WIND-BREAKING POWER OE FORESTS. 35 



and plateaus may, during the year, amount to from 50 to 80 

 inches, nay, in spots, 100 inches, while the rainfall (diminishing 

 in reverse ratio) over this area is from 30 to 12 inches and less. 



"Thus, in Denver, where the maximum annual precipitation 

 may reach 20 inches, the evaporation during one year was 69 

 inches. This deficiency of 49 inches naturally must be supplied 

 by waters coming from the mountains, where the precipitation 

 is large and the evaporation low. (On Pike's Peak alone there 

 may be 45.6 minus 26.8 or 18.8 inches to spare.)" 



Evaporation from the soil is dependent upon its covering, 

 and this is important, as the soil in forests is always covered 

 with dead branches, leaves, etc. In some experiments which 

 were carried on in Germany during the months of July and 

 August, 1883, to determine the amount of. evaporation from 

 different soils, it was found that from 1,000 square centimeters 

 of bare ground 5,730 grams of water were evaporated, and that 

 from the same area of similar soil covered with two inches of 

 straw 575 grams were evaporated. This shows that the naked 

 soil evaporated more than ten times as much as the covered 

 soil. It is evident then that the soil covering has an important 

 function in preventing evaporation. 



Wind-breaking Power of Forests. If the loss by evap- 

 oration from an open field be compared with that of a forest- 

 covered ground, as a matter of course it will be found to be less 

 in the latter case, for the shade not only reduces the influence 

 of the sun upon the soil, but also keeps the air under its cover 

 relatively moister, therefore less capable of absorbing moisture 

 from the soil by evaporation. In addition, the circulation of 

 the air is impeded between the trunks, and this influence upon 

 available water supply, the wind-breaking power of the forest, 

 must be considered as among the most important factors of 

 water preservation. Especially is this the case on the Western 

 plains and on those Western mountain ranges bearing only a 

 scattered tree growth, and where, therefore, the influence of shade 

 is but nominal. 



The evaporation under the influence of the wind is dependent 

 not only on the temperature and dryness of the same, but also 

 on its velocity, which being impeded, the rate of evaporation is 

 reduced. 



