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<br />In the early years, irrigation water was generally abundant, <br />except beyond the Cap Rock and in certain parts of Lamb, Lynn and <br />Terry Counties. It was supplemented by a fairly uniform area-wide <br />rainfall of 18 to 21 inches annually. Lamb County's 70-year aver- <br />age is 18.73, Lubbock's and Tahoka's long-term average is 18.50, <br />Plainview's 20.73, Brownfield's 17.21. Close to 65 percent of <br />these annual averages fall froiTl /lay through September at <br />a rate of about 2.45 inches each month. The attached Map IV-3 <br />shows mean annual precipitation for all of Texas. <br />The South Plains encounters two major problems with any surface <br />water supply availability. One is the two-fold climatological one <br />of normally high temperatures and normally high winds which con- <br />spire to evaporate essentially all rainfall except that minor <br />amount which reaches plants and is consumptively used in transpira- <br />tion. The other is the virtual absence of surface streamflow avail- <br />able for storing and off-stream diversion to plants. Map IV-4 <br />shows mean annual temperatures for all of Texas. For our study <br />area, these range from 57 to 61 degrees Fahrenheit. Unfortunately, <br />too. in the months of reasonably good rainfall, the temperature <br />range is highest. In Lubbock, the July average minimum is 66.9, <br />average maximum is 92.4, and average is a very high 79.7 degrees <br />Fahrenheit. <br />It is not surprising that the South Plains farmers developed <br />such an intensive irrigation economy, and perhaps it is surprising <br />that it was not so developed more rat)idly during the post-'/orld <br /> <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 />'I <br />. <br /> <br />IV-22 <br /> <br />I <br /> <br />Arthur D Little, Incl <br />