Drought

What a drought is, and the three kinds of shortage

A drought is a period of dry weather that is longer or more severe than normal. The definition is relative, not absolute: a drought is measured against what is normal for that place, so there is no single rainfall figure that counts as a drought everywhere. A dry month in the west of Ireland may bring more rain than a wet month in the Sahel, yet only one of them is a drought. That is why a weather station needs a long rainfall record before anyone can say a drought has begun, and why a dry spell only becomes a drought once the rainfall, the rivers and the soils have all fallen below their usual levels.

Three types of drought are described, and they do not start at the same time. A meteorological drought is simply rainfall that is well below the long-term average for that time of year, over weeks, months or years. A hydrological drought is when rivers, reservoirs, lakes and aquifers fall below their normal level; it appears later, because a reservoir or an aquifer keeps supplying water for a while after the rain has stopped. An agricultural drought is when soil moisture becomes too low for crops and pasture to grow, and it can arrive very quickly: a few weeks of hot, rainless weather in the middle of the growing season is enough to cut a harvest, even in a country whose annual rainfall is normally reliable.

The three types explain why the same dry period causes such different damage in different places. A city with a large reservoir and a piped supply rides out a meteorological drought for months and only suffers when the hydrological drought bites. A farming district with no storage suffers at once, because the water that matters to a crop is the water in the top metre of soil, not the water in the national rainfall record.

Drought is monitored with rainfall gauges and long records, soil moisture measurements, river flow gauges, reservoir and groundwater levels, and satellite images of vegetation. Indices such as the Standardised Precipitation Index combine the records into a single number, so a government can compare this year with past years and decide when to declare a drought and release emergency funds.

• Meteorological drought: rainfall well below the long-term average for that time of year.
• Hydrological drought: rivers, reservoirs, lakes and aquifers below their normal level, usually months after the rainfall deficit begins.
• Agricultural drought: too little soil moisture for crops and pasture to grow, which can happen within weeks.
• Drought is defined against the normal rainfall of that place, not against zero rainfall.

Why the rain fails: high pressure, El Nino and a warming climate

Rain falls when moist air rises, cools and its water vapour condenses into cloud droplets. The most common immediate cause of a drought is therefore persistent high pressure. In a high-pressure system air sinks instead of rising, and sinking air warms and can hold more water vapour, so cloud does not form and rain does not fall. The same block of high pressure also pushes rain-bearing fronts and the seasonal rains aside, so the rain that would normally arrive never reaches the region. Under clear skies the sunshine is intense, temperatures climb, and evaporation from the soil and transpiration from plants remove the last of the moisture, so dry ground heats up further and the drought deepens.

The failure of the rains is how most droughts begin in the tropics and subtropics. Seasonal rainfall is tied to the movement of the intertropical convergence zone, which brings the monsoon to South Asia and the long and short rains to East Africa. If the zone stalls, or the rains start late and end early, the growing season fails. One failed season is a bad year; two or three in a row turn a harvest failure into a food crisis, because farmers have no reserves and their animals have no pasture.

Droughts are also driven by the large-scale patterns of the Pacific Ocean. During an El Nino, the surface water of the eastern Pacific warms, the trade winds weaken and the pattern of rainfall shifts across the world: Indonesia, eastern Australia and southern Africa dry out, while Peru and coastal Ecuador receive unusually heavy rain. During a La Nina the opposite happens, with heavier rain over northern Australia and South-East Asia and drier conditions in East Africa and the southern United States. Knowing which phase is developing lets forecasters warn a region months ahead.

Climate change makes droughts more frequent and more severe in most regions. A warmer atmosphere increases evaporation and transpiration, so soils dry out faster and the same rainfall produces less usable water. It also holds more water vapour, so rain tends to arrive in fewer but heavier storms, with longer dry spells in between: wet seasons become wetter and dry seasons drier. Glaciers and mountain snowpacks shrink, which reduces the meltwater that many rivers rely on in the dry season, and an increase in extreme weather events means more places experience both floods and droughts within the same year.

When people turn a dry spell into a drought

Natural rainfall only explains part of a drought. People decide how much water is taken out of rivers and aquifers, how much land is left bare and how many users depend on the same supply, and those decisions decide how severe a dry spell becomes. Over-abstraction is the clearest example: when groundwater is pumped from an aquifer faster than rainfall recharges it, the water table falls year by year. The aquifer is a store that carries a region through a dry period, so emptying it in the good years means there is nothing left to draw on in the bad ones. Wells run dry, the land may subside, and in coastal areas salt water moves in and ruins the fresh water that remains.

Demand keeps rising. A city whose population grows, a farming district that converts to irrigated crops, and new industry all take more water from the same rivers and aquifers. The rainfall may be exactly as it was, but the same dry year now causes rationing, because there are more users and more storage is needed for each of them. A drought is therefore partly a measure of how much water a society has committed itself to using.

Deforestation, overgrazing and desertification reduce the land's ability to make the most of the rain it does get. Trees and other vegetation shade the soil, their roots open it up so water soaks in rather than running off, and the water they release as vapour helps to recycle rainfall locally. When the cover is cleared or grazed away, rain runs off the surface, carrying soil with it, and the exposed soil holds less water for the next dry spell. The result is desertification: land at the edge of a desert becomes less and less productive, and the dry, bare, hot surface makes the local climate harsher still.

Upstream water use adds a further pressure. Dams and irrigation schemes built further up a river can leave the lower course short of water even in a normal year, and reservoirs in hot climates lose a significant share of their contents to evaporation from the open surface. All of these effects feed back into one another: dry soil and dry air mean less evaporation, fewer clouds and less rain, so a drought that has started can help to keep itself going.

• Over-abstraction of aquifers: pumping faster than recharge lowers the water table and removes the store that buffers a drought.
• Rising demand: population growth, irrigation and industry all compete for the same supply.
• Deforestation and overgrazing: less infiltration, more runoff and erosion, and less moisture recycled into local rainfall.
• Desertification: degraded land holds less water and supports fewer plants, so each drought does more damage than the last.
• Upstream dams and abstraction: less water reaches the lower course, and reservoirs lose water to evaporation.

Impacts on water supply, energy, industry and money

The first impact of a hydrological drought is that water sources dry up. Rivers fall to a trickle or stop flowing, reservoirs shrink and expose a ring of bare shoreline, wells and boreholes fail, and lakes retreat from their shores. Cities introduce rationing and look for emergency supplies, and water has to be moved from wherever it can be found. Low rivers also stop navigation, which is a cheap way of moving bulk goods such as coal, grain and chemicals: when the River Rhine fell to very low levels in the dry summer of 2022, barges could carry only part of their normal load and freight had to move by road and rail instead.

Energy supply is affected in two ways. Hydroelectric power depends on the flow of water and the height it falls, so a shrinking reservoir cuts output directly. The Kariba dam on the Zambezi, which supplies a large share of the electricity used in Zambia and Zimbabwe, has repeatedly fallen to levels too low to generate at full capacity, and both countries have had to impose power cuts. Thermal, gas and nuclear power stations need water for cooling as well, and in hot summers with low river flows they have to reduce output or shut down; several French nuclear plants beside the Rhone and the Garonne have had to cut generation during heatwaves because the river was too low and too warm to absorb the cooling water.

Industry and the economy suffer next. Factories that use water for washing, cooling or processing - food and drink plants, breweries, paper mills, textile works and car plants - must cut production or stop. Farms spend more on pumping and lose income from failed crops and dead livestock, and rural businesses that depend on farming lose their customers. Governments spend money on tankering water, food aid and relief payments, while exports fall and imports of food rise, so the financial losses spread far beyond the people who work the land. Insurance and reinsurance costs rise after repeated droughts, and tourist businesses from safari lodges to ski resorts can lose a season.

These losses last longer than the drought itself. An orchard or a vineyard that dies has to be replanted and takes years to produce again, herds that are sold off cannot be rebuilt quickly, and factories that move away during a drought do not always come back.

Impacts on people: food, health, fire and migration

Crops fail when there is too little soil moisture for seeds to germinate and plants to grow, and yields fall long before the crop is lost completely. Livestock weaken and die because pasture and drinking water run out, and for pastoral families the herd is the savings account as well as the income, so losing it removes their whole livelihood. Yields of cereals drop, livestock are sold cheaply to pay for food, and prices rise. Once food becomes scarce and unaffordable, famine follows: children become malnourished, resistance to disease falls, and families depend on food aid. Soil left bare by a failed crop is then blown away by wind or washed away when the rain finally returns, so the drought damages the land as well as the harvest.

Ecosystems are damaged too. Fish die as rivers shrink, warm and lose oxygen; trees die of water stress, sometimes years after the drought as their reserves run out; and wetlands that support huge numbers of birds and other wildlife dry out. The loss of habitats and the death of organisms reduce biodiversity, and recovery can take a decade when a long-lived species such as a mature tree or a coral is lost.

Wildfire and air quality are closely linked to drought. Dry vegetation, high temperatures and low humidity make a landscape ready to burn, so fires start easily and spread quickly, destroying crops, pasture, forest, wildlife and homes and forcing evacuations. Smoke from large fires raises concentrations of fine particles over thousands of square kilometres and can reach cities far downwind, causing coughing, asthma attacks and heart and lung problems. Drought also raises dust: when a lake bed or a drying wetland is exposed, wind lifts fine sediment and salt into the air, which damages air quality and crops. The dried bed of the Aral Sea is the classic example, and Lake Urmia in Iran has produced dust storms that affect towns many kilometres away.

Health suffers through both too little water and poor quality. When supplies are rationed, less water is available for hygiene, so the risk of diseases spread by dirty hands and dirty water rises even as people try harder to save. Dust, smoke and heat add respiratory and heart problems, and heat stress and dehydration affect those who work outdoors; malnutrition weakens resistance to infection. People also move: farming families leave the land for towns and cities, adding to urban slums, and in the worst cases they cross borders. Competition for water and for grazing can turn into conflict, and the severe drought in Syria in the years before 2011 drove hundreds of thousands of farming families into already crowded towns, adding to the pressures in the country.

Managing a drought before and during it: monitoring, warning and using less water

Monitoring and warning work on the idea that a drought is predictable months in advance. National meteorological services keep rainfall records, measure soil moisture and river flow, and track reservoir and groundwater levels, while satellite images show how vegetation is faring over a whole region. Seasonal forecasts, and the ocean temperature patterns behind El Nino and La Nina, allow an area to be told that the coming rains are likely to fail. Organisations such as the Famine Early Warning Systems Network and the regional climate outlook forums in East Africa classify areas by how close they are to a food crisis, which allows water tankering, cash transfers and food distributions to be arranged before people are starving. The limitation is that a forecast is a probability, not a promise: acting too early wastes money, acting late costs lives, warnings do not reach everyone, and a warning that is not followed by money and supplies changes nothing.

Emergency water supplies are the during-the-drought response: tanker trucks delivering to standpipes and to households, bottled water, emergency boreholes, mobile desalination units and the organised distribution of relief. They save lives quickly, but they are expensive, they supply only small volumes, they depend on roads that may be cut and on security that may collapse, and they maintain consumption without protecting anyone's livelihood.

Water conservation reduces demand instead of chasing supply. Metering is the foundation, because charging by the cubic metre makes people notice how much they use, and tiered or rising-block tariffs charge the heaviest users the most. Restrictions follow when the situation worsens: bans on watering gardens, washing cars and filling swimming pools. Cities also manage the network itself, reducing pressure so that less leaks away, finding and repairing leaks, and replacing old mains, and they use treated wastewater for parks, golf courses and industry so that drinking water is not used on grass. Public information campaigns, with daily reports on reservoir levels, help people accept the restrictions.

Every one of these measures has a limit. Water lost through leaks is often a larger share of supply than households use, so restrictions may look busy while achieving little. Charging more for water raises the bills of poor households who use very little and can hurt the people the policy was meant to protect unless there is a cheap basic allowance. Restrictions are hard to enforce and can close car washes, nurseries and laundries. Agriculture usually takes the largest share of water, so a city that saves a little in gardens but leaves irrigation untouched has not solved its problem. Finally, and most simply, behaviour changes slowly and reverses as soon as rain returns.

• Monitoring and warning: rain gauges and long records, soil moisture, river flow and groundwater levels, satellite vegetation images, ocean and seasonal forecasts.
• Emergency water supplies: tankering, standpipes, bottled water, emergency boreholes.
• Water conservation: metering, tiered tariffs, restrictions, pressure management, leak repair, wastewater reuse and public information.
• Food and cash relief: food distributions, cash transfers and livestock feed, arranged through the early-warning system.

Managing drought over the long term: more supply, better farming and better land care

Increasing the supply makes a region less dependent on this year's rain. Dams and reservoirs capture wet-season flow for the dry season and can also generate hydroelectricity and supply irrigation. Aquifers and wells reach groundwater that is still available when every river has stopped, but they must be licensed and monitored, because a borehole that is pumped without limit simply empties the store faster. Water transfer schemes move water from a wetter basin to a drier one through canals and pipelines. Desalination produces fresh water from the sea and is completely independent of rainfall, which is why it is used in Israel, parts of Spain, Saudi Arabia and Australia. Rainwater harvesting collects rain from roofs into tanks and cisterns, and at a larger scale stores runoff behind check dams, in farm ponds and in sand dams in seasonal river beds.

Each of these carries a cost. A large dam is extremely expensive, floods land and displaces the people living there, traps silt that would have fertilised the floodplain, changes the river downstream, and in hot dry climates loses a large part of its contents by evaporation; in a run of dry years it may not refill at all. Water transfer schemes are costly to build and to pump, they take water away from the basin that had it, and they cause disputes between regions. Desalination uses a great deal of energy, produces greenhouse gas emissions unless the power is renewable, and leaves behind brine that must be disposed of without damaging marine life; the water it produces is expensive per cubic metre. Rainwater harvesting is cheap and can be done by one household, but a cistern holds only what the roof catches, so it empties in a long drought and needs storage and maintenance.

Adapting the farming may be the most effective long-term measure of all, because agriculture usually uses most of the water. Drought-resistant and early-maturing varieties of maize, sorghum, millet and cowpea can produce a crop on less water or finish before the rains fail. Conservation agriculture - minimum tillage, keeping crop residues on the field and mulching - protects soil moisture from evaporation. Drip irrigation delivers water slowly to the roots and uses far less than flooding a field, and scheduling irrigation to the actual water use of the crop saves more. The limitations are practical: improved seed must be bought every season, many farmers cannot afford drip systems or cannot get the clean water and spare parts they need, advice may not reach them, and no improved variety survives a drought that lasts several years.

Land management reduces desertification by helping the ground to hold the rain when it comes: terracing and contour stone bunds on slopes, planting pits that trap runoff around each plant, windbreaks and shelterbelts to cut erosion, tree planting, and controlled rotational grazing to let pasture recover. Bringing trees back onto farmland - farmer-managed natural regeneration in Niger is the best-known example, and the Great Green Wall is the continent-wide version of the same idea - shades the soil and improves infiltration, but results take years and can be lost in one bad season. Finally, international aid covers the emergency end: food distributions, cash transfers that let families buy from local markets, seed and livestock restocking, and index-based livestock insurance. Aid keeps people alive, but it is expensive to deliver, needs roads and security, can depress local food prices for the farmers who still have something to sell, and creates dependency if it becomes permanent. Long-term development aid that builds storage, irrigation and warning systems is slower and less visible, and it reduces the need for emergency aid in the first place.