Laserfiche WebLink
<br />~, <br /> <br />,< <br />, <br /> <br />Discussion <br /> <br />While floods and droughts are both naturally occurring phenomena, droughts have another <br />dimension that complicates the analysis. Flood flow frequency analysis looks at a series of annual <br />peak flows to determine the magnitude of floods for a range of average return periods, such as <br />the! OO-year flood. A statistical analysis of drought requires a frequency analysis of a series of low <br />flows occurring over successive years. Consequently, stream gage records lasting much longer than <br />the available instrumental records are required. <br /> <br />. <br /> <br />A true risk based drought analysis requires the use of statistical and stochastic techniques to <br />define stream flow characteristics during low flow periods. Because drought is some period of time <br />with below average stream flow, drought severity is a function of duration (in months or years) and <br />the magnitude of the deficits. A deficit is the difference or shortage in volume between the demand <br />and total available supply, including storage, for a given increment oftime (typically a month). The <br />design drought is a monthly or annual series oflow flows. It can be based on the recorded historic <br />drought experienced in the region or on estimated drought characteristics from a statistical analysis <br />of available stream flow or precipitation records with long term "synthetic" stream flow histories <br />using stochastic models. <br /> <br />As it turns out, one of the tasks in the design of the Colorado River Decision Support System is <br />to develop a series of IO,OOO-year stochastic data sets characterizing drought for key stream gage <br />locations on the west slope. The preliminary r~sults show, for example, that the 50-year drought <br />sequence for the Colorado River lasts 6.2 years and has a cumulative deficit of9.3 million acre-feet. <br />This task is still in progress, but the preliminary results based on synthetic stream flows should <br />complement the proposed drought planning fact sheet to be based on analysis of climate records. <br /> <br />. <br /> <br />Recommendation <br /> <br />This agenda item is provided for your information. The staff has been trying to identifY an <br />appropriate source of funds for the project. We are also trying to refine the scope of work to <br />eliminate uncertainty and reduce the costs. We would be happy to answer your questions. <br /> <br />L:\BOARDMEM\MAROO\000327c - Drought Planning fact Sheet.doc <br /> <br />.0 <br /> <br />2 <br />