The exceptional rainfall that occurred over December 2015 and the early part of January 2016 affected groundwater levels in many parts of the UK. Groundwater levels in the more responsive areas rose rapidly, but monitoring wells show these levels have now stabilised or started to fall. In areas where groundwater levels respond more slowly to rainfall, levels were relatively static during the rainfall events but are now rising.
Having started the 2015–2016 winter with average or below average groundwater levels (Figure 1, October 2015) across much of the country, the rainfall in the first few weeks took us to a situation where much of the country had above-average groundwater levels (Figure 2, December 2015). Many superficial aquifers (comprising relatively shallow, unconsolidated deposits) would have been saturated. This means that further periods of intense or prolonged rainfall could potentially lead to localised groundwater flooding.
The Chalk in Yorkshire had "notably low" groundwater levels at the end of October 2015 (Figure 1), reflecting conditions in the winter of 2014–2015 (the time of year when groundwater is usually replenished). During the summer months, plants take up most of the available moisture, causing soils to be dry, and little water passes down to aquifers. It is normal for groundwater levels to fall over this period: this is called the recession.
East Yorkshire received average to above average rainfall in October and November 2015, and above average rainfall in December 2015.
We can compare the groundwater level response in monitoring wells in two different environments:
Water infiltrates down from the surface until it reaches the water table, which is the top of the saturated unconfined aquifer. From there it flows to the confined aquifer. We thus expect water levels to respond to rainfall in the unconfined aquifer first, and in the confined aquifer later.
The hydrographs for these monitoring wells are shown in Figure 3, with some extra vertical lines delineating the boundaries between November (N), December (D) and January (J).
From Figure 3 we can see that groundwater levels at Wetwang started to rise very gently through November 2015, then rose quite rapidly throughout most of December 2015, with an even steeper rise in late December 2015/early January 2016. During mid-January 2016 the groundwater level at Wetwang stopped rising and began to fall.
The story is very different at Dalton Holme, which is less responsive. Here the groundwater level was still falling gently throughout November 2015, and fairly level for most of December 2015. Only in late December 2015 does the level start rising, and it is not until early January 2016 that we see a significant rise. We call this the "lag time" — the delay between the rainfall landing at the ground surface, and the response observed in the groundwater level. The water level is still rising at Dalton Holme in mid-January 2016, though it has only just reached the average level for the time of year (the dashed curve).
You can read more about these monitoring wells on our groundwater level pages:
You can download our report The Chalk Aquifer of Yorkshire for free.
The monitoring well at Houndean Bottom, near Lewes, in the Chalk of the South Downs, is a good example of a responsive well. The hydrograph in Figure 4 shows a steep rise throughout November 2015, December 2015 and the early part of January 2016. In early January 2016, water levels were rising rapidly towards levels at which flooding has previously occurred, and the Environment Agency issued a flood alert for Patcham on the 12 January 2016. Following a dry period, levels are currently falling, but are relatively close to ground level so further intense or prolonged rainfall could generate localised groundwater flooding.
You can download our report The Chalk aquifer of the South Downs for free.
Hydrograph data note: data up until the end of December 2015 are from the National Groundwater Level Archive (NGLA), and have been quality assured prior to submission, whereas January 2016 data are derived directly from the Environment Agency's telemetered boreholes and have not yet been quality assured.
This page updates the information given in Has groundwater influenced the recent UK floods?
Acknowledgement: the hydrographs include Environment Agency groundwater level data from the real-time data API (Beta).
Please contact Andrew McKenzie for further information.