1. Summary

January was a wet month with 146% of the long term average (LTA) rainfall recorded across the south-east of England. The first 6 days and the last 10 days accounted for an average of 94% of the monthly total. The soil moisture deficits (SMDs) rose steadily to the LTA up to around 19 January then dropped to zero to end the month below the LTA. Unsurprisingly the recharge was significant during the month, averaging about 160% of the LTA across the south-east. During January, the key indicator flow sites experienced flows ranging from normal to exceptionally high. Some groundwater-fed river sites recorded exceptionally high flows towards the Chilterns, including the Ver at Colney Street, Hertfordshire and North London (HNL). Rivers draining impermeable catchments recorded flows largely in the above normal category. There were 185 fluvial flood alerts, 32 flood warnings and 6 groundwater flood alerts issued during the month. Across the south-east of England, the groundwater levels varied from normal to exceptionally high. The expected steady rise of groundwater levels for this time of year continued during the month.

2. Rainfall

January was a wet month with 146% of the LTA rainfall recorded across the south-east of England. The first 6 days and the last 10 days accounted for an average of 94% of the monthly total. There was an average of 14 dry days during January, where the total rainfall for any day was less than 0.2mm. Storm Floriane moved across the south-east on 4 to 6 January bringing freezing conditions and heavy rainfall. Light snow fell overnight on 4 January and had melted by the end of the next day. There was also a light dusting of snow south of the M4 overnight on 8 January. The south-east of England was towards the calmer south of storm Éowyn on 24 to 25 January. The rainfall from storm Herminia impacted mainly the south coastal areas on 26 to 27 January.

The top 5 daily rainfall totals were recorded in Solent and South Downs (SSD). The top 3 were recorded on 4 January and the remaining 2 were on 26 January. The highest daily rainfall total was 41.3mm, recorded at Duncton, Western Rother Greensand, (SSD) on 4 January.

January had the highest monthly total rainfall since January 2014 for most of Thames (THM) and SSD. However, the rainfall totals for the 12 months ending January was the highest on record for a number of the areal units in THM, including Cotswolds West, Berkshire Downs and Cherwell. It was the highest January rainfall since 2001 for a high proportion of the units, including:

• Test Chalk, SSD, second highest on record
• Chilterns East, HNL, third highest on record
• Lee Chalk, HNL, third highest on record
• Loddon, THM, third highest on record
• Medway, Kent and South London (KSL), fifth highest on record.

These statistics are reflected in the exceptionally high rainfall displayed for most of the south-east of England in the rainfall map below for the last 12 months.

3. Soil moisture deficit and recharge

The SMDs rose steadily up to the end of month LTA around 19 January then dropped to zero to end the month below the LTA. They were maintained at zero or close to zero by the rainfall from the storms. Unsurprisingly the recharge was significant during the month, averaging about 160% of the LTA across the south-east. Small peaks of recharge occurred on 5 and 26 January coinciding with the heaviest rainfall.

4. River flows

During January, the key indicator flow sites experienced flows ranging from normal to exceptionally high. Some groundwater-fed river sites recorded exceptionally high flows towards the Chilterns, including the Ver at Colney Street, HNL. These rivers are supported by the high groundwater component of the flow. Rivers draining impermeable catchments recorded flows largely in the above normal category.

Groundwater-fed rivers responded to the rainfall in the early part of the month with peaks around 10 January, which then declined to 23 January and then rose again with the rainfall from the storms. This trend in the variation of the flows was matched across the south east, with rivers draining groundwater catchments showing more gentle variations compared to the impermeable catchments.

The Ver at Colney Street and the Mimram at Panshanger (both HNL) had the second and third highest January flows on record respectively, after 2001. There were 185 fluvial flood alerts, 32 flood warnings and 6 groundwater flood alerts issued during the month.

5. Groundwater levels   

Across the south-east of England, the groundwater levels varied from normal to exceptionally high, with:

• normal levels recorded on the North and South Downs
• notably high levels measured in the Test Chalk (SSD)
• exceptionally high levels were recorded in the Chilterns and Lee Chalk (HNL)

The expected steady rise of groundwater levels for this time of year continued during the month. Lilley Bottom (HNL) levels have been rising since September and have remained in the exceptionally high category for 4 months. They were the highest January levels on record. Levels at Ampney Crucis (Cotswolds West) and Stonor (Chilterns West), both THM, recorded their second and third highest January levels respectively.

6. Reservoir stocks

Reservoir storage for January was above the LTA at all reservoirs across the south-east with the exception of Arlington (SSD) which is well below the LTA and Darwell (KSL) which is just below 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.