170 CONSERVATION OF GROUND WATER 



tion study seem to us to warrant conclusions regarding burning 

 and water conservation there. 



EFFECTS UPON STORAGE IN GROUND-WATER RESERVOIRS 



Where recent gullies have been cut deeply enough to inter- 

 sect the water table, the water table has been lowered as ef- 

 fectively as it would have been by a drainage canal of similar 

 depth. 12 Otherwise, ground-water reservoirs are affected by 

 land use only to the extent that the rate of downward per- 

 colation from the soil is modified. Since only the water that is 

 surplus to the needs of the soil percolates downward, its quan- 

 tity is determined not alone by the amount of infiltration but 

 also by the return to the atmosphere through evaporation and 

 transpiration. The comparative return of water to the atmos- 

 phere before and after the land-use change is an important 

 factor in determining the effect of that change upon ground- 

 water reservoirs. 



The findings as to the effects upon ground-water reservoirs 

 of the agricultural use of the land in the past century or two 

 are based in part on interpretation of the meager historical 

 records but chiefly on experimental research in the past two 

 decades. 



HISTORIC RECORDS 



The popular notion is that there has been a general and 

 progressive decline of "water tables" (using the term to in- 

 clude the artesian-pressure surfaces of artesian aquifers) 

 throughout the nation. It is true that in many localities water 

 levels in wells are now considerably lower than in bygone 

 days and that these localities are widely distributed through- 

 out the nation. There are enough records to show also that the 

 condition is not universal, for the "water tables" in many areas 

 are higher today than in past decades and in some instances 

 higher than at any time in history. In most of the country the 



12 Bryan, Kirk, Change in Plant Associations by Change in Ground-water 

 Levels, Ecology, vol. 9, pp. 474-478, 1928. 



