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<br />n!\~I~ l3 <br />U...IU_" <br /> <br />Palmer Drought Severity Index <br /> <br />The Palmer Drought Severity Index (PDSO is a measurement tool for relative dryness <br />or wetness, indicating prolonged and abnormal deficiencies or excesses of moisture. <br />PDSI calculations are made from such indices as weekly precipitation totals, weekly <br />average temperatures, moisture content of the soil, evapotranspiration rates, and <br />previous indexes. The PDSI is based upon a scale of values, with value~ greater than <br />+4 (wet) and less than -4 (dry) representing extreme conditions, <br /> <br />The PDSI is an important climatological tool for evaluating the scope, severity, and <br />frequency of prolonged periods of abnormally dry or wet weather, The index does not <br />consider availability of snowmelt runoff, reservoir storage, or the availability of <br />ground-water resources which can significantly influence an evaluation of water <br />supply projections for a given area. The value of the PDSI is that it can provide <br />important information about the following concerns: Will irrigated crops require <br />more than the usual amount of water? Will precipitation run offinto water channels <br />or be absorbed into the soil? What is the degree of fire danger? <br /> <br />The PDSI maps on the following pages show soil moisture conditions at four intervals <br />during 1990, As illustrated, soil moisture conditions in some sections of the West <br />started dry and became even more arid as the season progressed, In particular, areas <br />of California and the Colorado River Basin were extremely dry, causing increased <br />water demand from storage for consumptive use, <br /> <br />14 <br />