Figure 7.5 



Average Annual Runoff 



Regional data not available for Alaska, Hawaii, and Puerto Rico. 



Still another area with important ground water 

 potential is that of the glacial deposits in the Great 

 Lakes area, extending from central Montana to 

 eastern New York. The deposits contains beds of 

 water-sorted permeable sand and gravel but consti- 

 tute an important source of water. 



Ground water supplies become depleted if recharge 

 of ground water aquifers does not equal or exceed 

 withdrawals. Because mining is substantial in some 

 areas of the nation, ground water levels have been 

 receding rapidly. For example, more than 14 million 

 acre-feet are mined annually in the High Plains 

 area. Thus, much of the nonrenewable ground water 

 in parts of the arid West is being exhausted at 

 a rate that will cause significant reductions in total 

 availability by the year 2000. 



The water supply available for use in a region is 

 the runoff into streams or other water bodies aug- 

 mented by the contribution of ground water to 

 streamflows, plus the amount that is available directly 

 from ground water aquifers on a long-term basis. 

 This supply can be calculated for a region by mea- 

 suring the flow of streams as they leave the region, 

 adding the volume of water consumed in the region, 

 and subtracting the volume of ground water depletion, 

 or the volume of mined ground water. Table 7.10 

 presents a general picture of the Nation's water supply 

 by water resource region. This is the supply expected 

 in a year of average precipitation; 80 years out of 

 100; and 95 years out of 100. This should closely 

 approximate an annual supply based on the stated 

 probabilities of occurrence. 



289 



