after 1903.
2.1 Soil moisture deficit and recharge
The SMDs were all but wiped out by Storm Darragh in the first week of the month. Thereafter, the drier weather throughout the rest of the month allowed the SMDs the build, albeit slowly to end the month with some deficits, but were still well below the LTA for the end of December. Whilst recharge was below average for December, for the winter period so far (October to December) the combination of a very wet start to the winter and the ongoing very low SMDs has allowed around one and a half times the LTA recharge to occur across the south-east.
3. River flows
River flows responded to the rainfall brought by Storm Darragh and then fell for the rest of the month. The additional rainfall and recharge ensured that the Chalk groundwater fed rivers across HNL, THM and SSD all ended December at notably high or exceptionally high flows. The Ver at Colney Street was the second highest flow on record after 2000 and the Mimram at Panshanger reached its highest flow ever recorded. Both sites are in HNL. Key indicator sites across the eastern part of SSD and across much of KSL ended December mainly at normal flows. There were 34 fluvial flood alerts issued across the south-east during the month.
4. Groundwater levels
Groundwater levels rose at most of the key indicator sites across the south-east as would be expected at this time of year. Levels did fall at three sites, at Jackaments (Oolites, THM), Carisbrooke Castle (Isle of Wight, SSD) and Riddles Lane (North Downs, KSL). Levels ended the month at exceptionally high and notably high levels across the Chilterns, Berkshire Downs and the Test Chalk. Stonor and Gibbet Cottages (both Chilterns West, THM) were at the second highest level on record since 2000 and Lilley Bottom (Lee Chalk, HNL) was the highest ever December level on record. Throughout SSD and KSL, levels ranged from above normal to below normal. The only site that was below normal for December was Carisbrooke Castle on the Isle of Wight, SSD.
5. Reservoir stocks
Reservoir storage for December was above the LTA at all reservoirs across the south-east with the exception of Arlington (SSD) which is well below the LTA and Bewl (both KSL) which is just at the LTA.
Author: groundwaterhydrology@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.