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<br />W <br />,.d. <br />I'" <br />...., <br /> <br />and semiarid regions are by definition areas of low annual precipitation <br />and high potential evaporation, and the natural waters of these regions <br />become more concentrated. Therefore, saline accumulations appear on the <br />surface and in the soils of drylands. The saline deposits are composed <br />of slightly soluble minerals (most carbonates), more soluble sulfates of <br />calcium, sodium, and magnesium, and highly soluble minerals--the <br />nitrates and'chlorides. <br />Runoff encountering these soluble saline accumulations acquires a <br />high salinity (Chebotarev, 19S5, Casey, 1972). This process is <br />particularly noteworthy when the underlying bedrock contains a high <br />soluble mineral content (SMC) and it is eroded. A high $MC <br />characterizes fine-grained, typically shaly marine formations, such as <br />the Mancos S~ale (Fisher et al, 1961). <br />Topography also affects the occurrence of saline surface water <br />(Holmes, 197J.). Steep highly erodible hillslopes that contain a high <br />$MC in conjunction with high intensity, erosive storm events may produce <br />highly saline runoff. <br />In summarY, aridity, steep topography, and the presence of erodible <br />saline surficial materials, will produce highly saline surface waters. <br /> <br /> <br />Distribution of Salinity <br /> <br />Arid to semiarid climates are closely associated with the occurrence <br />of salinity. Figure 1-2 shows areas of the USA where surface water <br />contains more than 1000 mg/l of solutes. Large parts of the semiarid <br />Midwest and ;arid Southwest, are characterized by saline surface water, <br />that is associated with saline soils. The saline surficial materials of <br />these regions are the source of the salinity in the waters that drain <br />from them. <br />Although considerable salt loading occurs in the Rio Grande and in <br />the Arkansas and Missouri Rivers, much of the downstream increase in <br />salinity in these basins is associated with irrigation and/or with <br />saline groundwater flow, the later being the principle source of <br />salinity in the northern Midwest. <br />The problem is aggravated in the Colorado River basin because the <br />ri ver flows, from high to low precipitation areas in contrast to the <br /> <br /> <br /> <br />5 <br /> <br />J <br /> <br />. <br /> <br />_c '-, '~"., .' A"~,_j~,,:',L:',,Jcic.' <br />