20 PLANT SUCCESSION AND CROP PRODUCTION 



Table 1^ 



CIRCULATION OF WATER ON THE EARTH'S SURFACE 



Cubic Miles 



Volume of ocean vapor escaping to the land. 

 Continental vapor over the land 



Total volume of vapor from which land precipitation is derived 



5,997 

 20,870 



26,868 



The present discussion is not concerned with the amount of 

 precipitation into the oceans. The precipitation and evaporation of 

 closed basins in the interior of continents is regarded as equal and 

 therefore omitted. The volume of ocean vapor escaping to the land, 

 5,997 cubic miles, is 7 percent of all the water evaporated from the 

 oceans. The volume of continental vapor furnishes 98 percent of 

 the continental precipitation. 



The Source of Land Evaporation 



Evaporation is a surface phenomenon. A surface which is 

 rough and which absorbs heat will give off vapor more rapidly than 

 one which reflects heat. Therefore, with equal insolation and equal 

 wind velocity a bare surface of moist soil will give off more water 

 per unit of time than will a water surface. This will continue dur- 

 ing a warm season as long as water rises through soil pore spaces 

 to resupply evaporation. A soil surface with only dead vegetation 

 as a cover does not evaporate as much moisture as a bare soil. A 

 soil area with a living cover of vegetation, however, evaporates 

 much more rapidly. Every living plant has an uninterrupted stream 

 of water passing through it. In the case of the flowering plants 

 with the leaves borne on elongated stems some distance above the 

 soil, there is a complicated mechanical structure and a series of 

 interrelated physical processes by which water is absorbed and 

 lifted to the heights of the tallest trees. Plant communities are 

 composed of individuals of different water requirements and capa- 

 ble of absorbing water from different soil levels. In the most highly 

 developed plant communities, the forests, evaporation as well as 

 absorption is taking place at different levels. For example, there 

 is a certain amount of evaporation from the ground cover vegeta- 



* The relation between continental vapor and continental precipitation. 



