28 ELEMENTARY FORESTRY. 



as an element of dissipation may be learned from the experi- 

 ments of Prof. T. Russell, Jr., of the U. S. Signal Service, 

 made in 1888. We learn from these that the evaporation on 

 the western plains 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 sup- 

 plied by waters coming from the mountains, where the precip- 

 itation 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 1000 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 import- 

 ant function in preventing evaporation. 



Wind-Breaking Power of Forests. If the loss by evaporation 

 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 mois- 

 ture from the soil by evaporation. In addition, the circu- 

 lation of the air is impeded between the trunks, and this in- 

 fluence upon available water supply, the wind-breaking power 

 of the forest, must be considered as among the most import- 

 ant 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. 



