CHAPTER XXIV. 185 



EVAPORATION. 



We have another subject to consider — that is, evaporation. Under 

 this head will be included transpiration from foliage. 



On the subject of evaporation there exist many experimental 

 data. Temperature and wind are the chief controlling elements. 

 Woods lower temperature and reduce the velocity of the wind. It is 

 to be expected, therefore, that evaporation in woods is much less than 

 in the open. Such is found to be the case. 



The observations of sixteen forestry stations in Germany show a 

 marked saving effected by the woods. Of the rainfall, an average 

 of 42 per cent, was evaporated in the open and 24 per cent, in the 

 torest. A clear saving of 18 per cent. The Prussian stations showed 

 the evaporation under trees to be about 43 per cent, of that from 

 fallow fields; this percentage for the previously mentioned sixteen 

 stations would be 57 per cent. The evaporation from water sur- 

 faces in woods was found to be about 38 per cent, of that from water 

 surfaces in the open. 



As an offset to the saving in evaporation comes the moisture 

 transpired through the foliage, and that retained in the substance of 

 the tree. The transpiration computed by various observers ranges 

 from an equivalent rainfall of one-fourth inch per annum for four-year 

 old firs, up to fifteen inches for cereals and thirty-seven inches for 

 grasses. Forests of mixed growth transpire about 6% inches. Ac- 

 cording to observations at the Austrian stations, deciduous trees trans- 

 pire during the period of vegetation 500 to 1,000 pounds of water per 

 pound of dry leaves, and the coniferous from 75 to 200 pounds. (This 

 suggests the natural selection of conifers for our mountain slopes.) 

 One remark of Hohnel, regarding the Austrian observatories, is very 

 suggestive. He says "a plant will transpire in proportion to the 

 amount of water which is at its disposal." This remark serves well to 

 mark the point that willows and other water-loving growths along 

 our streams consume more water than they save. 



It is estimated that a coniferous forest will transpire 8 per cent. 

 of a total rainfall of 20 inches, and a beach forest 48 per cent of the 

 same precipitation. The water annually absorbed into the structure 

 of the trees has been estimated as ranging from 19 to 52 per cent, of 

 the weight of the wood, and 54 to 65 per cent, of the weight of the 

 leaves. By the hardwood deciduous trees, 38 to 45 per cent; the soft- 

 -wood, 45 to 55 per cent, and the conifers, 52 to 65 per cent. These 



