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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />~ ~-." n~ '1 <br />:...' :,~ ~-- ".j ; -' <br /> <br />The Region's elevation, ranging from about 2000 feet above sea level on <br />the east to more than 5000 feet to the northwest, in combination with pat- <br />terns of precipitation, wind velocities, typically low humidities and high <br />average sunshine intensities, all contribute to the relative aridity of the <br />Region for agricultural pursuits, and help explain the importance of irriga- <br />tion to the Region. <br /> <br />Typical "free-water" (pan) evaporation rates for the High Plains vary <br />from less than 60 inches per year in the more sub-humid eastern parts of the <br />Plains, to more than 100 inches annually in the far western stretches of <br />eastern New Mexico and southwest Texas. <br /> <br />Regional temperature extremes tend to be typical rather than unusual. <br />Summertime temperatures, although averaging in the 80's, commonly range from <br />the 50's to over 100oF. Winter daytime temperatures average in the 50's but <br />range from well below OOF to the 80's. Average and maximum temperatures are <br />higher for the Southern plains than for the north, with the mean annual <br />number of days during which the daytime temperature exceeds 900F more than <br />2.5 times greater for the south than the north. <br /> <br />Surface and Ground Water Interactions <br /> <br />The High Plains aquifer system was formed by windblown sand and silt, <br />and from sediments deposited by streams flowing eastward from the Rocky <br />Mountains. It is the nonuniform manner in which these materials were <br />deposited that determines both the topography and ground water hydrology <br />of the area by controlling the structure of both the land surface and the <br />aquifer (USGS, 1980). The High Plains aquifer system consists of hydrauli- <br />cally connected geologic units of the late Tertiary or Quaternary age. The <br />Ogallala formation is the principal geologic unit underlying approximately <br />156,000 square mil es. The formation is essentially heterogeneous and made <br />up of randomly distributed forms of sediment (Breyer, 1975). This accounts <br />for the variations in well yield that occur throughout the area. <br /> <br />The saturated thickness and general sediments of the northern portion of <br />the Region (principally Nebraska) enable wells to yield as much as 1,000 <br /> <br />1-9 <br />