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<br />.;colorado Water Resources Research Institute~
<br />
<br />What is drought?
<br />
<br />Drought is unique among natural hazards because it is not
<br />a clear event, like a flood, earthquake, hurricane or tornado.
<br />These events strike, leave their mark, and are gone. A
<br />drought, however, sneaks up on us quietly disguised as
<br />lovely, sunny weather. Unlike a hurricane, we cannot
<br />follow its course on a map. We are never sure when a
<br />drought began until after it is already well underway, and
<br />we are often unsure when it ends. Any day when it
<br />doesn't rain or snow (which describes the majority of days
<br />here In Colorado) could be the beginning of the next
<br />drought.
<br />
<br />Which still does not answer our question: what is
<br />drought?
<br />
<br />Drought is a concept that is both simple and complex.
<br />Drought is a shortage of water, usually associated with a
<br />deficiency of precipitation. Drought occurs when the
<br />demand for water exceeds the supply of water.
<br />
<br />Simple, right?
<br />
<br />Yes, except that different types of demands require
<br />dIfferent amounts of water in different forms at different
<br />~imes of the year. Droughts are often defined by their
<br />ImpactS, e.g. on crops or on reservoir water levels and
<br />power availability. Thus, there is no universal definition of
<br />a drought. The more diverse the economy and the climate
<br />of a regi~n are, the harder it is to define drought. Drought
<br />means dIfferent things to different people; one person's
<br />drought can be another person's "sunny day."
<br />
<br />
<br />Several definitions of drought have
<br />been proposed by various agencies.
<br />The figure, which is from a publication
<br />by the National Drought Mitigation
<br />Center (http://enso.unl.edu/ndmc/),
<br />illustrates a set of definitions that show
<br />both causes and impacts of drought
<br />overtime,namely: meteorological
<br />drought, agricultural drought, hydrologi-
<br />cal drought, and socioeconomic (and
<br />environmental) drought. Another
<br />important defmition of drought con-
<br />cerns wildfire risk. This paper deals
<br />primarily with meteorological drought
<br />and the history of wet and dry periods
<br />in Colorado. However, the impacts of
<br />droughts of different durations, levels
<br />of severity, and areal extent, are also
<br />discussed.
<br />
<br />I Preclpllatlon tleflcleney
<br />ji (amount, Intensity, timing)
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<br />Plant water stress, retluCed
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<br />E i biomass and yield
<br />j: \ ----- ---------
<br />-If Reduced streamflow, InflOW to
<br />. reservoirs, lakes, and ponds;
<br />/ reduced wetlands,
<br />wildlife habi'ltlt
<br />
<br />Drought is also "relative" and must be defined in terms of
<br />what is "nonnal" for a particular region or time of year. To
<br />someone who has lived in Nevada or Arizona, Colorado
<br />looks mighty green and lush, Folks who just moved here
<br />from the eastern halfoflhe United States, however, may
<br />think that Colorado experiences perpetual drought. In fact,
<br />for most locations east of Omaha or Kansas City, the driest
<br />year on record may still be wett.er than the wettest year on
<br />record for most of Colorado's lower elevations. Coloradans
<br />have adapted to this dry regime.
<br />
<br />One important measure of drought is the impacts of water
<br />shortage on nature and society. These impacts may
<br />change over time as new mitigation measures are devel-
<br />f
<br />oped. For example, many water users in Colorado are able
<br />to store water over time, or transport water from other parts
<br />of the state, or use groundwater recharge to re-regulate
<br />streamflOw5, or make artificial "snow" for ski areas. Since
<br />C~lorado receives new water supplies only one way - as
<br />r~n, .snow, or other precipitation - some form of precipita-
<br />tIOn IS necessary to provide water to Colorado's population
<br />and economy. That precipitation, however, does not
<br />necessarily fall on the populations that use the water most
<br />or at the times when it is most needed.
<br />
<br />Understanding - and thus preparing for - a severe drouoht
<br />in Colorado requires an appreciation of how Colorado to
<br />receives its water supply and how different water demands
<br />use that supply. This paper presents a discussion of
<br />Colorado's water supplies and demands. and an analysis of
<br />the history of wet and dry periods in Colorado.
<br />
<br />Natural Climabl variability
<br />
<br />Reducecllnflftratlon, runoff,
<br /><I9l111119rcOlatlon, anti
<br />ground water recharge
<br />
<br />
<br />High temp.. high winds, low
<br />rei olive humidity, gresler
<br />sunshine. 1_ cloud C~
<br />
<br />Increased evallora1ion
<br />anti \ran trallon
<br />
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<br />
<br />SOCial Impacts Envi'onmonblllmll9C1S
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