Drought is the condition of critically low water supply caused by persistently below-normal precipitation. Drought begins with decreased soil moisture as well as surface and underground water supplies that, in turn, lead to suppressed plant growth and restrictions in the use of surface and underground water supplies for domestic, municipal, agricultural and industrial purposes.

Drought can also temporarily disturb the ecological regime, creating conditions favourable to the spread of certain insect pests and plant and animal diseases.

Drought-tolerant plants tend to displace conventional vegetation, and wild and domestic animals put stress on the reduced vegetation, thereby adversely affecting future plant growth. Drought also creates conditions favourable to forest fires.

Droughts have always occurred throughout the world. Prehistoric droughts can be inferred from evidence left in tree rings, lake and ocean sediments and ice cores from glaciers. Evidence is also available from ancient writings and diaries. A few examples of major more recent droughts include those in the Great Plains of North America in the 1930s; the Sahel (a semiarid region between Ethiopia and Senegal) in the 1970s, 1980s and early 1990s; Great Britain in 1975-76; California in 1976-77; southeast Asia in the 1990s; and Australia in the first decade of the 21st century.

Types of Droughts

There is no universally accepted definition of drought; definitions depend on how people are affected. Therefore, criteria may include factors such as precipitation, stream flow, lake and reservoir levels, groundwater supplies, soil moisture, crop yields, pasture conditions and economic hardship.

Meteorological Drought

Meteorological drought is measured by the deficiency in precipitation compared to the normal "expected" precipitation. Evaluation of such a drought requires consideration of factors such as total precipitation over a given period (week, month, year) and length of time between significant rains, as well as when they occur.

Agricultural Drought

Agricultural drought occurs when soil moisture supplies are not adequate for crop production. It is caused by meteorological drought, and its severity is often increased by high evapotranspiration losses. The practice of summer fallowing lessens the severity of an agricultural drought by carrying over soil moisture from the previous year. In areas where irrigation water is available and stored water supplies ample, a meteorological drought may not lead to an agricultural drought.

Hydrological Drought

Hydrological drought is measured by the decrease in available water supply in flowing streams and in surface and underground storage. Hydrological drought is primarily caused by a deficiency in precipitation and further intensified by high evaporation, but factors such as demand for water, availability of surface-storage reservoirs and development of wells to use underground storage affect its impact.

Socio-Economic Drought

If meteorological, agricultural and hydrological drought persists for an extended period of time, a socio-economic drought often results. This type of drought can be defined as an unusual shortage of water that produces an adverse effect on society and the economy.

Weather and Drought

Atmospheric circulation usually provides a progression of low- and high-pressure systems across a region. Low-pressure systems often produce widespread rains and trigger the formation of convective storms, which result in locally heavy thundershowers. Occasionally a situation known as blocking develops when a high-pressure system becomes anchored over a region, causing storm-producing low-pressure systems to be steered well to the south or north. Such blocking can prevent normal rainfall for weeks, months or even years.

While the above is a common means of drought production, recent research has also indicated that other circulations can also lead to drought. For example, an extended period of cold, northerly flow led to severe drought conditions over parts of the Canadian Prairies in 2009.

The causes of blocking and other flow conditions that lead to drought are not yet understood. Therefore, efforts to provide long-range drought predictions have been limited but it is a very active area of research.

Research on drought prediction includes numerical modelling with focal points being large-scale atmospheric circulations, persistent sea-surface temperature patterns, precipitation processes and land-atmosphere interactions; studies of drought cycles; identification of physical predictors; and the search for matching weather patterns or analogies from the past.

Canadian Droughts

Although most areas of Canada experience droughts, it is the agricultural region of the Canadian Prairies that is most susceptible. Written accounts of past droughts (in newspaper articles and in diaries of settlers and explorers) refer to major droughts during the 1800s on the Prairies. Paleo-records over this region indicate that droughts have occurred frequently and actually extended for decades. A more recent major drought was recorded for the region in the 1930s.

Local droughts of shorter duration have occurred since then in parts of Canada, especially in the Prairies (late 1950s/early 1960s, 1980s and 1999-2005). The drought in 2001-02 virtually spanned the entire southern half of the country stretching from British Columbia to the Maritimes.

In Canada the effects of droughts have been felt most severely in areas where agricultural activities allow little margin of safety in the water supply. One such area was identified by explorer Captain John Palliser as a result of his field work (1857-60). This dry area (later referred to as Palliser's Triangle) is an irregularly shaped area roughly defined by lines joining Cartwright, Manitoba; Lloydminster, Saskatchewan; and Calgary and Cardston, Alberta. Annual precipitation in the Triangle averages 380 mm, compared to 800 mm at Toronto and 1400 mm at Vancouver and Halifax.

The Canadian drought of the 1930s is of special importance because of its areal extent and severity, and because of the government policies, programs and farming practices that resulted. The drought began in 1929 and continued, with some respites, until midsummer of 1937. Some 7.3 million ha, one-quarter of the arable land in Canada, was affected. Severe wind erosion of the topsoil compounded the effects of the drought. Consequently, many farms were abandoned and farm families moved elsewhere (see History of Agriculture; Great Depression).

The drought of the 1930s brought into being federal and provincial government agencies to develop and manage drought-alleviation programs, the most notable being the Prairie Farm Rehabilitation Administration (PFRA) created by an Act of Parliament in 1935. PFRA was established to help prairie farmers cope with drought by providing financial and technical assistance in the building of water-storage reservoirs such as dugouts and small dams. Submarginal farming lands were taken over by PFRA, regrassed and turned into community pastures. The farmers were assisted in moving to more suitable areas, including areas on which farm irrigation was being practised.

The Dominion experimental farm system was expanded to provide research and advice to farmers on cropping and cultivation practices more suitable to drought conditions (see Agricultural Research Stations). The federal Indian Head Tree Nursery provided trees, free of charge, to prairie farmers for shelter belts to control soil drifting and to trap snow. New tillage machines were developed, which disturbed the soil as little as possible and therefore decreased soil susceptibility to wind erosion.

Alleviation programs developed during the 1930s have been modified and expanded on the basis of experience gained during subsequent dry periods. Agencies of the governments of Alberta, Saskatchewan and Manitoba developed programs, alone and in combination with federal agencies, to provide facilities enabling overland pumping of water to fill small reservoirs, cattle-feed assistance, and wells and irrigation projects.

Future Droughts

Based on recorded and inferred evidence, droughts are a recurring phenomenon. Given the ongoing issue of climate change and generally warmer temperatures, there is concern that droughts will increase in frequency in the future although there is considerable uncertainty.

The difficulty of forecasting the timing, extent and severity of droughts should not deter the development of contingency plans. In many areas of Canada, water uses for agricultural, municipal and industrial purposes are rapidly reaching the limits of natural supplies. Thus, there is always the possibility that a severe drought could cause a serious disturbance to the Canadian economy. Study and planning could alleviate many of the hardships of a drought.

One necessary prerequisite in planning for future droughts is the gathering of information. Precipitation, stream-flow and water-level data, gathered by Environment Canada in particular, are supplemented by information collected by provincial agencies. Statistical analyses can be made to determine long-term average conditions, the possible start of a drought and the severity of an ongoing drought. Mechanisms to alleviate drought hardships can then be activated.