Under varying conditions evaporation may lower a reservoir 

 of water IS inches (Ontario) to 8 feet (California) or even 

 12 feet (Egypt) in a year. 1 A given area of conifers will 

 transpire the equivalent of from 3 inches (pines) to 8.5 

 inches (spruces) of precipitation on that area; of hardwoods, 

 from 5 inches (oaks) to 10 inches (beech). 2 



b. Moisture-laden air is cooled as it moves upward, or as 

 it comes in contact with other cooler bodies of air, and the 

 moisture is dropped in the form of rain or snow (precipita- 

 tion). Fogs and dews are also forms of precipitation. 



c. Generally the precipitated water, when it strikes the 

 surface of the land, is absorbed and held by the surface soil 

 (absorption), and when the water content of this layer has 

 reached a certain point, depending on soil conditions, any 

 surplus penetrates by gravity to underground strata of soil, 

 gravel or porous rock (infiltration) where it is stored as 

 ground water. The surface of this underground reservoir is 

 called the "water table." 



d. When the rate of precipitation is greater than the rate 

 of absorption and infiltration, part of the water runs along 

 the surface of the ground directly into creeks and rivers, 

 and thence into the lakes and the oceans. This is called 

 "surface run-off." 



Thus absorption, infiltration and run-off are related. 

 Run-off varies according to the combination of such factors 

 as intensity of precipitation, duration of precipitation, and 

 the absorption and infiltration capacity of the soil. Absorp- 

 tion and infiltration vary according to numerous factors 

 which determine soil characteristics and conditions. With 

 respect to absorption and infiltration there are two dividing 

 points: (a) the soil surface which acts like a sieve and sepa- 

 rates surface run-off from absorption and infiltration; (b) the 

 absorption capacity of the top soil, which does not permit 

 water to pass downward to underground storage until the 

 top-soil reservoir is filled to its field-moisture capacity, a 

 point generally considerably below the point of saturation. 



1 Robert Follansbee, Evaporation from Reservoir Surfaces in Proceedings of the American 

 Society of Civil Engineers, Vol. 59, No. 2, February 1933, pp. 258-262. 



2 Unpublished manuscript by Oran Raber, Water Relation of Trees with Special Refer- 

 ence to the Economic Species of the North Temperate Zone, Forest Service, U. S. Depart- 

 ment of Agriculture. 



