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<br />. <br /> <br />. <br /> <br />~ <br />t;,) <br />N <br />N <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />.. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />as semi-arid or arid, maximum monthly precipitation occurs during <br /> <br />July, August, and September and are associated with violent thunder- <br /> <br />shower events. Higher elevations receive most of their precipitation <br /> <br />during late October to April in the form of snow or spring rain, though <br /> <br />summer thundershower activity also occurs. The snow pack in the <br /> <br />higher Upper Basin mountains usually exceeds 100 inches and serves as <br /> <br />the principle source of water for the Colorado River. <br /> <br />The temperature variability on the basin covers a wide range de- <br /> <br />pending on elevation and seasonal effects. In the desert sections during <br /> <br />summer months, maximum temperatures in the excess of 1000F are <br /> <br />common. The diurnal temperature fluctuations reach a maximum in <br /> <br />summer with a variation of 300 to 400F. Cool temperatures in summer <br /> <br />are associated with elevations above 6,000 feet. These regions also <br /> <br />record the lowest temperatures in the basin. <br /> <br />o <br />Temperatures of - 30 to <br /> <br />_500F are not uncommon during severe cold spells. <br /> <br />The range of the frost- free period in the Colorado River Basin de- <br /> <br />pends on elevation and the nature of surrounding terrain. At 8,500 feet, <br /> <br />the frost-free periodaverages.20 days or less while in the Yuma area of <br /> <br />Arizona the growing season extends to 300 days or longer. <br /> <br />The combination of high temperatures and low humidity result in <br /> <br />high rates of evaporation and transpiration in the semi- arid to arid re- <br /> <br />gions of the basin. Estimates of annual evaporation from reservoirs and <br /> <br />lakes range from 30 inches at .higher elevations to more than 80 inches <br /> <br />3 <br /> <br />, <br /> <br />1 <br />