KELATION OF EVAPORATION TO FORESTS. 



101 



since the plant takes up little or no water in tliat Avay. We liave, there- 

 fore, to add to the other forest additions to the vapor of the air 0.30 x 

 2.02 = O.Gl per cent of evaporation from a free water surface in the open 

 air. To this should be added the amount of dew which is evaporated, 

 but this is uidvuown, and as it is condensed from the adjacent air at 

 night to be added to it again in the daytime, its omission will not make 

 the result less significant. The moisture added to the air is, therefore, 

 roughly, for the warm season : 



By trauspiratioii, .77 per cent of the free surface evaporation. 



By interior evaporation, .13 per cent of the same. 



By evaporation of rainfall, .61 per cent of tlie same. 



Total, 1.51 per cent of the evaporation from a water surface in open fields. 



To get the total in terms of the i)recipitation, this result is to be 

 divided by 2.02, giving 0.75. 



As to other forms of vegetation various estimates are given, but Dr. 

 Wollny within a few years has made a series of careful measurements. 

 From them, by reducing, we get — 



For mixed crops and fallow, 1.44 of evaxtoration. 

 For clover, Avheat, oats, etc., 1.73 of same. 

 For sod, 1.92 of same. 



These values are conservative, other authors giving generally larger 

 ones. The evaporation from bare soil varies much with the kind of 

 soil and its degree of saturation Avith water. 



As Dr. Ebermayer kept the soil constantly saturated, his results do 

 not represent natural conditions. The evaporations from soil in open 

 fields will be cut down by general lack of sutficient moisture to satu- 

 urate them. Schulder's value of soil evaporation has, therefore, been 

 used. By it the soil evaporates O.GO as much water as a water surface 

 sheltered from sun and wind, but otherwise freely exposed to the air. 

 Tabulating the results, we find that the percentages of additions of 

 moisture to the air over different kinds of vegetation are about as fol- 

 lows for the warm season : 



Evaporalious from various livds of vegetation. 



Sod 



Cereals . . . . 



Forest 



Mixed, etc 

 Bare soil. .. 



Austrian measurements of Ricgler, published in the Journal of the Forest Experiment 

 Stations in 1870, from which it would api)ear that the rainfall reai liin<; the soil is 

 increased by from 3.1 to li).fii)er cent through the water rnnnin<; oft" alonjj; the trunlvs, 

 accordinjLC to the kind of irees; the lirst fij^are referrini^ to spruce, the la.st to beech. 

 The total loss of water by interception may then be averaged to be 12 per cent of the 

 rainfall instead of 30. See also p. 134 of this bulletin.— B. E. F. 



