Drought - a meteorological feature of all climates
Drought, like aridity, is a meteorological phenomenon. However, while aridity is a permanent climate feature of a region, droughts are best described as “extreme events”. On a year-to-year basis, droughts are one of the most costly natural hazards, affecting many of the essential elements of coupled human-natural systems. Palmer defined droughts as an “interval of time, generally on the order of months or years in duration, during which the actual moisture supply at a given place rather consistently falls short of climatically expected or climatically appropriate moisture supply.” In other words, as Redman succinctly noted, droughts represent periods where there is “insufficient water to meet needs.”
Droughts occur in all climates, from wet to very dry. Since they are temporary aberrations, persisting over months or years, they vary significantly from one region to another and even within a given region. The nature of a drought may be exacerbated or moderated by air temperature, antecedent soil moisture, frequency and duration of prior droughts and so forth. Drought can be perceived as being more severe in drylands as it is easier to exceed tipping points where total crop failure is possible. The exact manifestation of any type of drought depends on the sector (biophysical or socio-economic) analysed and the related processes and their impacts. Depending on the prevailing impacts, various types of droughts are recognised, for example: meteorological drought (precipitation deficiencies against some norm for a given period); agricultural drought (soil moisture deficiencies that adversely affect crop plant development and yields); hydrologic drought (reduction of streamflow, reservoir storage and lowering of groundwater levels); and socio-economic drought (demand for an economic good exceeds supply as a result of a weather-related shortfall in water supply).
What is a drought?
From a climatic point of view, a drought results from a shortfall in precipitation over an extended period of time or from the inadequate timing and consequently the ineffectiveness of the precipitation. This situation may be exacerbated by high temperatures, strong winds, atmospheric blocking patterns and antecedent conditions in soil moisture, reservoirs and aquifers, for example. If this situation leads to an unusual and temporary deficit in water availability, it is called a drought. It is distinguished from aridity, a permanent climatic feature and from water scarcity, a situation where the climatologically available water resources are insufficient to satisfy average long-term requirements. Droughts have significant impacts Unlike other extreme events or natural disasters, droughts develop slowly over time and over large areas. Their impacts cascade through the hydrological cycle, affecting soil moisture, reservoirs, river flows and groundwater. Ultimately, droughts impact many different sectors of human societies and the natural environment (e.g. wildlife habitats) and over varying time frames (some immediate, some long-term). Prolonged or repeated droughts may lead to progressive land degradation, especially in semi-arid and arid regions characterised by fragile ecosystems with naturally limited and highly variable water resources. Whereas the human populations in these regions are especially vulnerable, the processes involved are not simple. They are characterised by complex interdependences of coupled socialenvironmental systems, which include economics, public policy, antecedent rangeland conditions, management and human behaviour. This is illustrated in an study of the impacts of shortand long-term droughts on grazed Australian rangelands over the past century. Seven major ‘degradation episodes’ were found to share three common sets of events:

1. a period of favourable climate and economic conditions that encouraged increased stocking rates;
2. these favourable periods were followed by a major drought with a commensurate decline in markets, which paradoxically made reducing livestock stocking rates financially unattractive to ranchers, further exacerbating the pressure on rangeland ecosystems;
3. invariably, rangeland degradation occurred with declines in grazing production. Recovery (where it occurred and its rate) depended on rangeland condition, the type of subsequent seasons (wet versus dry), market forces and social conditions.

How do we measure drought?
Droughts are relatively easy to monitor, largely due to their slow onset, which allows enough time to observe changes in precipitation, surface water, snowpack, groundwater supplies, temperature and soil moisture in a region. The World Meteorological Organization (WMO) has complied a handbook of drought indicators to help track droughts and guide early warning and assessment. Like other hazards, droughts can be characterised in terms of their severity, location, duration and timing. Drought indicators are usually normalised against the longterm (> 30 years) statistical distribution of the measured variable in order to derive anomalies. This allows for the comparison of data from different climatological regimes and the relation of the results to probabilities and return periods. Indicators range from very simple ones (e.g. precipitation) to complex formulations that may include anomalies of the photosynthetic activity of vegetation, river flows and other meteorological, hydrological and satellitebased indices. The WMO identifies three general methods for monitoring droughts: single indices, multiple indices and composite or hybrid indicators or indices.
Three simple examples from the handbook are briefly described below:

1. Standardised Precipitation Index (SPI): The SPI is an indicator that can be calculated for different rainfall accumulation periods (e.g. 3, 6, 9, 12 months). SPI values are given in units of standard deviation from the standardised mean. Negative values correspond to drier periods than normal, and the magnitude of the departure from the mean is a probabilistic measure of the severity of a dry event.
2. Standardised Precipitation Evaporation Index (SPEI): The SPEI is calculated in a similar manner to the SPI, including however the effect of temperature on the evaporative demand.
3. Soil Moisture Anomalies (SMA): Uses precipitation and PET values in a simple water balance equation.

Exposure and vulnerability
The environmental and socio-economic impacts of a drought stem from the duration, severity and spatial extent of the precipitation deficit, but also from the exposure of different goods, assets and activities to a drought event and from the environmental, social and economic vulnerability of the affected regions and societies. Key factors that exacerbate the impacts are inadequate land-use practices, unsustainable management of water resources and inadequate risk management.
Drought and climate change
Global warming is expected to increase the frequency, duration and severity of droughts in many parts of the world. Although there is some uncertainty, increasing evidence suggests that this is due to higher temperatures, which enhance dry conditions and thus evaporation if moisture is available. Drier soils and less recycled moisture in the atmosphere is a recipe for increased intensity, frequency and duration of droughts. In a modelling study with an ensemble of 35 models, it was found that there is a high probability of increases in the global severity of drought by the end of 21st century, especially in South America and Central and Western Europe, where the frequency of drought may increase by more than 20 %. Such changing conditions will undoubtedly impact social-environmental systems in fragile drylands in the world. In spite of the fact that droughts are complex phenomena involving various phenomena (such as precipitation, temperature, soil moisture, snowpack and responses of the human and natural system on a variety of timescales) there is a growing consensus that even specific extreme weather and climate events like drought can be attributed to anthropogenic climate change.