Research and analysis

Devon and Cornwall water situation: September 2024 summary

Updated 11 October 2024

Applies to England

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

1. Summary - September 2024

Devon and Cornwall received 163% of the September long term average (LTA) rainfall, which was above normal for the time of year. September marked the end of the wettest water year in a record starting in 1871. Soil moisture deficit (SMD) decreased overall in September and ended the month lower (wetter) than the LTA for the time of year. Monthly mean river flows ranged from normal to exceptionally high for the time of year across the area. Groundwater levels at most sites decreased overall during September, but still ended the month at above normal to exceptionally high for the time of year. Total reservoir storage across Devon and Cornwall ended the month at 72% net storage, with Wimbleball, Colliford and Roadford reservoirs at 58%, 67% and 83% of net storage respectively at the end of September.

2. Rainfall

Devon and Cornwall received 151mm of rain during September (163% of the September LTA), which is classed as above normal for the time of year. The highest rainfall totals occurred on 5 to 7 September and 21 to 29 September, with drier periods in the remainder of the month. In September, rainfall was above normal in most hydrological areas apart from the Exe and North Cornwall, where rainfall was notably high for the time of year, and the Otter, Sid, Axe and Lim, where rainfall was exceptionally high for the time of year. It was the 4th wettest September on record in the Otter, Sid, Axe and Lim catchment (in a record starting 1871).

September marked the end of the wettest hydrological water year in a record starting in 1871. In the last 3 months, cumulative rainfall totals were normal in 5 reporting catchments, above normal in 5 reporting catchments, and exceptionally high in the Otter, Sid, Axe and Lim catchment, for the time of year. Cumulative rainfall over the last 12 months has been exceptionally high.

3. Soil moisture deficit

SMD decreased (soils became wetter) overall in September. By 1 October, the average deficit for Devon and Cornwall was close to 10mm, and was lower (soils were wetter) than the LTA for the time of year. The SMD was lower (soils were wetter) than the same time in 2023.

4. River flows

September monthly mean river flows ranged between normal to exceptionally high for the time of year. Whitford on the River Axe and Dotton on the River Otter recorded notably high and exceptionally high monthly mean river flows respectively, reflecting higher rainfall in these catchments. All reporting stations experienced peaks in daily mean river flow at the beginning and end of the month in response to the increased rainfall. The highest peaks were seen at the end of the month. On 30 September, most reporting sites recorded notably high or exceptionally high daily mean flows for the time of year.

5. Groundwater levels   

Levels at all groundwater sites receded overall in September, except for Winnards Perch, which is the fastest responding site and whose seasonal rising hydrograph limb has begun. Groundwater levels were classed as follows:

• exceptionally high at Whitlands (monitoring the Upper Greensand), Winnards Perch (monitoring the Staddon Formation) and Woodbury Common No2 (monitoring the Budleigh Salterton Pebble Beds)
• notably high at Branscombe Lane (monitoring the Dawlish Sandstone) and Bussels No7A (monitoring the Dawlish Sandstone)
• above normal at Coleford Production (monitoring the Permian Breccias and Sandstones) and Woodleys No1 (monitoring the Otterton Sandstone Formation)

6. Reservoir stocks

Total reservoir storage was 72% at the end of September, which is a reduction of 2% of storage since the end of August. This is higher than the total storage at the same time last year and in 2022. At the end of the month, storage at Wimbleball, Colliford and Roadford was 58%, 67% and 83% of net storage respectively, compared to 74%, 52% and 54% this time last year.

Author: Environment Agency, hydrology.dandc@environment-agency.gov.uk

All data are provisional and maybe subject to revision. The views expressed in this document are not necessarily the views of the Environment Agency. Its officers, servants, or agents accept no liability for loss or damage arising from the interpretation or use of the information, or reliance upon views contained in this report.