294 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



that any changes which may have occurred in the total quantity of 

 external water have not had effects of major importance within the 

 period of definite geologic record. It also appears that no changes 

 are in prospect that will affect appreciably the future affairs of man. 

 Although the total quantity of external water has apparently not 

 changed significantly, fluctuations in sea level, probably due to other 

 causes, have been of primary importance throughout geologic history, 

 including relatively recent time. Indeed, many of the large cities of 

 the earth are at present situated below the levels of strands that have 

 been washed by the sea since the human race began to live on the 

 earth. 



The hydrologic cycle. — The water that is of principal concern to man 

 is the land water — the water in the lakes and ponds and in the brooks 

 and rivers, the water that forms the soil moisture, the water in the 

 rocks that supplies the springs and streams and wells. Any natural 

 or artificial change that increases the supply of land water where it 

 is needed, eliminates it where it is destructive, improves its quality, 

 or increases its availability is a distinct human gain; any change in 

 the opposite direction is human impoverishment. 



The land water is not a stationary supply but forms a part of an 

 ever-recurring circulation of great complexity and variation, which is 

 known as the hydrologic cycle. The prime mover in this cycle is the 

 Sim, which, in the last analysis, furnishes the energy that evaporates 

 water from the sea and conveys it as vapor to higher elevations on 

 the land, where it is precipitated, chiefly as snow or rain, with poten- 

 tial energy that tends, through the force of gravity, to carry it back 

 to the sea. The solar energy is also applied in causing evaporation 

 from the lakes, ponds, swamps, and streams, from the land surface, 

 from objects on the surface, from the soil, and, by transpiration, from 

 the leaves of growing plants, including the native and cultivated trees, 

 shrubs, and herbs. Indeed, the records of precipitation and run-off 

 show that only about a third of the water that falls as rain or snow 

 in the United States reaches the sea as run-off — about one-half in 

 the eastern part of the country, only a small percentage on the Great 

 Plains, and none in the Great Basin. 



In its return course, the water flows over the land surface and 

 through the stream channels and percolates through the interstices of 

 the soil and the water-bearing formations. In its course it performs 

 work of great variety, some of which is beneficial and some injurious 

 to man, and much of which has been modified for better or worse by 

 the intelligent or unintelligent activities of civilized man. 



Thus the hydrologic cycle consists of two phases — one phase includ- 

 ing evaporation, atmospheric movement of the water vapor, and ulti- 

 mately its condensation and precipitation upon the land; the other 

 phase including the movement and temporary storage of the precipi- 



