Wednesday, 20 August 2014

Hydrological Outlook from August 2014

The latest Hydrological Outlook states that the groundwater levels in August will generally be unaffected by the amount of rainfall we have.  This is because large soil moisture deficits have built up over the summer and rainfall will not reach the water table while these deficits exist.

Looking further ahead, the prediction for precipitation over the next 3 months is not a clear "wet" or "dry" scenario:  "The probability that UK precipitation for August-September-October will fall into the driest of five categories is around 20% and the probability that it will fall into the wettest category is 25-30%".  The timing of the soil moisture deficit being overcome by rainfall (and the reduced efforts of sunshine and plants in evaporation and transpiration), and thus the groundwater recharge season starting, will depend upon when the rain falls, and how persistent it is.

The general outlook is for groundwater levels to be in the near-normal range across the majority of aquifers.

For further information see the full Hydrological Outlook for August.

[Apologies for not posting this sooner.]

Tuesday, 19 August 2014

July 2014 overview

Groundwater levels, July 2014
July was a pleasant summer month over most of the UK.  The rainfall total was slightly below average, but the distribution varied significantly.  Thunderstorms brought downpours to areas of the Midlands, the south-east and East Anglia.  Wales and parts of England were notably dry.

Groundwater levels in the very responsive Carboniferous Limestone aquifer of south Wales fell as a result of the low rainfall there.  This is the only site with below normal groundwater levels on our map this month.

Across the rest of the UK, groundwater levels are in the normal or above normal range for the time of year.  Levels are falling at the moment as we are in the recession phase of the annual cycle of groundwater fluctuations.

Below is the hydrograph for Wetwang, a 46 m deep borehole used to measure groundwater level in the Chalk aquifer in Yorkshire.  The dashed line shows the average groundwater level by month.  The observed level (solid black line) indicates the low levels at the start of the year, followed by a rapid recovery during the wet months of January and February.  The steepest section of the recovery shows almost 10 m rise in water level in a little over a month.  The recession, which started in late February, was initially steep, followed by a period of rather stable level during May and June.  During July the groundwater level at Wetwang has closely followed the monthly average line.  Groundwater levels are expected to continue to fall until the autumn.

Groundwater levels at Wetwang, unconfined Chalk