FIFTH NATIONAI, CONSERVATION CONGRESS 



193 



Balance Sheet — Circulation of water on the earth's surface. 



Cubic 



miles 



Depth 



Per 



cent 



An analysis of these figures discloses the fact that one-fifth of the entire 

 vapor on the earth's surface comes from evaporation on land ; that only 7 per cent, 

 or 5,997.5 cubic miles, of all the water evaporated from the oceans enters into 

 the precipitation over land, and that 78 per cent of all the precipitation that falls 

 over the peripheral land area is furnished by this area itself. 



Where is evaporation on land greatest ? The evaporation from a moist, bare 

 soil is, on the whole, greater than from a water surface, especially during the 

 warm season of the year, when the surface of the soil is heated. Soil covered only 

 with a dead vegetable cover evaporates moisture much more slowly than a bare 

 soil or an open water surface. On the other hand, a soil with a living vegetal 

 cover loses moisture, both through direct evaporation and absorption by its vege- 

 tation, much faster than bare, moist soil. 



The more highly developed the vegetal cover the faster is moisture extracted 

 from the soil and given of? into the air. In this respect the forest is the greatest 

 desiccator of ground moisture. The experiments of Otozky, which have been 

 fully confirmed by many observers in other countries, have conclusively shown 

 that the forest, on account of its excessive transpiration, consumes more moisture, 

 all other conditions being equal, than a similar area bare of vegetation or covered 

 with some herbaceous growth. 



The amount of water consumed by the forest is nearly equal to the total an- 

 nual precipitation — in cold and humid regions less and in warm and dry regions 

 somewhat greater. This enormous amount of moisture, which is later given off into 



1 The difference between the amount of vapor that escapes from land to the ocean and 

 from the ocean to land. 



