MICHIGAN EXPERIMENT STATION 19 



FORESTS AND MOISTURE. 



Much has been written about the relation of forests to rainfall; some 

 claiming that rain is increased by forests, and others that there is no con- 

 nection between them. 



Geo. P. Marsh in his admirable work "Man and Nature," has pointed 

 out how forests restrain the rapid escape of the rain; preserving springs 

 and small streams, retarding floods, and mitigating drought. Their influ- 

 ence in preventing the drifting of snow and keeping an even covering of 

 that wonderful blanket for the sleeping earth, is well recognized. 



The influence of forests in diminishing evaporation by restraining the 

 velocity of the wind at the surface of the ground is worthy of considera- 

 tion. I do not refer to the well-known influence of tall forests in restrain- 

 ing the violence of the wind as a body, but to the influence of forests, 

 shrubs and vegetable growth of every kind in keeping the air quiet at the 

 ground level, where alone soil evaporation takes place. 



To obtain some measure of the relative rapidity of evaporation in a 

 draft and in comparatively still air; the following experiment was 

 tried: The air in one part of the chemical lecture room was so nearly still 

 that a feather would not move perceptibly; by opening windows) in an 

 other part of the room a strong draft could be secured at one window, ■ 

 the wind blowing 12 miles an hour. Two square pieces of Turkish towel- 

 ing of same size, were thoroughly wet, just short of dripping, then weighed 

 separately, one suspended in the still air of the room tor an hour, while 

 the other was similarly hung in the draft by the window, or in the 

 mouth of the ventilating shaft. Both were left for an hour, then weighed 

 again and the loss in weight showed the amount of water evaporated in 

 each case. The trial was made seven times, with the result that the evap- 

 oration was four times greater in the draft than in the still air. The 

 actual amount evaporated was not the same for each hour, but the ratio 

 of evaporation was almost identical in the whole series, viz: four times 

 as much in the draft as took place in the still aii". 



The reason for this is not hard to find. A volume of perfectly still air 

 surrounding a wet body wjll take up moisture with progressive slowness 

 till the air is saturated; but if this damp air is blown away and replaced 

 by relatively dry air, evaporation will go on with increased rapidity, and if 

 the air is constantly renewed as in a draft of wind, the evaporation will 

 be more rapid. Every washer-woman knows that the clothes will soon 

 blow dry when hung out in a stiff breeze, but will take hours if lined up in- 

 doors. 



The principle of my laboratory experiment will hold in the broad open 

 of Nature. The air in contact with the moist ground will take up water 

 till it is saturated and then evaporation will be suspended, provided the 

 air remains still and undisturbed. But if this bottom air is swept away 

 by wind, evaporation will be renewed and the drying of the soil will again 

 go on. The influence of trees, shrubs and even the grasses in preserving 

 in some degree this shallow pool of quiet air at the ground level and thus 

 diminishing evaporation from the soil may seem a trifling matter at first 

 thought, but becomes of great moment on the large scale of nature. 



RELATION OF SOIL TO MOISTURE. 



The capacity of a soil to imbibe and retain water is intimately connected 

 with its composition. The open and porous soils of a sandy character, 



