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<br /> <br />EFFECTS OF HUMAN ACTIVITIES <br />ON THE INTERACTION OF <br />GROUND WATER AND SURFACE WATER <br /> <br />Human activities commonly affect the distri- <br />bution, quantity, and chemical quality of water <br />resources. The range in human activities that affect <br />the interaction of ground water and surface water <br />is broad. The following discussion does not <br />provide an exhaustive survey of all human effects <br />but emphasizes those that are relatively wide- <br />spread. To provide an indication of fhe extent to <br />which humans affect the water resources of virtu- <br />ally all landscapes, some of the most relevant <br />structures and features related to human activities <br />are superimposed on various parts of the concep- <br />tuallandscape (Figure 25). <br /> <br />The effects of human activities on the quan- <br />tity and quality of water resources are felt over <br />a wide range of space and time scales. In the <br />following discussion, "short term" implies time <br />scales from hours to a few weeks or months, and <br />"long term" may range from years to decades. <br />"Local scale" implies distances from a few <br />feet to a few thousand feet and areas as large as a <br />few square miles, and "subregional and regional <br />scales" range from tens to thousands of square <br />miles. The terms point source and nonpoint source <br />with respect to discussions of contamination are <br />used often; therefore, a brief discussion of fhe <br />meaning of these terms is presenfed in Box M. <br /> <br />Agricultural Development <br /> <br />Agriculture has been the cause of significant <br />modification of landscapes throughout the world. <br />Tillage of land changes the infiltration and runoff <br />characteristics of the land surface, which affects <br />recharge to ground water, delivery of water and <br />sedimenf to surface-water bodies, and evapotrans- <br />piration. All of these processes either directly or <br />indirectly affect the interaction of ground water <br />and surface water. Agriculturalists are aware of <br /> <br />the substantial negative effects of agriculture on <br />water resources and have developed methods to <br />alleviate some of these effects. For example, tillage <br />practices have been modified to maximize reten- <br />tion of water in soils and to minimize erosion of <br />soil from the land into surface-water bodies. Two <br />activities related to agriculture fhat are particu- <br />larly relevant to the interaction of ground water <br />and surface water are irrigation and application of <br />chemicals to cropland. <br /> <br />54 <br /> <br />