Groundwater levels

Central Region

*The long term average (LTA) has been calculated statistically, using the long term data record for this hydrograph. The respective drought zones are used as operational triggers, and have been derived using the LTA.

May rainfall was 22 % of the LTA, making it the third consecutive month with below average rainfall. The Met Office has confirmed that May 2020 has been the sunniest calendar month on record in the UK, and the driest May in England on record. Warm temperatures and low rainfall have meant that soil moisture deficit (SMD) has continued to increase and is well above the LTA. Whilst SMD remains high, any rain which falls during the coming months is unlikely to result in groundwater recharge.

June was wetter than the previous months, with 131% of LTA rainfall in Central region. This followed the second driest April/May period since 1995 for Central region, surpassed only by spring 2011. A significant amount of the rain in June fell during relatively heavy rainfall events, resulting in a high proportion of runoff. Soil moisture deficit (SMD) remained above average throughout June. Due to these factors, warmer temperatures and increased plant growth, effective precipitation is generally low during the summer months. Groundwater levels continued to decline through the month and are likely to do so until the next recharge season begins.

Groundwater levels are forecast to remain above drought zone 1 through the summer if we receive greater than 60 % of the monthly LTA rainfall, potentially tracking the LTA curve through the seasonal recession if we receive average rainfall (100 % of the LTA). Under a very dry scenario (continued 60 % LTA rainfall), we could dip into drought zone 1 in early autumn.

The Chalk derived baseflow of streams across the Chilterns has remained relatively healthy and in the normal range (as classified by the Environment Agency) due to the above average Chalk groundwater levels. In catchments where the rivers are also strongly supported by shallow aquifers (Cam, Rib, Mimram, Beane,), flows remain lower, due to the persistent dry weather experienced during the spring. The Cam and Ely Ouse river catchment in particular is experiencing low flows, and this has been reflected in the Environment Agency announcement that it is now in developing drought (prolonged dry weather) status. Most rivers have experienced short-term increases in flow and level following rainfall events but no significant flow recovery, due to a higher proportion of runoff associated with the high SMD.