Drought
From HFA-PEDIA
Introduction
Drought is a normal part of climate; an extreme climatic event, often described as a natural hazard. Drought by itself does not trigger an emergency. Whether it becomes an emergency depends on its impact on local people. And that, in turn, depends upon their vulnerability to such a 'shock'. Drought results in substantial impacts in both developing and developed countries, although the characteristics of these impacts differ considerably. The ability to cope with drought also varies from country to country and from one region, community or population group to another.
Many definitions of drought exist because the characteristics of drought differ between regions. Drought impacts also vary significantly between locations because of differences in economic, social, and environmental characteristics at the micro and macro scales. Drought definitions should be impact or application specific and region specific. All droughts originate from a deficiency of precipitation.
Types of Drought
Three types of drought are commonly noted: meteorological, agricultural, and hydrological. Meteorological drought is principally defined by the deficiency of precipitation from expected or "normal" amount over an extended period of time. This type of drought is focused on the physical characteristics of drought, i.e., the departure of precipitation from normal, rather than on the impacts associated with this departure. Agricultural drought may be characterized by a deficiency in water availability for crop or plant growth. Although precipitation deficiencies are important, agricultural drought severity is usually more closely associated with deficiencies in soil moisture - the most critical factor in defining crop production potential. Agricultural drought lags the occurrence of meteorological drought because it is associated with the level of soil water reserves in the soil profile. Some soils are more resilient to drought because of high water holding capacity. Vulnerability is highest on soils with a low water holding capacity, although appropriate soil management practices can reduce the impacts of drought on crops. Hydrological drought is best defined by deficiencies in surface and subsurface water supplies, which lead to a lack of water availability to meet normal and specific water demands. Hydrological or water supply drought lags the occurrence of agricultural drought because considerable time elapses between precipitation deficiencies and declines in ground water and reservoir levels. Likewise, these components of the hydrologic system are usually the last to recover from longer-term droughts. There are clearly strong relationships between the three types of drought especially during prolonged periods of rainfall deficiency, although with leads and lags in terms of their respective onsets and departures.
Drought Early Warning Systems (DEWS)
Drought has some unique characteristics that require different approaches to monitor its development and cessation and assess potential impacts on people and society at the local, regional, and national level. Common indicators of drought include meteorological variables such as precipitation and evaporation, as well as hydrological variables such as stream flow, ground water levels, reservoir and lake levels, snow pack, and soil moisture. Numerous climate and water supply indices are in widespread use to identify the severity of drought conditions, and to represent it in a probabilistic perspective. Each index has strengths and weaknesses, which need to be clearly understood as they are integrated into drought early warning systems. In many countries, especially in Africa, early warning systems for drought are also coupled to those developed for early warning of famine and food shortages more generally. In these cases, many additional variables are monitored as indicators of stress on lives and livelihoods.
Because of the slow-onset nature of drought, it is essential that early warning systems have the capacity to detect the early emergence of rainfall deficiencies, which will normally be the best indicator of an incipient drought period. There is a need for the application of climatic indices to evaluate the status of climate and water supply conditions and potential impacts in specific sectors (e.g., agriculture, wildfires). This information should be supplemented by long range or seasonal forecasts whenever possible. A drought early warning system (DEWS) must not only encompass mechanisms and procedures for the collection and analysis of information in a timely manner, but also for the dissemination of that information through locally appropriate channels to potential end users. Training end users about the value of this information to decision making is essential. Once an incipient drought period is identified or forecast, there should be continuous information flow on expected onset and timing, intensity, cessation, duration, spatial extent and changes in spatial coverage through time, and the estimation of economic, social, and environmental impacts. Meteorological, agricultural, and hydrological scientists need to work together and with social scientists to design products that better communicate information to decision makers in climate-sensitive sectors in order to reduce the risks associated with drought. These scientists should also solicit input from end users to better understand their need for information so climate-based products can be tailored to their specific needs.
See full document: Living With Risk: An Integrated Approach to Reducing Societal Vulnerability to Drought; ISDR Ad Hoc Discussion Group on Drought
