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<br />4.0 Definition of Dry (Drought) and Wet Periods <br />Drought is fundamentally a deficit of water in the sense that demand for water is greater <br />than the supply of water. This can apply to any source of water so there is not a single definition <br />that meets all information needs. As a result several definitions could be constructed which each <br />apply to a specific aspect of water supply. Most of the effort to define drought has been <br /> <br />concerned with the supply of water. The demand for water is not as well defined. One definition <br /> <br /> <br />proposed by Palmer (1965) has been used extensively in the United States for the past 40 years. <br /> <br /> <br />The Palmer Drought Severity Index (PDSI) has been used to monitor agricultural drought and <br /> <br /> <br />also as a general measure of drought. Unfortunately, the PDSI does not answer most of the <br /> <br /> <br />questions asked about drought in Colorado or other places. These questions include: What is the <br /> <br />water deficit in terms of probability, percent of average, or precipitation deficit? How bad is it <br /> <br />now? When did it start? How does it compare with past droughts? When did it end? Other <br /> <br />questions relate to the impact of drought on natural ecosystems or on the economy and society. <br /> <br /> <br />The word drought and dry are used interchangeably in this study. The opposite of dry is wet arid <br /> <br /> <br />the definition of dry has an exact opposite which is used for wet periods. <br /> <br />There is no single definition of drought that satisfies most needs. Wilhite and Glantz <br /> <br /> <br />(1985) discuss definitions but don't conclude there is any universal definition. However, for a <br /> <br /> <br />quantitative description of drought or wet periods a definition is required. The definition of <br /> <br /> <br />drought used here is one that will answer most of the questions about water supply in terms of <br /> <br /> <br />precipitation and snowpack. It will allow a determination of beginning and ending along with <br /> <br />intensity. It can be applied to streamflow, reservoirs, soil moisture and ground water but the first <br /> <br />two of these are affected greatly by the management of them and the last two are not observed <br /> <br />11 <br />