Ryan's Weather Disasters-Droughts

INTRODUCTION

Figure 1. Propagation of precipitation defecits through other components of the hydrologic cycle. (Modified from Changnon, 1987).

A drought is a complex phenomenon that can be defined from several perspectives (Wilhite and Glantz, 1987). The central theme in the definitions of a drought is the concept of a water deficit. A drought is difficult to define because of the need to specify the component(s) of the hydrologic cycle affected by the water deficit and the time period associated with the deficit. The simultaneous occurrence of a long-term deficit in deep ground-water storage and a short-term surplus of soil water in the root zone is an example of the complexity encountered in defining a drought.

Changnon (1987) illustrates how the definitions of a drought are related to specific components of the hydrologic cycle and how precipitation deficits are related to drought (fig. 1). Figure 1 shows how the effects of two hypothetical precipitation deficits are propagated over time through the surface-runoff, soil-moisture, streamflow, and ground-water components of the hydrologic cycle. From this perspective, precipitation can be considered to be the carrier of the drought signal, and streamflow and ground-water levels can be considered to be the last indicators of the occurrence of a drought (Hare, 1987; Klemes, 1987).

If precipitation is the carrier of the drought signal, then climate describes the long-term characteristics of this signal. The climatic factors associated with drought, including a description of the local climate in areas that have precipitation deficits, are described in the following section. These factors are then related to atmospheric circulations that extend well beyond the local area. Finally, possible causes of drought-related atmospheric circulations and the relation of these causes to nonatmospheric factors are described.