58 



drying up of streams and springs and by a diminution in the mi- 

 nimum flow of rivers is a well established fact. The forest is the 

 most effective agent known in regulating the disposition of the 

 precipitation after it reaches the ground. 



Rainfall escapes from the ground upon which it falls in five 

 ways — through evaporation, transpiration, surface run-ofl", seepage 

 run-off and deep seepage. By evaporation is meant the moisture 

 which passes into the atmosphere in the form of vapour from 

 water and soil surfaces and from objects resting upon such sur- 

 faces, including vegetation. Transpiration is that portion of the 

 rainfall which sinks into the soil, and which is later taken up by 

 the vegetation through the roots and given off to the atmosphere 

 through the stems and foliage. To this latter should be added, 

 although not actually a part of it, the comparatively small amount 

 of moisture taken up by the vegetation, but which through chemi- 

 cal change becomes a part of the organic vegetable structure. By 

 surface or superficial run-off is meant that portion of the precipi- 

 tation which, from the time of falling until its exit from the drain- 

 age 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. By deep see- 

 page 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 to- 

 gether 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 EVAPORATION ? 



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 vege- 

 tation, 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 per- 

 sistency in the rate of evaporation by the fact that heat and 

 abundant rain seldom occur at the same time. Consequently, in 

 a wet season, the lower temperature and more or less saturated 

 atmosphere prevent excessive exaporation ; 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 



