The Environment Agency has published its monthly Water Situation Report for the South East and despite record rainfall, groundwater levels in most aquifers are lower than normal for this time of year in the West Thames area.

Summary – April 2012

Rainfall totals for West Thames Area were exceptionally high in all catchment areas in April 2012. The rainfall was sustained throughout much of the month leading to exceptionally high daily mean river flows at most key indicator sites. Groundwater levels in the Upper Thames, Ock and Cotswolds responded with some rise in levels however ground water levels were generally lower than normal for the time of year.

Rainfall

The overall average rainfall total for West Thames Area was 128 mm, 258 % of the April long term average (LTA). All areal rainfall units in West Thames area had exceptionally high end of month rainfall totals for April. The Wey –greensand unit had the highest rainfall total with 161 mm of rainfall, which was 288 % of its April LTA. This was the highest April rainfall total in this unit since records began in 1920. Similarly, the neighbouring Loddon unit also had its highest April rainfall record with 253 % of its LTA. The Enborne and The Cut units also recorded their highest April rainfall totals, although their records only extend as far back as 1961. The Berkshire Downs areal rainfall unit had the lowest April rainfall total with 113 mm, but this was still more than double its April LTA with 222%.

Soil Moisture Deficit (SMD) /Recharge

As a result of the high rainfall totals in April effective rainfall was exceptionally high in all West Thames Areal rainfall units too. The highest effective rainfall was in the Wey – greensand unit with 540 % of the April LTA. The Cotswolds West had 508 % of its LTA, and the Upper Thames had 500 % of its LTA. On average the areas total effective rainfall was 374 % of the April LTA. By the end of the month all areal rainfall units had soil moisture deficits to 0 mm. This means that at the end of April soils were saturated.

River Flows

With the exceptionally high rainfall in April, daily mean river flows responded rapidly with most key indicator sites going from exceptionally low to exceptionally high. At the end of April most rivers were still rising with 11 of the key indicator sites having exceptionally high flows. Owing to the low flows and dry soils at the start of April the rivers in West Thames Area were able to contain most of the rainfall and surface water runoff within their channel capacity and floodplains. As a result of the increase in river flows in all catchments the April month mean flows suggest that most rivers have responded to the rainfall with 6 key indicator sites now being normal for the time of year. The Upper Wey month mean for April was notably high and the Lower Wey and Cherwell was above normal. However the River Coln month mean remained exceptionally low and the River Kennet was notably low. It is expected the recovery in river flows will be short lived unless we see significantly heavy rainfall in May.

Groundwater Levels

Groundwater levels at some West Area key observation boreholes showed a response to the April rainfall. In the Oolitic Limestone of the Cotswolds and Upper Thames and in the Corallian of the Ock catchment groundwater levels increased slightly. Despite this, groundwater levels in most aquifers are lower than normal for this time of year. Only the Great Oolites as measured at Ampney Crucis in the West Cotswolds are normal for the time of year and the groundwater levels at Marcham in the Corallian of the Ock are above normal. At the end of April Stonor Park observation borehole in the West Chilterns remained dry. Environmental Impact There were 27 flood alerts issued in April but no flood warnings were needed.

With the increase in river flows the number of active constraints on abstraction licences steadily declined throughout April. By the end of the month there were only 5 flow constraints in force preventing abstraction to two public water supply licences owned by Thames Water and three significantly smaller private licences. The high rainfall quantities and increase in rivers flows allowed the Environment Agency to cease running the Letcome Brook augmentation scheme in April. The scheme has been running almost continuously over the past 18 months.