WATER CIRCULATION AND ITS CONTROL 



417 



nounced in dry climates than where 

 there is much rain. 



The amount by which the water ta- 

 ble may be depressed varies from a 

 fraction of a meter to as much as three 

 meters. It is observed in the level 

 steppes of southern Russia, where the 

 rainfall is moderate, that springs are 

 found outside of the forests rather than 

 in them. In citing these results, it is 

 important to insist upon the distinction 

 to be made between level and sloping 

 surfaces. A comparison of the ground- 

 water level beneath the forest with that 

 beneath a flat field in which percolation 

 is at a maximum gives an entirely dif- 

 ferent result from a similar compari- 

 son, in case the surface is so inclined 

 as to permit run-off from the bare 

 ground. 



According to very careful and accu- 

 rate observations made in Switzerland 

 from 1903 to the present time and still 

 in progress, it is found that the run-off 

 from a wooded watershed is but sixty 

 per cent, of that from a cleared water- 

 shed, all other conditions being the 

 same ; and that the maximum high- 

 water level of the stream from the 

 former is thirty to fifty per cent, lower 

 than that of the stream from the latter. 



The same observations establish the 

 reciprocal fact that there is a larger and 

 more continuous percolation of water 

 into the ground and steadier flow of 

 springs during drought from the wood- 

 ed watershed than from the cleared 

 watershed.* 



Forests in relation to evaporation. — 

 At the surface of the soil moisture 

 passes into a state of vapor and is car- 

 ried away by the air ; this is evapora- 

 tion. At the surface of a leaf moisture 

 is given off by the living organism; 

 this is transpiration. Evaporation is a 

 physical phenomenon which increases 

 with the dryness, temperature, and 

 movement of the air; transpiration is 

 a physiological phenomenon which de- 

 pends upon the kind of plant and its 



response to the conditions to which it is 

 exposed. Evaporation from the soil 

 draws upon the moisture contained in 

 the capillary spaces between the soil 

 grains and extends down to the point 

 where the soil is saturated — that is, to 

 the water table in humid regions, or in 

 arid regions to the point where the sub- 

 soil is so dry that the capillary films 

 are no longer in contact. It continues 

 until the available moisture in the soil 

 is so reduced that the tension of the 

 films about the soil grains becomes 

 great enough to resist the tendency to- 

 ward vaporization at the surface, and 

 evaporation then practically ceases, al- 

 though diffusion into the air spaces of 

 the soil continues even at some depth. 

 Transpiration from the surface of the 

 leaf responds in some measure to sun- 

 light and shadow, to humidity and dry- 

 ness, and to temperature. Among 

 these humidity is the most important. 

 The rate of transpiration is, however, 

 subject to physiological regulation, the 

 mechanism of which is not understood. 

 In certain experiments conducted by 

 the desert laboratory it was found that 

 in plants under examination the water 

 loss due to transpiration was checked 

 at temperatures above seventy-nine de- 

 grees Fahrenheit, whereas the checking 

 action disappeared at lower tempera- 

 tures.^ Thus, some plants possess a 

 means of defense against excessive 

 transpiration during hot weather and 

 drought, whereas the inert soil must go 

 on yielding moisture almost to the last 

 atom. 



There is a certain amount of evapo- 

 ration from the leaf surface, but it is 

 exceedingly slight. It differs from 

 transpiration in that it takes place 

 through the tissue itself, whereas trans- 

 piration occurs through the stomata. A 

 grape which has no stomata gives a 

 good illustration of the very small 

 amount of evaporation from such sur- 

 faces. 



Underneath the trees in the forest 



'Engler: Der Einfluss des Waldes aiif den Stand der Gewasser. Centralblatt fiir das 

 gosammte Forstwesen, Jan., 1907, pp. 35-40. 



'MacDougal, D. T. : Botanical features of North American Deserts. Carnegie Institu- 

 tion, Publication 99, 1908, p. 86. 



