312 The Farm Woodlot 



only one in the region — is destroyed and irrigation in a 

 larger part of that district is rendered impossible. While 

 this silting up of the reservoir is going on, the fine particles 

 not deposited in the basin are carried down the ditches 

 and spread over the fertile fields, dulling their productive 

 capacity. 



These are the results of cutting forests as expressed in 

 the terms of erosion. A well-placed forest prevents these 

 results. 



LESSENING OF EVAPORATION 



Distribution of stream flow and the prevention of erosion 

 are not, however, the only influences that the forests 

 exercise over the surrounding country. They lessen 

 evaporation, which is a very active agent in drawing the 

 moisture from the soil. T. Russell, Jr., of the United 

 States Signal Service, made some experiments in 1888 

 to learn the amount of evaporation on the western plains. 

 During the year this evaporation amounted to 50-80 and, 

 in some spots, even to 100 inches, while the rainfall over 

 this area is 30-12 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 precipi- 

 tation 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.)" 



To understand better the application of this, we must 

 examine the most important factors determining the 

 amount of evaporation. The first of these factors to be 

 considered is the soil cover. In experiments made in Ger- 



