HYDROLOGIC PRINCIPLES 19 



for determining the extent and value of the minerals therein, 

 from which an estimate of the total reserves in the deposit may 

 be made. Thereafter the computations of depletion by min- 

 ing and of reserves still underground are bookkeeping opera- 

 tions, continued until the mineral deposit is exhausted. 



Water is different. Except in a very few places it is a transient 

 resource which cannot be conserved for the future merely by 

 not using it. On the other hand, utilization of water does not 

 necessarily mean that the Nation's reserves are being depleted, 

 for the great majority of uses are made possible by diversion 

 of water which would otherwise continue through the hydro- 

 logic cycle to be discharged into the atmosphere or into the 

 ocean. Because our water resources are both renewable and 

 mobile, the available reserves at any one place are continually 

 changing, and an inventory runs into problems similar to those 

 met in determining the density and flow of traffic along a sys- 

 tem of highways. 



A complete inventory of water resources requires continu- 

 ing maintenance of representative check points at the land 

 surface to determine the gross water supply from precipita- 

 tion and the return to the atmosphere by evaporation and 

 transpiration; in the soil zone to determine changes in soil 

 moisture and amount of downward percolation; in the ground- 

 water reservoirs to determine changes of storage, lateral move- 

 ment, and ultimate discharge; in streams to determine outflow 

 from drainage basins and the gain or loss within these basins; 

 in the developmental works of man to determine consump- 

 tive use and waste as well as nonconsumptive use and disposal; 

 and throughout the cycle to determine changes of chemical, 

 physical, or biological quality of the water that might affect 

 subsequent utilization. 



For the country as a whole no phase of the hydrologic cycle 

 is completely inventoried. The data concerning precipitation 

 and stream flow are sufficiently comprehensive to provide rea- 

 sonable estimates of the gross water supply and of the surplus 

 carried by streams flowing from the Nation's borders, by long- 

 term averages as well as annually, seasonally, or for shorter 



