Drought

Definition

Drought is a severe natural hazard that affects more people than any other natural disaster. It is usually only recognized as a natural hazard when social, economic, or environmental impacts become apparent. Drought is different from many other natural hazards in that it lacks easily identified onsets and terminations (Maybank et al., 1995). It is also unusual in that it is a hazard of scarcity rather than one of excess. Drought is a natural, recurring pattern of climate that occurs within nearly all climatic regions. However, it is not just a physical phenomenon or natural event caused by changes in climatic conditions. Rather, drought results from a connection between the natural event of lower than expected precipitation, and the demand of human usage on water supplies (Wilhite, 2000). Anthropogenic activities can exacerbate the severity and impacts of drought, but within a natural variability range.

Drought does not have a universal definition; rather it has hundreds, as Wilhite and Glantz (1985) discussed in their classification study. Despite the number of definitions, many are not useful to policy makers or scientists. This causes some uncertainty in declaring whether a region is suffering from drought, and its degree of severity. There are three major characteristics of drought – intensity, duration, and spatial extent. Drought is never small scale or short term (by definition). The effects of drought can build up over lengthy periods of time, and these effects may be felt for years after the drought has “broken,” making the onset and conclusion of drought difficult to define. “Seasonal” droughts are frequent and predictable, as distinguished from “supra-seasonal” droughts, which are aberrant and unpredictable (Bond et al., 2008). It is generally accepted that drought can be divided into four categories based on disciplinary viewpoints: meteorological, hydrological, agricultural, and socioeconomic (Wilhite and Glantz, 1985). Figure 1 (modified from Wilhite, 2000) shows the relationship between the various categories of drought and their durations. Each discipline incorporates different physical, biological, and/or socioeconomic factors in its definition (Wilhite, 2000), but common to all these is inadequate precipitation.

Drought, Figure 1

The relationship between types of drought and duration of drought events (Figure modified from Wilhite, 2000, p. 10.).

A working definition of meteorological drought is “an extended period (season, year, or several years) of deficient rainfall relative to the statistical multiyear mean for a region” (Druyan, 1996a). A lack of rainfall does not necessarily constitute drought. It must be distinguished from aridity, which occurs in areas where there is a high probability of low rainfall for indeterminate periods of time (Druyan, 1996b). Meteorological drought must be defined on a regional basis, as deficiencies in precipitation are specific to local atmospheric conditions. Once meteorological drought establishes itself, both agricultural and hydrological drought usually follows.

Hydrological drought is associated with the effects of a persistent scarcity in rainfall on the capacity and availability of surface water (e.g., rivers, lakes, reservoirs) and groundwater supplies. The frequency and severity of hydrological drought is often defined on a catchment or basin scale. The commencement and finishing of groundwater drought both usually lag well behind that of surface water drought (Bond et al., 2008), and both are usually out of phase with meteorological and agricultural drought. As a lack of expected rainfall continues, water levels in temporary water bodies decrease, and they eventually dry up. The drought also decreases water levels in “perennial” surface water bodies, and if it continues long enough, these may also disappear. As surface waters are depleted by ongoing drought, groundwater levels may also decrease over time in those aquifers influenced by modern recharge. This can exacerbate the effects of drought in surface water systems in which groundwater forms the base flow. After a return to normal rainfall conditions, surface water drought usually breaks well before groundwater drought.

Agricultural drought is associated with a shortage of available water for plant growth. It is assessed as insufficient soil moisture to replace evapotranspiration losses, and links meteorological and hydrological drought to impacts on agriculture. Most regions can be affected by agricultural drought, but its duration and intensity varies greatly between climatic zones (Wilhite, 2000). There are many definitions of agricultural drought, but in general they account for the varying susceptibility of crops during development to deficient topsoil moisture.

Scientists tend to frame the broad social dimensions of droughts into a general category called “socioeconomic drought” (Kallis, 2008). Socioeconomic drought associates supply and demand of economic goods with at least some elements of meteorological, hydrological, and agricultural drought (Wilhite, 2000). It can result when the demand for economic goods exceeds supply because of a shortfall in water related to variations in climate. It can also occur when the demand for goods increases due to population increase and/or per capita consumption.

Drought indices

Measurements of the frequency and severity of droughts are important in the development of mitigation strategies and preparedness plans. As in drought definitions, it is generally agreed that there can be no universal drought index or operational definition (Kallis, 2008), and so numerous indices have been developed to monitor and measure drought. Droughts differ in three major ways: intensity, duration, and spatial coverage (Wilhite, 2000). Intensity is related to the precipitation deficit, and several indices measure how precipitation has deviated from historical norms. The duration of drought is a discerning characteristic, which along with intensity and timing, is closely related to the level of impact. The spatial characteristics of drought also differ as the degree of severity evolves across areas and through seasons.

Drought and its severity can be numerically defined using indices that integrate temperature, precipitation, and other variables that effect evapotranspiration and soil moisture (IPCC, 2007a). The simplest of these assess meteorological drought using a measure of the precipitation deficit over a particular time period, whereas those more complicated use models that incorporate soil moisture conditions and land-use parameters (Oladipo, 1985).

One of the main difficulties with using indices that measure precipitation deficiency is setting the threshold below which the onset of drought is defined (Wilhite, 2000). The Palmer Drought Severity Index (PDSI; Palmer, 1965) is one of the most extensively used meteorological drought indices across the world, and particularly in the USA. PDSI is a reflection of how much soil moisture is currently available compared to that for normal or average conditions (Cook et al., 2007). The PDSI was one of the first methods to successfully quantify the severity of droughts across different climates. The index is based upon a primitive water balance model which accounts for the difference between precipitation required to maintain a water balance and the actual precipitation. The PDSI also incorporates calculations that attempt to account for climatic differences between locations and seasons of the year (Wells et al., 2004). Despite its popularity, the PDSI has been widely criticized for its empiricism (Keyantash and Dracup, 2002). It does not incorporate variables such as wind speed, water vapor, or solar radiation into its calculation of potential evapotranspiration. Commonly, it is said that calculated PDSI values are not comparable between diverse climatological regions. This led to the development of a self-calibrating version of the PDSI by Wells et al. (2004) to ensure consistency with the climate at any location. A relatively new precipitation deficit index used in the USA is the Standardized Precipitation Index (SPI) developed by McKee et al. (1993, 1995) in recognition of the impacts that precipitation deficit has on groundwater, soil moisture, streamflow, and other water resources. It was designed to quantify precipitation deficit for multiple timescales, allowing for the determination of the rarity of drought as well as the probability of precipitation necessary to break a drought. In Australia, the drought definition is based on the Rainfall Deciles method (Gibbs and Maher, 1967), chosen because it is relatively simple to calculate, and requires fewer assumptions than the PDSI (Smith et al., 1993).

Similar to the PDSI is the Palmer Hydrological Drought Severity Index (PHDI), with the primary difference being stricter criterion for the ending of a drought (or wet spell). This is considered more appropriate for hydrological drought assessment, as it is much slower to build up than meteorological drought (Keyantash and Dracup, 2002). Shafer and Dezman (1982) developed the Surface Water Supply Index (SWSI) to account for snowpack and delayed runoff, and it is useful in providing a measure of hydrological drought in areas where snow makes up a significant component of the hydrological budget.

Agricultural drought is specifically related to cultivated crops rather than natural vegetation (Keyantash and Dracup, 2002), and it is characterized by short-term changes to volumetric soil moisture in the root zone. The Crop Moisture Index (CMI) was developed by Palmer (1968) and uses a meteorological approach to monitor agricultural drought. The CMI was developed from procedures within the PDSI, but it was designed to measure short-term moisture conditions across crop-producing regions, rather than monitoring long-term meteorological drought like the PDSI (Hayes, 2009).

Many parts of the world have not adopted clear indices for agricultural drought and hydrological drought, making attempts to compare the impacts of drought between places and between times difficult (Bond et al., 2008). In the context of climate change and increasing land degradation, it is becoming increasingly important to be able to calculate drought impacts if the consequences of climate change are to be understood (Vicente-Serrano, 2007).

Impacts of drought

Drought, as one of the most complicated yet least understood natural hazards, is associated with many other kinds of hazard, and these play out in impacts on economic, social, and environmental systems (Kallis, 2008). The onset of drought is difficult to identify or even recognize, although predictive capabilities are increasing. The study of past droughts can indicate what onset might look like, how drought develops and the kinds of impacts that follow. The palaeo record shows that severe droughts of the last century were greatly eclipsed by megadroughts in the past (Maybank et al., 1995; Woodhouse and Overpeck, 1998). These will occur again, and are likely to be exacerbated by greenhouse warming.

There are methodological problems in assessing the impact of droughts due to the difficulty of defining it. However, the most obvious first-order impacts are through drought impact on agricultural production, water supply, and forestry. Forests are usually less sensitive to drought as they tend to occur in wetter regions.

Reduction in crop and animal production has secondary affects on food prices and may feed through to global markets and consumer demand (Kallis, 2008). Reduction in river flows may have consequences for water supply, hydroelectricity generation, and the amount of potable water. Poor quality water can have significant negative health outcomes for affected populations.

Drought takes a heavy toll on life in Africa, causes social disruption in Asia, and has economic impacts in Western countries. Exposure and vulnerability have strongly regionalized patterns; where drought coincides with war, poverty, or recession, the impact is magnified and exposed population are made more vulnerable (Kallis, 2008).

There is a significant difference between aridity and drought. Deserts occur in areas where there is extreme heating of the surface, and/or lack of moisture. These are created when subsiding air, which becomes compressed and thus heated, forms subtropical high-pressure zones. The deserts of Australia, Peru-Chile (Atacama), southwestern USA, Namib, Sahara, and Kalahari are of this type. Additionally, deserts occur in the lee of major mountains: Patagonia, Middle East, central Asia, Ethiopia, and the Thar (India) are examples of these.

Deserts are naturally dry most of the time, and thus drought is not a hazard in them in the strictest sense. However, droughts can be a normal weather pattern in all regions.

Projected precipitation anomalies estimated from regional climate models depend heavily on the scenario applied for the simulations. The IPCC (2007b) has applied a relatively large number of simulations (21) and these show a high degree of consistency. Drought conditions will be exacerbated whenever simulations suggest a decline in precipitation, especially outside the natural regions of aridity. Figure 2 (modified from IPCC, 2007b) shows that the main areas of predicted precipitation decrease are:

• 1. Annual decrease in the Mediterranean region, northern Africa, Central America, and SW USA.

• 2. Winter decrease in SW Australia, eastern French Polynesia, southern Africa.

• 3. Winter and spring decreases in southern Australia.

• 4. Decrease in snow season length and likely snow depth in Europe and North America.

• 5. Summer decrease in the southern Andes, southern SE Asia, SE South America, central Asia, central Europe, and southern Canada.

Drought, Figure 2

The fraction of 21 atmosphere–ocean global climate model simulations that predict a decrease in mean precipitation in a model grid cell (comparing the period 2080–2099 with control period 1980–1999) (Figure modified from IPCC, 2007a, p. 859.).

The effect of decreased precipitation will be enhanced with higher temperatures. The main regions of impact are outside the tropics and high latitude zones, and areas with winter and spring dominated rainfall patterns will be particularly disadvantaged, as will midlatitude areas dependent on snow melt for water supply. The burden of enhanced drought will fall quite unevenly across the nations of the world. These same regions will need to develop robust adaption and mitigation strategies to reduce future vulnerability to drought.

Historical impact of drought

Severe drought can have serious consequences for exposed societies. The degree of exposure depends on the kind of drought and the resilience of the society. All but the least resilient of societies can weather single seasonal droughts when they occur at some kind of recurring interval. They often do this by building reserves that can be drawn upon in times of need. Societies meet the most challenging of situations when long sequences of unexpected drought conditions occur. These may be due to rare and essentially unanticipated sequences or due to a shift in climate pattern. It is expected that climate change will, as it always has, alter the geographical patterns and severity of droughts.

In the past, many societies have encountered unexpected enduring drought conditions and they have had to adjust and adapt, migrate, or they collapse. One can wonder at the thought processes that accompanied these circumstances. Initially, a poor season would have placed strains that would have been endured with the expectation that “normality” would return the next season. After all, this was what experience shows to be the case. A string of poor seasons challenges this experience, and the longer the sequence the more challenging this becomes. When do societies accept that conditions have indeed changed and adjustments must be made? This dilemma has been met before and will be visited upon many societies in the future.

An example of how this occurred in the past concerns the Classic Maya Civilization. The Maya occupied the Yucatan Peninsula region of Mesoamerica from 250 to 850 AD. The Late Classic culture (550–850 AD) was known for being a highly stratified society; there were vast trade networks, and widespread construction of urban centers and monuments. Complex language, belief systems, sports, and mathematics were embedded as elements of society. This all came to a sudden end when society seemed to be at its peak. Many potential factors have been cited for the collapse and include deforestation, overpopulation, warfare, and social upheaval for political reasons.

Recent research suggests prolonged drought was at least a contributing factor. Lake sediments reveal substantially lowered water levels and changes from freshwater to saline conditions (Hodell et al., 2001), reduction in forest cover according to pollen diagrams (Mueller et al., 2009), and increased soil erosion is recorded in marine sediments (e.g., Gischler et al., 2008). These indicate substantial environmental change, which coincides with the main phase of collapse of the Maya in terms of buildings and the desertion of urban centers. In the latter phase of the Maya, Sun God worship was evident, and this may have been an attempt to appease the Sun as the cause of ongoing drought.

Similar fates are thought to have transpired to the Harappan people of NW India as an arid phase developed over the region and made extensive irrigation systems become dysfunctional, and the base which supported a huge urban population was swept away (Staubwasser et al., 2003).

In some cases, human activities have appeared to exacerbate the impact of drought. In northern China, there is an environmental boundary between the loess and desert. Loess is windblown dust deposited by the Westerlies and is a highly productive soil where there is sufficient rain or irrigation that can be applied to it. In the northern region the loess gives way to desert sand-dominated soils, these are mobile and rarely watered by the monsoon rains which sweep in from the Pacific Ocean. The border region of the Northern Loess Plateau and Chinese deserts west of Beijing supported many Neolithic villages in the mid-Holocene. It appears that monsoon rain reached the region and provided sufficient water for millet-based agriculture and animal husbandry. By about 3,000–4,000 years ago villages were abandoned as desertification set in (Zhou et al., 2002). This may have been due to drought resulting from failure of the monsoon rains reaching the region, perhaps in concert with anthropogenic driven land degeneration. In any case, the desert sands shifted some hundreds of kilometers south, and so did the villages.

The observational record of drought

Observations of drought based on meteorological records indicate that they have become more intense, of longer duration, and occurred over wider areas of the tropics and subtropics since the 1970s (IPCC, 2007a). Reliable meteorological records for much of the world only exist for the last 100 years or so, but they provide a basis to investigate possible causes for drought.

Since the 1950s, the number of heat waves and warm nights has increased. These have contributed to the area under drought, although the drivers of changes in precipitation are also very important. While increases in continental temperatures are important for some regions, changes in snowpack and sea surface temperatures related to phenomena, such as El Niño–Southern Oscillation, are also strong drivers for climate in other regions. Extreme events such as the drought for western North America (Canada to Mexico) in 1999–2004 seem to be strongly related to a diminished snowpack and hence runoff (McCabe et al., 2004; Stewart et al., 2004). These in turn may be driven by sea surface temperatures in the tropical Pacific (Herweijer et al., 2007). Recent Australian droughts correlate well with higher continental temperatures, and the 15% decline in precipitation for southwestern Australia since the 1970s (mostly a failure of early winter precipitation) is related to sea surface temperature variation in the tropical Indian Ocean (Samuel et al., 2006).

Drought in the Sahel is due to failure of rainfall. Simulations have been good at reproducing the decadal variations in Sahel rainfall (Held et al., 2005) and these suggest sea surface temperatures of the Indian Oceans and Mediterranean are significant drivers of this, as is sulfate aerosol concentration (Rosenfeld et al., 2008).

Overall, the increased risk of drought, as measured by the Palmer Drought Severity Index suggests that the anthropogenic fingerprint is there, but simulations have this as a weaker component than the observed occurrence of drought (IPCC, 2007b).

Future vulnerability to drought

Observations show drought has already increased. Models can be used to simulate possible future drought intensity, frequency, and extent. In general, these suggest that the trends already seen can be expected to intensify, and the increase will be between 1% and 30% of land area in the next few decades – with greatest increase in midlatitude areas. The Mediterranean, western USA, Southern Africa, and northeastern Brazil are all expected to see intensification of drought. Russia, Mongolia, China, southern SE Asia will see drought intensification due to higher temperatures in the summer and drier months and due to changes in ENSO (IPCC, 2007b), and poleward migration of annular weather modes (Yin, 2005; Menéndez and Carril, 2010).

The impact of drought will be intensified due to human population increases. About one sixth of the world’s population relies on meltwater, and reduced snowfall and snowpack will result in a reduction in delayed runoff. People in Bolivia, Ecuador, Peru, and the Hindu Kush – Himalaya are particularly vulnerable to this (Barnett et al., 2005). Soil moisture deficits will reduce pasture growth in the eastern South Island and Bay of Plenty regions of New Zealand (Mullan et al., 2005). There will be increased fire danger in seasonal environments of the midlatitudes (Gonzalez et al., 2010).

The high cost of drought, for example, in Australia in 1982–1983 ($2.3b), 1991–1995 ($3.8b), and 2002–2003 ($7.6b) (IPCC, 2007b), has already driven measures for adaption. A range of options have been used or are being considered for vulnerable areas. These include increased rainwater harvesting, adjustment of silvicultural techniques, channel and pipe leakage reduction and modifying crop planting dates, and choosing varieties which are more drought resistant.

Models also suggest precipitation extremes will be more prevalent, but the gaps between high magnitude events will increase, and so will the likelihood of drought. Of course the scale of these changes will depend on the willingness of nations to reduce the size of the anthropogenic fingerprint on global warming.

Mitigation

As a natural element of climate, the recurrence of hydrometeorological drought is inevitable. However, drought can also be exacerbated by anthropogenic influences such as rapid population growth, excessive water demand, and land degradation, and vulnerability to these impacts can be mitigated by appropriate drought plans (Rossi et al., 2005).

The uncertainty about drought definition leads to uncertainty about its characteristics and impacts, which contributes to poor drought management and mitigation across many parts of the world (Wilhite et al., 2007). A key element in any drought plan is a set of indicators that characterize drought conditions, and location-specific triggers (indicator values) which prompt some kind of response. Unfortunately, drought plans often contain ad hoc indicators and triggers that lack scientific validation or operational relevancy, and this can weaken the effectiveness of the mitigation plan (Steinemann and Cavalcanti, 2006). Other factors contributing to the difficulty of developing an effective drought plan include the changing spatial and temporal scales of drought impacts, the unique characteristics of each region or watershed, and changing societal structures and demands, to name just a few. Even though an existing drought may be of similar intensity and duration to one that has occurred in the past, changes in socioeconomic structures and environmental conditions can result in strikingly different impacts, and therefore changing vulnerability (Wilhite et al., 2007).

According to Wilhite (2000), drought mitigation is “short and long-term actions, programs, or policies implemented during and in advance of drought that reduce the degree of risk to human life, property, and productive capacity.” These measures can be classified as either proactive or reactive. The proactive measures are prepared according to a planning strategy rather than in an emergency situation (Rossi et al., 2005). The most effective actions are long-term measures taken in advance of drought, such as building infrastructure to increase the reliability of water supply under increasing demand and drought conditions (Dziegielewski, 2003). Short-term measures are taken after the onset of drought, and these are aimed at mitigating impacts within existing infrastructure and management policies. An effective mitigation strategy will contain an appropriate mix of long- and short-term actions to reduce the vulnerability of human life, property, and production to future droughts.

Mitigating agricultural drought

Vulnerability, and therefore appropriate mitigation actions, differs significantly between the developing world, where drought can lead to livelihood loss, famine, and even death, and the developed world, where impacts are usually economic or asset losses. Numerous mitigation measures have been formulated to reduce the impacts of drought, and especially that of agricultural drought because of its huge environmental, economic, and social costs (Maybank et al., 1995).

Approximately, 80% of the world’s agricultural land is rainfed (Rockström, 2003), so developing mitigation strategies to build ecological resilience in drought-prone and semiarid agricultural land is very important for food security. To a certain extent, water harvesting through small-scale systems such as farm ponds and subsurface tanks, can help mitigate the impacts of drought or dry spells in these areas. The building of ecological resilience to drought also requires strategies such as conservation farming (minimal or no tillage), improved crop varieties, and soil fertility management.

Decreases in agricultural production can have a roll-on effect leading to financial disaster for farmers and higher food prices for all consumers, unemployment, and even migration. Water is frequently wasted in agriculture practices through over-irrigation, poorly designed canals, and inefficient irrigation systems, and this waste can be reduced through adoption of improved channeling and irrigation practices (Le Houerou, 1996). A multidisciplinary approach of genetic improvement and physiological regulation to increase crop water productivity is another way to help achieve efficient and effective use of water (Cattivelli et al., 2008). Combining these biological water-saving measures with engineered solutions (e.g., water-saving irrigation methods) and agronomic and soil manipulation will contribute to an effective drought mitigation strategy for agriculture (Ali and Talukder, 2008).

Mitigating hydrological drought

Mitigation of hydrological drought primarily involves optimal water supply management under drought conditions, that is, making water more productive. This requires a contingency plan that includes a systematic evaluation of drought conditions with associated responses.

Traditionally, mitigation has focused on increasing water supplies through the construction of dams and reservoirs to capture and store increasing fractions of surface runoff. High levels of surface storage can effectively buffer against low runoff periods, especially in regions that experience high interannual variability in river flows (Bond et al., 2008). This practice was carried out with little analysis of how water was actually being used or of the impacts of this practice on the aquatic ecosystems. As new fresh surface water supplies for exploitation have dwindled, governments have turned to groundwater to augment supplies, especially during drought. However, the increased dependence on groundwater resources is leading to dwindling reserves and/or quality degradation. More and more countries are turning to nonconventional water sources to boost supplies. Desalination and waste water treatment and recycling are usually more expensive options than traditional water sources, but the associated environmental benefits can compensate for some of the costs.

Summary/Conclusions

Drought is a severe natural hazard that affects more people than any other natural disaster. It is difficult to define and recognizing its onset and termination is also difficult. It can be expressed in meteorological, hydrological, agricultural, and socioeconomic terms. The severity and extent of drought has increased in recent decades, and regional climate models suggest these will increase further in the future. The burden of dealing with drought will be unevenly distributed. Midlatitude regions and those heavily dependent on snow melt will have the greatest challenges. Multidisciplinary approaches will need to be developed to mitigate the extreme impacts of drought.

Cross-references

Adaptation

Climate Change

Costs (Economic) of Natural Hazards and Disasters

Desertification

Disaster

Dust Bowl

Hazard

Historical Events

Land Degradation

Loess

Mitigation

Models of Hazard and Disaster

Natural Hazard

Risk

Vulnerability