HYDROLOGIC PRINCIPLES 33 



saturated zone — fluctuates in direct response to rainfall or to 

 stream flow and to the movement of water toward the areas of 

 discharge. Some ground- water reservoirs may be susceptible to 

 recharge from the land surface over their entire area. 



Portions of many aquifers are covered by relatively imper- 

 meable material, which not only impedes downward percola- 

 tion of water from the soil zone but also confines the water 

 within the aquifer. Commonly the confined water has moved 

 laterally from areas where the confining layer is not present 

 and where it has moved into the aquifer by percolation from 

 the surface. Water so confined may develop sufficient artesian 

 pressure to cause it to rise above the bottom of the confining 

 layer or even to flow at the land surface if that layer is punc- 

 tured by a well. Under these artesian conditions the rate of 

 movement at any point fluctuates with the changes of pressure 

 gradient, which in turn may reflect changes in storage in the 

 recharge area or changes in rates of discharge. The storage of 

 water in these artesian portions of an aquifer remains rela- 

 tively constant as long as the aquifer remains "full." However, 

 slight changes result from changes in pressure, and these slight 

 storage changes may represent considerable volumes of water 

 in an aquifer extending over many square miles. 



Discharge from a well, whether by pumping or artesian flow, 

 inevitably lowers the water table or artesian pressure in the 

 vicinity. If by this lowering the water is diverted from a point 

 of natural discharge, or if a greater amount of water is induced 

 to enter the recharge area to replace the water withdrawn, 

 the well can take its place in the natural circulating system 

 and yield water perennially. Thus a declining water table or 

 artesian pressure is not necessarily a cause for alarm. 



Most instances of declining water levels result directly from 

 ground-water development. Some indicate marked depletion 

 in storage in a ground-water reservoir; others have developed 

 in reservoirs which show no evidence of over-all depletion in 

 storage and result from the inadequate capabilities of the rock 

 materials to transmit the water quickly enough to the points 



