All types of drought originate from a deficiency of precipitation (Wilhite and Glantz 1985). When this deficiency spans an extended period of time (i.e., meteorological drought), its existence is defined initially in terms of these natural characteristics. However, the other common drought types (i.e., agricultural, hydro-logical, and socioeconomic) place greater emphasis on human or social aspects of drought, highlighting the interaction or interplay between the natural characteristics of the event and human activities that depend on precipitation to provide adequate water supplies to meet societal and environmental demands (Fig. 2.1). For example, agricultural drought is defined more commonly by the availability of soil water to support crop and forage growth than by the departure of normal precipitation over some specified period of time, ttere is not a direct relationship between precipitation and infiltration of precipitation into the soil. Infiltration rates vary according to antecedent moisture conditions, slope, soil type, and the intensity of the precipitation event. Soils also vary in their characteristics, with some soils having a high soil water holding capacity while others have a low water holding capacity. Soils with a low water holding capacity are more drought-prone.
Socioeconomic & Political
Decreasing emphasis on the natural event (precipitation deficiencies)
Increasing emphasis on water/natural resource management
Increasing complexity of impacts and conflicts
Time/duration of the event
Fig. 2.1. Interrelationships between meteorological, agricultural, hydrological, and socio-economic drought. (Source: National Drought Mitigation Center, University of Nebraska-Lincoln, USA.)
Hydrological drought is even further removed from the deficiency of precipitation since it is normally defined in terms of the departure of surface and subsurface water supplies from some average condition at various points in time. Like agricultural drought, there is not a direct relationship between precipitation amounts and the status of surface and subsurface water supplies in lakes, reservoirs, aquifers, and streams because these components of the hydrological system are used for multiple and competing purposes (e.g., irrigation, recreation, tourism, flood control, hydroelectric power production, domestic water supply, protection of endangered species, and environmental and ecosystem preservation), ttere is also considerable time lag between departures of precipitation and the point at which these deficiencies become evident in the components of the hydrologic system. Recovery of these components is also slow because of long recharge periods for surface and subsurface water supplies. In areas where the primary source of water is from snow pack, such as in the western United States, the determination of drought severity is further complicated by infrastructures, institutional arrangements, and legal constraints.
Socioeconomic drought differs markedly from the other types because it associates the supply and demand of some economic good or service with elements of meteorological, agricultural, and hydrological drought. Socioeconomic drought is associated directly with the supply of some commodity or economic good (e.g., water, maize, hay, hydroelectric power) that is dependent on precipitation. Increases in population can alter substantially the demand for these economic goods over time, ttis concept of drought supports the strong symbiosis between drought and its impacts on human activities, ttus, the magnitude of drought impacts could in crease because of a change in the frequency of meteorological drought, a change in societal vulnerability to water shortages, or both.
tte interplay between drought and human activities raises serious questions about our attempts to characterize it and define it in a meaningful way. Drought results from a deficiency of precipitation from expected or "normal" that is extended over a season or longer period of time and is insufficient to meet the demands of human activities and the environment. Conceptually, this definition assumes that the demands of human activities are in balance or harmony with the availability of water supplies during periods of normal or mean precipitation. If development demands exceed the supply of water available, the result can be that demand exceeds supply even in years of normal precipitation, ttis can result in a situation of human-induced drought that is separate from the drought types previously discussed. When this situation exists, development can only be sustained through mining of ground water and/or the transfer of water into the region from other watersheds. Is this practice sustainable in the long term? Should this situation be defined as "drought" or unsustainable development? Some would define this as a "water shortage" drought because it is not necessarily the result of a deficiency of precipitation but rather the result of an overallocation of water supplies.
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