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<br />002308 <br /> <br />. <br /> <br />washes are pI'ominent features of the landscape. In some places volcanic necks, <br /> <br />buttes, and lava flows are abundant. <br /> <br />The basins and low mountain ranges are distinctive for their uniformity and <br /> <br />roughly parallel one another. Their bulk is fairly continuous and the crests are <br /> <br />quite uniform. The slopes are fairly straight and do not'tend to flatten at the <br /> <br />base. The abrupt meeting of the valley floor and mountain side and the uniform <br /> <br />slope of the latter are characteristic of the Basin and Range Provinces. <br /> <br />C. Climate <br /> <br />. <br /> <br />The important determinants of climate within the study area are elevation <br /> <br />and the pattern of mountain ranges and distances from large bodies of water. <br /> <br />In general, rainfall is light and hurriidity low.. The percentage of possible <br /> <br />sunshine received is high, as is the diurnal variation in temperature. <br /> <br />As would be expected, the southernmost areas experience the least rain- <br /> <br />fall and the highest temperatures. At Yuma, Arizona, the average annual rain- <br /> <br />fall is less than 3 inches, while as much as 30 inches per year is received in <br /> <br />some sections of the mountains. The average annual precipitation for the State <br /> <br />of Arizona is 12 1/2 inches. . The seasonal distribution of precipitation differs. <br /> <br />There is no marked seasonality at Yuma and other extremely arid sections. <br /> <br />The mountain areas experience summe; thundershowers and winter snows which <br /> <br />result in summer and/or winter precipitation highs. Over most of the region <br /> <br />there are two general periods of rainfall: December to February and July to <br /> <br />September, As is true in most semiarid regions, extreme departures from <br /> <br />8 <br /> <br />