Types of Drought
As previously stated, 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 the precipitation deficit, although temperature and other factors can also have a significant influence on its severity. Drought results from persistent large-scale disruptions in the global circulation pattern of the atmosphere (see Chapter 6). Exposure to drought varies spatially, and there is little, if anything, we can do to alter drought occurrence. However, the other common drought types (i.e., agricultural, hydrological, 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 the human activities that depend on precipitation and water management to provide adequate supplies to meet societal and environmental demands (see Figure 1.1). For example, soils play an important role in determining how drought conditions will affect agricultural production because no direct relationship exists 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 water-holding capacity and others a low water-holding capacity. Soils with a low water-holding capacity are more drought prone.
The characterization of hydrological drought is associated less with the precipitation deficiency because it is normally associated with the departure of surface and subsurface water supplies from some average condition at various points in time. Like agricultural drought, no direct relationship exists 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). The use and management of surface and subsurface water supplies is a major factor that determines their availability when drought occurs. There is also considerable time lag between a deficiency of precipitation from average and when these deficiencies become evident in other components (e.g., reservoirs, groundwater, and streamflow) of the hydrologic system. Recovery of these components is also slow because of long recharge periods for surface and subsurface water supplies and how they are managed. In areas where the primary source of water is snowpack, such as in the western United States, the determination of drought severity is also complicated by infrastructures, institutional arrangements, and legal constraints. For example, reservoirs increase this region's resilience to drought because of their potential for storing large amounts of water as a buffer during dry years. However, the operating plans for these reservoirs try to accommodate the multiple, often conflicting, uses of the water (e.g., protection of fisheries, hydroelectric power production, recreation and tourism, irrigation) and the priorities set by governments when the funds were appropriated to construct the reservoir. The allocation of water between these various water use sectors is generally fixed and inflexible, making it difficult to manage a drought of unforeseen duration. Also, legal agreements between political jurisdictions (i.e., states and countries) concerning the amount of water to be delivered from one jurisdiction to another impose legal requirements on water managers to maintain flows at certain levels. During drought, conflicts heighten because of multiple values being advocated and because of limited available water. These shortages may result from poor water and land management practices that exacerbate the problem.
Socioeconomic drought differs markedly from the other types because it associates human activity with elements of meteorological, agricultural, and hydrological drought. It may result from factors affecting the supply of, or demand for, some commodity or economic good (e.g., water, forage, and hydroelectric power) that is dependent on precipitation. It may also result from the differential impact of drought on different groups within a population, depending on their access or entitlement to particular resources, such as land, and/or their access or entitlement to relief resources. Drought may fuel conflict between different groups as they compete for limited resources. A classic example in the Horn of Africa is the tension, which may become violent in drought years, between nomadic pastoralists in search of grazing and settled agriculturalists wishing to use the same land for cultivation. The Integrated Drought Management Program (discussed in Chapter 3) provides new efforts to inform drought management in this and other regions.
The interplay between drought and human activities raises a serious question with regard to attempts to define it in a meaningful way. The concept of socioeconomic drought is thus of primary concern to policymakers. It was previously stated that drought results from a deficiency of precipitation from expected or "normal" that is extended over a season or longer time period 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 average precipitation. However, if development demands exceed the supply of water available, then demand will exceed supply even in years of normal precipitation. This can result in human-induced drought or what is commonly referred to as water scarcity. In this situation, water supply for development is sustained only through mining of groundwater and/or the transfer of water into the region from other watersheds.
Drought severity is not only aggravated by other climatic factors, such as high temperatures, high winds, and low relative humidity, but also by the timing (i.e., principal season of occurrence, delays in the start of the rainy season, and occurrence of rains in relation to principal crop growth stages) and effectiveness of the rains (i.e., rainfall intensity and number of rainfall events). Thus, each drought event is unique in its climatic characteristics, spatial extent, impacts (i.e., no two droughts are identical), and likelihood of amelioration or demise. The area affected by drought is rarely static during the course of the event. As drought emerges and intensifies, its core area or epicenter shifts and its spatial extent expands and contracts. A comprehensive drought early warning system is critical for detecting emerging precipitation deficiencies and tracking these changes in spatial coverage, severity, and potential impacts, as explained below.