OUR WATER SUPPLY— MEINZER 297 



cipitation because some of the swamp water that would normally 

 be evaporated and reprecipitated is drained away into the sea. On the 

 other hand, the diversion of water from streams that flow into the 

 sea and the use of this water for irrigation should tend to increase the 

 precipitation. Moreover, any changes incident to cultivating and 

 cropping the land or to grazing the land should decrease or increase 

 the precipitation according as they increase or decrease the run-off 

 into the sea. It is generally believed that these artificially produced 

 changes in run-off cannot be quantitatively competent to produce 

 appreciable changes in precipitation. However, it is pertinent to in- 

 quire whether they may have significant effects in some critical areas. 

 It would seem that the subject deserves serious investigation. 



SECOND PHASE OF THE HYDROLOGIC CYCLE 



Return flow and storage en route. — The other phase of the hydrologic 

 cycle consists of the flow of the precipitated water toward the sea or 

 toward places of reevaporation from the land, and its storage en 

 route, chiefly as ice and snow, as surface water in the lakes, ponds, 

 and swamps, as moisture held by molecular attraction in the soil, and 

 as ground water in the subterranean reservoirs formed by the porous 

 rocks. To the extent that the storage facilities are inadequate, the 

 precipitated water is rapidly discharged into the sea through the 

 natural drainage channels as direct run-off. This direct run-off is of 

 little value to man and it produces most of the destructive floods and 

 most of the destructive erosion and sedimentation. If there were no 

 natural storage facilities there would be virtually no springs, no 

 perennial streams, and no trees, grass, or crops, and all stream chan- 

 nels would be subject to sudden and violent floods. 



Near the close of the last century it became evident in this country 

 that accurate continuous records of stream flow were essential for 

 efficient utilization of the water resources and for effective flood con- 

 trol. Since that time a large amount of systematic stream gaging has 

 been done, with accuracy increasing from year to year. Intensive 

 studies have also been made to differentiate between the direct run- 

 off and the run-off derived from the several kinds of storage, and to 

 determine the laws of each and their relation to precipitation. In some 

 of the coastal regions large quantities of ground water are also dis- 

 charged into the sea without appearing at the surface, such discharge 

 being controlled by the geologic structure, the permeability of the 

 water-bearing formations, and the balance between the head of the 

 ground water and the back pressure of the heavier sea water. 



Civilized man has made a notable achievement by supplementing 

 the natural storage facilities with many artificial reservoirs, both 

 great and smaU. Unfortunately, this achievement is likely to prove 

 less substantial in the long run than is popularly supposed, chiefly 



