Chapter II 



HYDROLOGIC PRINCIPLES 

 AND BASIC DATA 



The Hydro-logic Cycle 



Water suitable for irrigation or for industrial or public use 

 is obtained from many sources, including streams, reservoirs, 

 lakes, wells, springs, drain tunnels, infiltration galleries, cis- 

 terns. These sources will yield a perennial supply to the ex- 

 tent that they are replenished by precipitation seasonally, 

 annually, or at less frequent intervals. However, only a frac- 

 tion of the water from precipitation may be expected to re- 

 plenish these surface and ground-water resources. The rest 

 returns to the atmosphere by evaporation from soils or ponds, 

 etc., and by transpiration through the leaves of plants; that is, 

 by processes collectively called "evapotranspiration." The net 

 water supplies available for our continuing use in desert areas 

 are a very small percentage of the gross supply that falls as 

 precipitation. In cool, humid areas the available water supplies 

 may be more than half the gross supply. 



Precipitation and evapotranspiration are processes in the 

 natural circulation of water which is going on at all times. The 

 term "hydrologic cycle" has been applied to the march of 

 events marking the progress of a particle of water from the 

 atmosphere to the land masses and oceans and its return to the 

 atmosphere. The two basic forces that keep the cycle going are 

 solar energy and the gravitational pull of the earth. Gravity 

 is the controlling force not only in the precipitation but in the 

 downward movement of water thereafter in streams or below 

 the land surface. Underground, molecular attraction becomes 

 an important force that resists the movement of water by 



15 



