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weighted and scaled alternate hydrology, and water use relative to potential changes in forest <br />and climate scenarios. <br />Background: <br />The recent drought has emphasized that the principal influence on water availability is the <br />amount of natural runoff in a basin, and that the principal influence on natural runoff is climatic <br />conditions. There are growing concerns that global climate is changing, that those changes will <br />be reflected in the regional climate, and changes in the regional climate will bring corresponding <br />changes in hydrology and streamflows. Additional concerns come from recent and continuing <br />insect outbreaks that are causing considerable tree mortality in Colorado forests, and from <br />increased fire risk during hot, dry.spells {which may be temporarily compounded by insect- <br />caused tree mortality.) <br />For this Study, the CWCB requested development of a data set representing an alternate <br />hydrology of climate change and forest change. <br />Projections of future climate are made with global climate models .(GCMs). Each model may be <br />used to make a number of runs, comprising an ensemble of projections, with each run using <br />different boundary conditions. Different GCMs and different runs of a single GCM provide <br />different projections of future climate. These differences reflect scientific uncertainty about the <br />processes that affect climate, imperfections in the model representations of those processes, and <br />imperfections in the state of knowledge regarding boundary conditions. These uncertainties are <br />reflected in a broad range of results. In the Colorado River Basin published work based on <br />projections of future climate provides evidence that future water availability could be as little as <br />one half of current supplies or could increase. This range of results is too broad for use in a "best <br />case/worst case" analysis. Consequently, some means of selecting or weighting projections is <br />required in order to generate information useful to a planning process. <br />There is also currently no evidence to indicate that the annual sequences of climate conditions <br />generated by GCMs are reliable. There is some evidence to indicate that, when translated into <br />hydrologic variables, the GCMs do not replicate either the annual variability of hydrologic <br />conditions or the statistics of wet and dry spells. If this is true, the only information that can be <br />used reliably from GCMs (in the context of water resources planning} is the projections of mean <br />conditions and the seasonal pattern of those conditions. Accordingly, approaches that use GCM <br />projections in combination with observed data and palea reconstructions of streamflows will <br />probably be recommended. In such a hybrid approach, the sequences of years come from <br />observed or reconstructed data while the projected changes in mean conditions and seasonal <br />conditions come from the GCMs. <br />The alternate hydrology developed in this task will serve as input to the water rights models to be <br />used in assessing water availability. <br />Colorado River Water Availability Study Scope 18 8/28/2008 <br />CRWAS Phasel Contract Scope EX A Final <br />