INTRODUCTION 9 



the work of natural forces, the forces which have in geologic 

 time reduced mountain ranges to low-lying plains and accu- 

 mulated sediments to thicknesses of thousands of feet. Gen- 

 erally man can claim credit only for hastening or increasing 

 the natural processes of aggradation and degradation. 



In the absence of actual records of ground-water levels in 

 wells since the beginning of settlement, there is a broad field 

 for argument as to the effects of these changes in vegetative 

 cover upon ground-water storage. It is certain that where the 

 earth materials were saturated to within a few feet of the sur- 

 face, the cutting of deep gullies has lowered the water table 

 as effectively as ditches have done for numerous drainage proj- 

 ects. Beyond that, the historic records are inconclusive. The 

 declining trends of water levels in areas of pumping are poor 

 evidence of the effect of land use because they can so obviously 

 be due to the pumping. On the other hand, records of water 

 levels in wells unaffected by pumping do not extend back far 

 enough into the past to show whether or not there has been 

 depletion of ground-water storage in the past two centuries, or 

 to what extent long-term climatic trends might be responsible. 



Scientific research has documented the relation of the land 

 surface to the replenishment of subsurface water supplies and 

 also the great variety of factors involved in that relation in 

 various localities. Through this research effective methods 

 have been developed for reducing or eliminating overland 

 flow of water from precipitation and for increasing the infiltra- 

 tion of water into the ground. These methods have been ap- 

 plied in many areas with resulting increase of storage of water 

 in the soil and in the entire zone above the shallowest imper- 

 meable material; the yields of wells and springs have been in- 

 creased also in numerous localities. In addition, overland flow 

 of water has been reduced, with consequent reduction of soil 

 erosion, of turbidity in streams, and of sedimentation in stream 

 channels. 



The increased infiltration and reduced overland flow is not 

 achieved without cost, however. Any vegetative cover uses 

 water, and in some instances that use may be a high proportion 



