Research and analysis

South-east England water situation report: September 2024 summary

Updated 11 October 2024

Applies to England

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

1. Summary

The south-east of England received 249% of the long term average (LTA) rainfall for September, making it one of the wettest months on record for many areas. The highest daily rainfall was 93.2mm, recorded at Stokenchurch (Thames, THM) on 22 September. Soil moisture deficits (SMDs) decreased sharply due to rainfall between 21 and 25 September, resulting in some areas ending the month at or near zero. River flows responded strongly, with several sites recording their highest September flows in decades. A total of 96 fluvial flood alerts and 27 flood warnings were issued throughout the month. Groundwater levels increased due to the heavy rainfall. Reservoir stocks remained above average at most reservoirs across the south-east.

2. Rainfall

The south-east of England received 249% of the LTA rainfall for September. Slow moving Atlantic fronts resulted in some record rainfall totals across 22 and 23 September. The heaviest rainfall was towards the north of THM and the north-east of Hertfordshire and North London (HNL), and accounted for an average of 24% of the monthly total across the south-east. THM received 35% of the monthly total recorded in the 2 days. Prestwood Reservoir rain gauge (HNL) recorded 75% of its monthly total across these 2 days. The maximum rainfall recorded over the 2 days was 126.7mm at Chipping Norton (THM).

There were some exceptionally high rainfall totals. These included:

• Stokenchurch (THM) registered the highest daily rainfall total of 93.2mm on 22 September, with the highest hourly intensity of 24.6mm occurring in the hour ending 3am Greenwich Mean Time (GMT) on 23 September
• Wheatley (THM) followed closely with a daily rainfall total of 92.8mm
• Whitwell (HNL) recorded 86mm, with nearly half of that total (30.66mm) falling within a 90 minute window ending at 5:15pm GMT on 22 September

September marked the end of the hydrological summer when the south-east received around a third more than the LTA total rainfall. The water year October 2023 to September 2024 has been one of the wettest on record for THM and 18 areal rainfall units across the south-east and the second wettest after 2000 to 2001 for 19 other areal rainfall units. Overall, this month marked the wettest on record for many areal units, particularly in THM and HNL areas. Despite the large monthly rainfall totals, there were, on average, 11 dry days (days with less than 0.2mm of rainfall) during September across the south-east.

3. Soil moisture deficit and recharge

In September, SMDs across the south-east peaked around 20 September, before rapidly decreasing in response to rainfall between 21 and 25 September. By the end of the month, most areas in THM had SMDs at or near zero. Similarly, SMDs in Chilterns East and Lee Chalk (HNL) fell to near zero.

In the Solent and South Downs (SSD), 11 out of 14 areas recorded SMDs at or close to zero by the end of the month. Meanwhile, the drier Kent and South London (KSL) had only 3 areas with SMDs near zero by the end of September.

Unusually there has been significant recharge during the month, particularly after the heavy rainfall on 22 and 23 September. This was due to the SMDs returning to zero before the end of the hydrological summer (April to September). There was almost double the expected recharge by the end of the summer.

4. River flows

In September, key indicator flow sites displayed a range of responses.  Rivers affected by the exceptional rainfall on 22 and 23 September responded very quickly, including normally slower responding groundwater fed rivers. Examples included:

• Coln at Bibury and Thames at Farmoor, both largely groundwater fed in THM
• Ver at Colney Street and Mimram at Panshanger, both groundwater fed in HNL
• Arun at Alfoldean and Ouse at Goldbridge, both draining impermeable catchments in SSD
• Medway at Teston and East Farleigh and Rother at Udiam, both draining impermeable catchments in KSL

Of the 21 indicator flow sites across the south-east of England, the monthly mean flows for 15 were in the exceptionally high category and 6 were notably high during September. Rivers which were exceptionally high were widespread across the south-east and included the:

• Thames at Farmoor (THM)
• Mimram at Panshanger (HNL)
• Test at Broadlands (SSD)
• Teise at Stonebridge (KSL)

The following sites recorded their highest September flows since 1968, the:

• Thames at Kingston and the Loddon at Sheepbridge (THM)
• Lee at Feildes Weir (HNL)
• Itchen at Allbrook and Highbridge (SSD)
• Medway at Teston and East Farleigh (KSL)

Both the Ver at Colney Street and the Mimram at Panshanger, groundwater fed rivers in HNL recorded their highest ever September flows.

Overall, the September flows across the south-east reflected a response to the significant rainfall during the month, with several rivers recording their highest flows in decades. A total of 96 fluvial flood alerts and 27 fluvial flood warning were issued during September.

5. Groundwater levels   

In September, groundwater levels increased across several sites, driven by the substantial rainfall during the month. Of the 16 indicator sites, 11 had notably high or higher levels during the month. The exceptionally high levels were distributed largely in the Chilterns and the Cotswolds. They were recorded at:

• Ampney Crucis (THM) and Lilley Bottom (HNL) both recorded their highest groundwater levels for September since records began
• Ashley Green (HNL) and Wolverton (KSL) ranked second, with the highest levels since 2008 and 2007, respectively
• Jackaments (THM), and Clanville Lodge Gate (SSD) recorded their highest levels since 2008
• Stonor Estate (THM) had the highest levels since in 2001

6. Reservoir stocks

The reservoirs remained above average for September at all the reservoirs across the south-east with just 3 exceptions. that ended the month just below the monthly LTA:

• Farmoor (THM)

• Ardingly (SSD)

• Arlington (SSD)

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.