Clear definitions of drought are elusive. Drought is generally understood in terms of the definition offered in Webster's Dictionary: dryness; want of rain; or a prolonged period of dryness. Drought is a normal part of climate in nearly all of the United States but it is of special concern in arid regions of the western United States, where precipitation is often in short supply and where one thus might say drought exists much of the time.
Drought can be defined in different terms, including meteorological, agricultural, hydrologic, and socioeconomic (Wilhite and Glantz, 1985). Hydrologic definitions of drought are of particular interest within this report, as Colorado River water managers generally define drought in terms of reservoir inflows. The Colorado River basin drought of the early 21st century saw well below normal inflows into Lake Powell for the 5-year period 2000-2004. It should be noted, however, that 1999 and 2005 both had only slightly above-normal inflows, and one or two years of slightly above normal inflows do not end a drought of such magnitude. For 1999-2005, average inflows into Lake Powell were below normal. The 2006 water year is likely to extend this trend.
A basic concept invoked in understanding drought is that of a water budget. Water is held in storage buffers such as soil root zones, aquifers, lakes, reservoirs, and surface stream flows. These buffers act as water supplies, are subject to demands, and are replenished and lose water at varying rates. When losses exceed replenishment, impacts are experienced and, at lower storage levels, become increasingly severe. In essence, drought is defined by its impacts on both natural and manmade environments because without impacts there is no drought, no matter how dry it might be. Drought infers a relationship between supply rates and demand rates; drought is not simply a supply-side phenomenon, but also depends on water demands. Without demands, there is no drought, whether a given supply of water is big, small, or even zero.
It can be difficult to determine exactly when a drought has begun or ended, and there can be differences of opinion over whether a drought actually exists. Droughts begin slowly. They may be interrupted by wet periods, during which it is not clear if precipitation will continue or if dry conditions will return. A drought may not be widely recognized until it has been under way for several months or longer, and it can be particularly difficult to recognize in arid regions that experience seasonal dry periods. Recognizing that drought began in some parts of the Colorado River basin in the late 1990s, and that it is ongoing in many areas and may not abate any time soon, this report uses the descriptors of drought of the early 21st century and drought of the early 2000s to refer to the drought that has affected Colorado River hydrology in this period.