162 CONSERVATION OF GROUND WATER 



Effects of Agricultural Use of Land 



The major changes wrought by settlement of the country 

 have been those resulting from development and use of agri- 

 cultural land. Almost everywhere, cultivation has been pre- 

 ceded by clearing of native forest, brush, or other cover, or by 

 plowing of grasslands. Grazing, logging, and burning have also 

 made significant changes in the vegetative cover. 



This wholesale change of one of the renewable resources — 

 plant life — has of course changed the pattern of the animal 

 life dependent upon it. It has also resulted in some major 

 modifications in the resources of soil and water. The natural 

 vegetative cover protected the soil from the eroding effects 

 of running water or of wind, and its removal facilitated the 

 removal of soil and the development of gullies. Some of the 

 eroded material has been carried out to the oceans, but much 

 of it has remained to create other problems, in sediment- 

 laden stream channels and reservoirs. There have also been 

 many historic changes in flow of springs and in the flood flows 

 of streams, which some observers long ago described as due in 

 part to changes in the vegetal cover. 1 In recent years research 

 in small watersheds widely distributed over the country has 

 provided a quantitative relation between the rate of flood 

 runoff and the type of vegetative cover. 2 



These rather appalling effects of man's use of the land are by 

 no means universal. Throughout the country there is great 

 diversity in all the factors that could have any bearing on the 

 infiltration or overland flow of waters. The earth materials 

 at the land surface in some places are so exceedingly permeable 

 that water from precipitation is absorbed as fast as it reaches 

 them; they may be devoid of vegetation because water drops 

 out of reach before plants can use it. In other places dense 



i Marsh, G. P., "The Earth as Modified by Human Action," 2d ed., pp 203- 

 274, Scribner Armstrong and Co., New York, 1877. 



2 Rates of Runoff for the Design of Conservation Structures, A Series of 

 Reports Covering the Basic Land-resource Areas of the United States, U.S. 

 Soil Conservation Service, Tech. Pubs. 60-69, July 1946-August 1948. 



