A NATIONAL PLAN FOR AMERICAN FORESTRY 309 



The following information on retardation of snow melt was ob- 

 tained in the snow studies just cited. In Idaho the snow cover 

 disappeared in the forest from 3 to 10 days later than in the open; at 

 least 10 days later in Nevada; " several weeks" later in Arizona, the 

 snow occurring, however, merely as drifts in the timber ; and from 1 to 

 5 weeks later in Washington. Ashe 9 reported that 20 inches of snow 

 falling at an elevation of^GOO feet in Maryland during March 1906 

 was 9 days longer in melting beneath a cover of Virginia pine than in 

 the open, and also longer though by a smaller interval beneath an oak 

 forest than in the open. 



REDUCTION OF EVAPORATION FROM THE SOIL 



In addition to intercepting precipitation and retarding snow melt, 

 the crowns and trunks of trees greatly reduce the rate of evaporation 

 from the soil, just as they have been seen to lessen evaporation or 

 sublimation of snow. In regions of low rainfall, where the forest is 

 open and litter is not continuous or deep on the forest floor, reduction 

 in evaporation from the soil is very much to be desired. W. C. 

 Lowdermilk found, in an analysis of factors affecting the yield of 

 water from watersheds in southern California, in 1930, that if all rain 

 in southern California were to occur as 0.5 inch storms one week apart 

 evaporation would account for practically the total supply of meteoric 

 waters. Although half an inch of rain may penetrate the soils of this 

 region to a depth of about 8 inches, when the surface is dried by sun- 

 light and wind, the moisture is drawn up by capillary action and is 

 evaporated. Burr 10 also found on cultivated ground in Nebraska 

 that a half-inch rain was of no storage value unless it fell on a surface 

 still moist. 



Fortunately, all the rain does not occur in California, Nebraska, or 

 anywhere else in the United States in small storms at weekly intervals, 

 and evaporation from the soil is universally influenced by a forest 

 cover which not only shades the ground but greatly reduces wind 

 movement. In Arizona, according to Pearson, 11 summer evaporation 

 a few feet above the ground within a forest of ponderosa pine may be 

 only 70 percent of the evaporation in the open. G. M. Jamison found 

 that during July and August 1931, evaporation beneath a dense virgin 

 forest of western white pine and hemlock in Idaho was only 22 percent 

 of that in an area clear-cut and burned, and in a similar stand from 

 which about 65 percent of the cover had been removed it was only 47 

 percent. Bode 12 states that in a heavy oak stand in Iowa summer 

 evaporation was 47 percent, and in a reproducing cut-over area 74 

 percent of that in the open. O. M. Wood found that evaporation 

 during one spring in a rather open, short-bodied stand of mature pine 

 and oak in southern New Jersey was only 65 percent of that in the 

 open. 



It is impossible to state what effect these very substantial reductions 

 in evaporation rate within the forest have upon soil moisture. There 

 are almost no American data on seasonal evaporation from a bare soil, 



9 Ashe, W. W. " Relation of soils and forest cover to quality and quantity of surface water in the Potomac 

 basin." U. S. Geol. Sur. Water Supply Paper No. 192. 1907. 



10 Burr, W. W. " The storage and use of soil moisture." Nebraska Agri. Exp. Sta. Research Bui. no. 5. 

 1914. 



" Pearson, Q. A. " Forest types in the Southwest as determined by climate and soil." U.S.D.A.Tech. 

 Bui. no. 247. 1931. 



13 Bode, 1. T. " Relation of the smaller forests area in nonforested regions to evaporation and movement 

 of soil water." Proc. Iowa Acad. Sciences. 1920. 



