Introduction

Drought, a normal feature of the climate for virtually all portions of the United States, is one of the defining characteristics of the Great Plains region. Early maps referred to this region as the Great American Desert, a belief attributed to the explorations of Zebulon Pike in the early 1800s (Brown, 1948). The region's past is firmly rooted in the drought of the 1890s and, in particular, the Dust Bowl years of the 1930s (Hurt, 1981). More recently, droughts have occurred at regular intervals, affecting all portions of a region that stretches from Texas and New Mexico northward through the Dakotas and Montana into the Prairie Provinces of Alberta, Saskatchewan, and Manitoba. In reality, it is rare for drought not to occur in the region each year, a fact that has forced considerable adjustments from a predominantly agricultural economy. Irrigation development and many other technological adjustments in the post-1930s era have improved the resilience of the region to the ravages of drought, but drought continues to produce devastating and widespread impacts.

The purpose of this chapter is to discuss drought in the context of the Great Plains. In that process, I will review some of the basic concepts of drought. A grasp of these concepts is essential to understanding the history and impacts of drought in the Great Plains and the region's continuing vulnerability to this insidious natural hazard. Current and future attempts to lessen drought impacts in the region are also discussed.

Handbook of Weather, Climate, and Water: Atmospheric Chemistry, Hydrology, and Societal Impacts, Edited by Thomas D. Potter and Bradley R. Colman. ISBN 0-471-21489-2 © 2003 John Wiley & Sons, Inc.

2 CONCEPT OF DROUGHT: DEFINITION AND TYPES

Drought is the consequence of a natural reduction in the amount of precipitation received over an extended period of time, usually a season or more in length, although other climatic factors (such as high temperatures, high winds, and low relative humidity) are often associated with it in many regions of the world and can significantly aggravate the severity of the event (Wilhite, 1992, 2000). High winds and low relative humidity aggravated the effects of the drought of the 1930s in the Great Plains. Drought is also related to the timing (i.e., principal season of occurrence, delays in the start of the rainy season, occurrence of rains in relation to principal crop growth stages) and the effectiveness (i.e., rainfall intensity, number of rainfall events) of the rains. Thus, each drought is unique in its climatic characteristics and impacts. Likewise, society is changing in response to increasing and shifting population, new technologies, government policies, and social behavior. These factors alter vulnerability, a fact that will be discussed in greater detail later in this chapter.

Drought is a temporary aberration that occurs in high- and low-rainfall regions (Wilhite, 1992). Although droughts are commonly associated with the Great Plains and other semiarid regions, it is difficult for many people to visualize drought occurring in more humid regions such as the eastern United States, Southeast Asia, Brazil, or western Europe. This fact emphasizes both the regional and relative nature of drought.

Drought differs from other natural hazards (e.g., floods, tropical cyclones, and earthquakes) in several ways. First, drought is a slow-onset natural hazard. It is often referred to as a creeping phenomenon (Tannehill, 1947). The effects of drought accumulate slowly over a considerable period of time and may linger for years after the termination of the event. As a result, the onset and end of drought are difficult to determine. Even today, with more sophisticated monitoring technology, climatologists struggle to recognize the onset of drought, and scientists and policymakers continue to debate the basis (i.e., criteria) for declaring an end to a drought. Second, the absence of a precise and universally accepted definition of drought adds to the confusion about whether or not a drought exists and, if it does, its degree of severity. Realistically, definitions of drought must be region-specific and application (or impact) specific. Wilhite and Glantz (1985) analyzed more than 150 definitions in their classification study, and many more definitions exist. Although the definitions are numerous, many do not adequately define drought in meaningful terms for scientists or policymakers. Third, drought impacts are nonstructural and spread over a larger geographical area than are damages that result from other natural hazards. For example, a recent analysis of drought occurrence by the U.S. National Drought Mitigation Center for the 48 contiguous states in the United States demonstrated that severe and extreme drought affected more than 25% of the country in 27 of the past 100 years.

Because drought affects virtually all regions of the world and many economic and social sectors, scores of definitions exist. Impacts are complex, vary on spatial and temporal scales, and depend on the societal context of drought. The impacts of drought in the Great Plains will differ from those experienced in the southeast, northeast, or far western portions of the United States. As a result, it is not possible to formulate a definition of drought that is universally acceptable in each of these settings. Wilhite and Glantz (1985) concluded that definitions of drought should reflect a regional bias since water supply is largely a function of climatic regime.

Drought has been grouped by type as follows: meteorological, agricultural, hydrological, and socioeconomic (Wilhite and Glantz, 1985). Meteorological (or climatological) drought is expressed solely on the basis of the degree of dryness (often in comparison to some normal or average amount) and the duration of the dry period. The Encyclopedia of Climate and Weather (Schneider, 1996) defines drought as an extended period—a season, a year, or several years—of deficient rainfall relative to the statistical multiyear mean for a region. This definition identifies two critical components that must be accounted for in a viable definition—intensity and duration. Meteorological drought definitions must be considered as region specific since the atmospheric conditions that result in deficiencies of precipitation are climate regime dependent. In the Great Plains, the distribution of precipitation is seasonal: Approximately 70% of the precipitation in this region occurs during the 6-month period from April to September. Definitions that differentiate meteorological drought on the basis of the number of days with precipitation less than some specified threshold (e.g., for Britain, 15 days, none of which received as much as 0.25 mm of precipitation; British Rainfall Organization, 1936) rather than the magnitude of the deficiency over some period of time would be inappropriate for the Great Plains.

Agricultural drought links various characteristics of meteorological drought to agricultural impacts, focusing on precipitation shortages, differences between actual and potential évapotranspiration (ET), soil water deficits, and so forth. Rosenberg (1980) defined agricultural drought as a climatic excursion involving a shortage of precipitation sufficient to adversely affect crop production or range productivity. A definition of agricultural drought should account for the variable susceptibility of crops at different stages of crop development. The impacts of drought are crop specific because the most weather-sensitive phenological stages vary between crops. Planting dates and maturation periods also vary between crops and locations. A period of high-temperature stress that occurs in association with dry conditions may coincide with a critical weather-sensitive growth stage for one crop while missing a critical stage for another crop. Agricultural planning can often reduce the risk of drought impact on crops by altering the crop, genotype, planting date, and cultivation practices. Considerable progress has been made in the Great Plains and elsewhere in applying various types of adaptive strategies to reducing the impacts of drought on crop and rangeland (Rosenberg, 1980, 1986).

Agricultural droughts usually take 3 months or more to develop, but this time period can vary considerably, depending on the timing of the initiation of the precipitation deficiency. For example, in the Great Plains a significant dry period during the winter season may have few, if any, impacts for many locales. However, if this deficiency continues into the growing season, the impacts may magniiy quickly since low precipitation during the autumn and winter season results in low soil moisture recharge rates, leading to deficient soil moisture at spring planting. Although the region's agriculture is usually the first to feel the effects of drought, a prolonged dry period can result in significant disruptions in other sectors, particularly water-based transportation, energy production, municipal water supplies, and recreation-based businesses. Also, a short-lived drought of less than 3 months can have serious impacts on crop yields if it occurs during the critical crop growth stages and is accompanied by high temperatures.

Hydrological droughts are associated more with the effects of periods of precipitation shortfall on surface or subsurface water supply (i.e., streamflow, reservoir and lake levels, groundwater) than with precipitation shortfalls directly (Dracup et al., 1980; Klemes, 1987). Hydrological droughts are usually out of phase or lag the occurrence of meteorological and agricultural droughts. Meteorological droughts result from precipitation deficiencies; agricultural droughts are largely the result of soil moisture deficiencies. More time elapses before precipitation deficiencies are detected in other components of the hydrological system (e.g., reservoirs, groundwater). As a result, impacts from hydrological drought are out of phase with those in other economic sectors. Water in hydrological storage systems (e.g., reservoirs, rivers) is often used for multiple and competing purposes (e.g., power generation, flood control, irrigation, recreation), further complicating the sequence and quantification of impacts. Competition for water in these storage systems escalates during drought, and conflicts between water users increase significantly. For example, changing water-use patterns and the series of drought years that occurred in the Missouri River basin between 1987 and 1992 resulted in significant conflicts between upstream and downstream water users.

Finally, socioeconomic drought associates the supply and demand of some economic good or service with elements of meteorological, hydrological, and agricultural drought. For example, the supply of some economic good (e.g., water, hay, hydroelectric power) is highly sensitive to the vagaries of weather. In most instances, the demand for that good is increasing as a result of increasing population and/or per capita consumption. Therefore, drought could be defined as occurring when the demand for that good exceeds supply as a result of a weather-related supply shortfall (Sandford, 1979). This concept of drought supports the strong symbiosis that exists between drought and human activities. Thus, the incidence of drought could increase because of a change in the frequency of the physical event, a change in societal vulnerability to water shortages, or both. For example, poor land-use practices such as overgrazing can decrease animal carrying capacity and increase soil erosion, which exacerbates the impacts of and vulnerability to future droughts. Overdrafts of groundwater, such as have been occurring in the southern portions of the Ogallala aquifer, will affect future vulnerability to drought in the Great Plains since access to groundwater is a primary adaptive strategy employed to alleviate the effects of drought.

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