176 CONSERVATION OF GROUND WATER 



to the lake has decreased. Records from several places on 

 the Great Plains indicate a downward trend in precipita- 

 tion and an upward trend in temperature prior to 1940, 

 and these long-term climatic trends may have resulted in 

 reduction of the net water supplies, both of surface and 

 ground water. A basic hydrologic question still to be 

 answered is how much of the inflow to the lake has been 

 derived from ground water and how much from overland 

 flow that has been unable to infiltrate into the ground. 



It might be expected that changes in land use would have 

 affected the flow of streams to the extent that those changes 

 have resulted in modification of the amount of overland flow. 

 However, most of the stream-flow records pertain to relatively 

 large drainage basins. In general for these large drainage 

 basins, as for the major ground-water reservoirs, the effects 

 of land use are not clearly discernible in the available records, 

 partly because the records are not long enough to cover the 

 entire period of settlement and land development by the 

 white man, partly because such effects are masked by the more 

 pronounced effects of climate and of man's use of the water. 



EXPERIMENTAL RESEARCH 



Most of our knowledge concerning the effect of land-use 

 practices upon ground-water reservoirs has been developed in 

 research by the U.S. Department of Agriculture, and to an 

 increasing degree in recent years, by the Tennessee Valley Au- 

 thority and numerous state agencies and educational institu- 

 tions. The research has been done with lysimeters and small 

 plots, and also in small watersheds up to several thousand 

 acres in extent. In these watersheds the studies have been 

 directed toward isolating the various factors that control the 

 relationships of precipitation, interception, infiltration, evap- 

 oration, transpiration, and soil moisture. In a few areas re- 

 search has also included analysis of changes of water levels in 

 wells, but in most the changes in ground-water storage have 

 been derived as the difference between infiltration to the soil 



