It has been the wettest April on record for England and Wales, yet in the past few weeks, I have often heard people asking the question: how can we be told we’re in a drought in the UK, when at the same time we’re being told many areas of the country are experiencing floods? Since I have some water science expertise, I thought I should write this blog on the subject.
Furthermore, drought is not just caused by shortage of rainfall, but also the type of rainfall, the length of time without rainfall, and it can occur not just in the summer, but in the winter too, when it may not be so noticeable. However, most criteria on declaring an area a drought area involve insufficient rainfall, which will result in a shortage of water for people, the environment, agriculture and industry.
Most of our freshwater supply comes from rivers and lakes, but approximately one third of it comes from groundwater. A few months of above average rainfall is necessary in order to replenish the groundwater reserves, i.e. the water-saturated soil layer. If rainfall is insufficient, the groundwater will not be adequately replenished and a drought might be declared. The main reason we've been told we're still in a drought in parts of the UK is that we had two unusually dry winters in the past two years and winter is normally when we get the steady rainfall, which is ideal to replenish groundwater reserves.
The UK Environment Agency (EA) has criteria to determine drought areas, taking into consideration the area’s hydrological and environmental triggers. The main aspects of drought and the impacts, risks and triggers monitored by the EA are set out in their drought plans and include the following aspects that tend to occur in the order below:
- Meteorological – shortage of rainfall and actual evaporation;
- Hydrological – reduced river flows and low groundwater levels;
- Environmental – dry soils, stress on habitats and wildlife;
- Agricultural – dry soils, need for irrigation and stress on plants;
- Public supply – low reservoir storage, high demands, customer appeals and restrictions and drought permits and orders.
The EA published a report entitled Managing drought in England and Wales, which explains the agency’s drought management actions.
As for floods, heavy rainfall and poor drainage can cause rivers to fill up quickly and overflow causing floods. A heavy downpour will not necessary replenish the groundwater, especially when the soil is a heavy clay, because most of the vast amount of water falling over the soil surface over a short period of time will not freely pass through the soil and into the water table. Some of the water will accumulate on the surface, some will run off to rivers and sea, accumulating where it is not able to be absorbed by the soil-river-streets drainage systems and therefore cause floods. Where the soil is sandy and freely drained, some of the water will be absorbed quite quickly, but in some areas not much water will be absorbed and the area will still remain in a drought. For more information about groundwater, the US Geological Survey has produced a ‘Water Science for Schools’ page, which explains it with diagrams.
A combination of lack of rain or sparse and insufficient rain for a few months, low groundwater levels, dry heavy compacted soils, irrigation needs, low reservoir storage and high demand will result in an area being declared a drought area, even though it is currently experiencing heavy rainfall and floods.
Effective water resources and drought management starts with planning, both in the long-term and short-term. Throughout a drought, the water companies monitor water availability by collecting data about the state of their water resources. As no one knows for certain when a drought will end; there is a need to conserve supplies so they last as long as possible.
Given the length of time we have been experiencing rainfall in the UK recently, the EA is probably re-assessing the drought areas status too. The UK, especially the South of England, had more rain during April than it did during the whole period from January to March, as shown in the rainfall anomalies graph, from the UK Met Office, below. The figures are percentage of the 1971-2000 average. Hopefully, this amount of rain will go some way to replenish the UK water reserves!
Graph source - http://www.metoffice.gov.uk/climate/uk/anomalygraphs/
So, are we still in a drought?
The EA released a briefing paper last week, 4th May, saying that "despite the amount of rain received throughout April, East Anglia, the Southeast of England, South and East Yorkshire, the Soutwest of England and the Midlands still reamain in drought." The briefing added that the continuous April rainfall has indeed eased the situation for farmers, gardeners and wildlife, and has increased water levels in a number of public water supply and farm reservoirs. After two years of exceptionally dry weather, the continuous rain in April will have started to restore water levels below ground and has given us a better start to the summer than anticipated, says the briefing. Nevertheless, it will take more time and more rain to undo the effects of two dry winters on groundwater stores.
Will there be more droughts under climate change?
Dr Richard Betts, from the Met Office Hadley Centre, explains, in a You Tube video, the different definitions of drought and what we might expect to see under climate change. With regard to drought, whether or not there will be more droughts under a hotter climate, Dr Betts points out that high temperatures tend to promote evaporation, which means a tendency to move toward a drought status. However, on the other hand, carbon dioxide (CO2) in the atmosphere may increase the efficiency of water use by plants, plants tend to extract less water from the soil under high CO2 concentrations and that to some extent may offset some of the effects of drought.
Link to the EA drought page.
Link to EA report: Managing drought in England and Wales.
Link to EA Briefing Paper.
Link to transcript of video.