140 FOREST INFLUENCES. 



It is, tlieu, the ijrotccHon against evapnraiion alone, duo to greater 

 relative huniidity of the forest air, to the shade, to the breaking of 

 the winds, and especially to the ])rotective soil cover, which males the 

 forest a conservator of moisture evcri/where, even where it does not by 

 its peculiar location increase the amount of precipitation. 



Springs, then, may be influenced in the amount of their discharge by 

 a removal of the forest, not because the forest sup])lies them directly 

 with more water, but because by its removal the rate of evaporation is 

 increased. 



SUMMATION OF THE CONSERVATIVE AND DISSIPATIVE INFLUENCES. 



The total conservative action of the forest with reference to available 

 water supplies, aside from an increase of precipitation, is expressed by 

 the difierence between the elements of dissipation and those of conser- 

 vation; the former comprised in the loss of the water by retention or 

 interception, evaporation, and transpiration, the latter in the protec- 

 tion against evaporation. This balance is known to be in favor of the 

 forest cover in some localities and under certain given conditions; but it 

 will have become apparent that a general statement or quantitative 

 expression of the amount of benefit is in general quite impossible. Yet 

 in an ingenious manner a calculation for one of the Prussian mountain 

 districts is proposed by Dr. Weber as follows: Using the figures which 

 are exhibited in the table on page 113 he argues that the amount of 

 water left, over and above the amount evaporated in the open at low 

 altitudes, deducted from the amount left over and above evaporation in 

 the forests of high altitudes, will suffice to cover the amount of trans- 

 piration; thus, in the spruce forest at the station of Sonnenberg, the 

 surplus of precipitation above the water needed for evaporation had 

 been 1,093.8 mm.; deducting from this the quantity which was found 

 remaining in the open at Schoo, and which would suffice for purposes 

 of transpiration and plant growth, a balance for drainage of 771.3 mm. 

 results; for the beech forests at Melkerei and Hadersleben, the calcula- 

 tion gave a balance of 1,176.8—495.8=681 mm. for drainage. On the 

 average, therefore, 700 mm. of the precipitation in the mountain forest 

 in this locality are saved for the " run-off," that is, 100,000 cubic feet of 

 water per acre. 



To get a conception of what these 100,000 cubic feet mean in the river 

 flow, it may be stated that with average water level the Rhine above 

 Manheim has a flow of 47,700 cubic feet per second, an amount which 

 would be yielded by 40,000 acres of mountain forest, provided all water 

 is drained into the river; and to keep the river continually flowing at 

 this rate would require, on the basis of these figures obtained experi- 

 mentally, a forest area of 23,472 square miles, a calculation which by no 

 means leads to absurd results for practical probability, since the drain- 

 age area of that part of the river is in reality about 30,000 square miles, 

 largely in forest. 



