What is drought? A simple question; and a simple answer would be that it is an extended period of no or very low rainfall, caused by natural variation in the climate. However, things are actually more complex.
For a start, there are different types of drought depending on what or who is being affected. Meteorological drought is the first way that drought is identified. This varies from country to country, but in the UK if we have 15 or more consecutive days with less than 0.2 mm of rainfall, then we’re in a drought. In the summer, as well as the lack of rain, drought is often accompanied by higher temperatures. This means there is also an increase in moisture transpiring from plants and evaporating from soil and open water back up into the atmosphere. Therefore, the ground starts to dry out.
As the ground dries out, we start to experience agricultural drought. Plants show signs of water stress, growth slows and they start to wilt. Therefore crop production falls. Farmers and gardeners are often some of the first to feel the effects of drought. What might be a period of pleasant dry warm weather to some could actually be a period of stress and anxiety to others. Drought means different things to different people.
As the agricultural drought continues, we start to experience hydrological drought. This is a period when the lack of rainfall, potentially increased evaporation and a reduction in soil moisture cause water levels in rivers, ponds, wetlands, lakes and reservoirs to decline. How is this linked to moisture in the soil, you might ask? Soil is a bit like a sponge: rain which falls on the ground and soaks in can be held in the soil for quite a while. Some of this moisture is used by plants or evaporates, but gravity continually pulls the moisture downward so that some of the water moves deeper down through the soil into water-holding rocks called aquifers. This ‘groundwater’ then moves towards streams and rivers. If the rock below cannot hold water, the soil water will move directly towards the stream or river. When the soil is very wet (or if the rainfall is very intense), it isn’t able to hold (or soak up) more water, and therefore any rainwater will runoff quickly over the surface towards the nearest river. Conversely, as the soil dries out, the amount of surface runoff decreases and the amount of water slowly making its way underground towards rivers decreases. Therefore water levels in rivers, lakes and wetlands start to fall, causing stress on the natural environment.
When water levels are falling, many people start to notice the effects of the drought and see pictures of dry rivers and part-empty reservoirs in the news. This is also the period when we experience socio-economic drought. Drought starts affecting day-to-day activities. Recreational activities which rely on water such as fishing and water sports may be affected. Industrial usage, for example for hydro-power generation or cooling, may also be affected. Reduced water availability and possibly increased temperatures may start to affect people’s health. It may be necessary for water companies to introduce water restrictions such as hosepipe bans to help save water. Some soils shrink when they dry out and this can lead to problems with building subsidence. Again, drought means different things to different people and as the drought slowly builds up over time it can also mean different things at different times to the same person.
Drought varies in other ways. It may occur across the whole country or just in a particular region. It can vary in time. It may be a longer drought, extending over more than one year, caused by lower rainfall in the winter which isn’t enough to replenish water in the soil, aquifers, rivers and reservoirs. Or it may be a short drought caused by a particularly hot and dry summer. It can even be a combination of the two, like the drought of 1975-76. Droughts vary in intensity. Since 1800 there have been 11 major droughts in the UK, mainly lasting for more than one year. There have also been over 20 other moderate droughts in the same period1. The longest drought in this period lasted from 1890 to 1910 and is known as ‘The Long Drought’. The impact of drought also varies, depending on who or what is being affected, whether preparations for drought have been made and the level of response.
A related question worth considering is ‘What is the difference between drought and water scarcity?’ Luckily this question is a bit easier to answer. As we’ve seen above, drought is a lack of water supply caused by natural climatic variability and is usually a temporary phenomenon. In contrast, water scarcity is a situation in which demand for water exceeds supply. This is driven by human factors and usually occurs over longer time periods. Although drought is viewed as a natural phenomenon, it is also affected by climate change caused by humans. Similarly, although water scarcity is usually longer-term, it can be influenced by short-term factors such as a temporary population increase due to summer tourism. There is clearly a complex relationship between the two and neither element should be considered on its own.
1 Marsh, T. et al. (2007) Major droughts in England and Wales, 1800–2006, Weather , 62(4), p.87-93
by James Blake (Centre for Ecology and Hydrology)