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<br />J0j~47 <br /> <br />Snow Water Equivalents <br /> <br />Most Reclamation projects rely on winter precipitation for project water supplies and <br />replenishment of reservoir storage, The winter snowpack that accumulates at higher <br />elevations is criticaL Snowpack indicates the amount of water that will be captured <br />in the lower elevation reservoirs, and, thus, available for storage and release during <br />the dry summer season, Low snowpack conditions indicate low reservoir inflow; very <br />high snowpack conditions, particularly when associated with high carryover storage, <br />indicate the potential for spring floods, Reservoir operations--the water release <br />schedule-must consider both the amount and timing of the spring snowmelt runoff. <br /> <br />Snowpack can be thought of as "inches of water" temporarily stored in a frozen <br />reserve, Two factors are considered: (1) the depth of the snow and (2) its moisture <br />content. Deep snow may actually yield very little water if it is dry and powdery, <br />Therefore, both factors are used to determine "snow water equivalents" which are an <br />indication of the amount of reservoir inflow to be expected for the year, <br /> <br />Reservoir storage availability is largely based upon snowfall, and Reclamation begins <br />its storage availability forecasts as early as possible each winter, Nevertheless, in <br />many areas, a prediction of water supply for the following summer contains high <br />levels of uncertainty until the winter storm season ends, <br /> <br />The following maps illustrate the snow water equivalents at two points during the <br />winter of 1989-1990, Both maps summarize conditions as percentages of average. <br />Numbers on the subbasins correspond to numbers on the following table, which <br />provides a more detailed tabulation of the index measurement. Subbasins were <br />chosen for illustration based upon availability of data and applicability to <br />Reclamation projects. <br /> <br />In the West, most snow typically accumulates from January through ApriL Thus, the <br />February 1, 1990, map shows the snowpack available near the beginning of the <br />season, and the May 1, 1990, map shows the snow water equivalents near the end <br />of the season. As shown, the percentages were extremely low in many areas. <br />Particularly severe were areas of Oregon, California and Arizona. (Drought <br />conditions in Arizona were relieved later in the season by a series of rain storms,) <br /> <br />15 <br />