RELATION OF FOEESTS TO STREAM FLOW. 281 



and deep seepage. By evaporation is meant the moisture which passes 

 into the atmosphere in the form of vapor from water and soil surfaces 

 and from objects resting upon such surfaces, in chiding vegetation. 

 Transpiration is that portion of the rainfall which sinks into the soil, 

 and w^hich is later taken up by the vegetation through the roots and 

 given olf to the atmosphere through the stems and foliage. To this 

 latter should be added, although not actually a part of it, the compara- 

 tively small amount of moisture taken up by the vegetation, but which 

 through chemical change becomes a part of the organic vegetable 

 structure. By surface or superficial run-off is meant that portion of 

 the precipitation which, from the time of falling until its exit from the 

 drainage basin, passes over the surface without gaining access to the 

 soil. On the other hand, by seepage run-off is meant that portion of 

 the rainfall which sinks into the earth, but which later reappears on 

 the surface at lower elevations, and with the surface run-off escapes 

 from the drainage basin in the streams. B}^ deep seepage is meant 

 that portion of the precipitation which sinks into the soil, but to such 

 depths that it does not reappear later on the surface of the drainage 

 basin. 



Evaporation and transpiration are frequently classed together as 

 evaporation. In the irrigated parts of the West they are together 

 known as ' ' fly-off. " So, also, the rainfall which does not escape through 

 evaporation and transpiration or through deep seepage is often classed 

 as run-off or stream flow. 



DO FORESTS INFLUENCE EYAPORATION? 



In order that the moisture which falls to the earth in the form of 

 rain and snow should be most efficient in sustaining vegetation and in 

 feeding streams, as little as possible should escape in the form of 

 evaporation. Under the best of conditions a very large part of the 

 ^annual rainfall is returned to the atmosphere through evaporation. 

 For humid regions, bearing the same types of vegetation, the amount 

 does not vary much from year to year, no matter what the fluctuations 

 in rainfall are — a fact first made known by Messrs. Lawes, Gilbert, 

 and Barrington in the classical Rothamsted investigations. These 

 gentlemen explain this persistency in the rate of evaporation by the 

 fact that heat and abundant rain seldom occur at the same time. Con- 

 sequently, in a wet season, the lower temperature and more or less 

 saturated atmosphere prevent excessive evaporation; while in a dry 

 season, although the temperature is higher and the air drier, there is 

 less water to evaporate, and the two extreme conditions balance each 

 other so far as the amount of evaporation is considered. This is not 

 true, however, in arid and subarid regions, because during years of 

 minimum rainfall the upper layers of the soil are often so dr}^ for 

 months at a time that there is very little moisture to evaporate, while 



