856 
Journal of Agricultural Research 
Vol. XXVIII, No. 9 
sites in the aspen-fir zone. The data are presented in figures 1 and 2. The 
changes in the evaporation on these sites in 1919 and 1920 and in the soil tem¬ 
perature at a depth of 1 foot in 1920 are shown in figures 3 and 4. 11 A study of 
figures 1 and 2 and Tables V and VI shows that the surface foot of soil is fre¬ 
quently dried out for considerable periods below the limit of availability for 
plant growth. The soil moisture determinations, which were made in con¬ 
nection with studies of forest planting and were therefore confined to the upper 
foot of soil, indicate that on the more severe sites the vegetation must, for ex¬ 
tended periods, draw the requisite amount of moisture from the lower layers of 
soil in which the plants are rooted. The upper stratum supplies soil moisture 
only intermittently following heavy rains which are of infrequent occurrence 
during the height of the growing season. 
Fig. 3.—March of evaporation jn Big Cottonwood watershed, central Utah, season of 1919 
(Seasonal averages shown by horizontal lines at left of graph) 
The deficiency of summer precipitation at low altitudes is aggravated, from the 
standpoint of forest planting, by rapid evaporation, which shows a marked 
response to aspect. The mean daily evaporation rates are also shown for each 
site. In comparing the moisture relations of the different sites the evaporation- 
soil moisture ratio has been used. This ratio is the quotient of the evaporation 
divided by the mean seasonal moisture content of the soil (in this case a depth 
of 6-12 inches). A systematic gradation occurs from the more or less xerophytic 
southern aspect, through the four intervening sites, to the mesophytic north¬ 
eastern aspect covered with a good stand of aspen. The aspen succeeded a 
forest of Douglas fir and white fir which was removed by logging and fire. 
n The two years’ record of evaporation was obtained by means of Shive’s improved form of the Living¬ 
ston standardized rain-correcting porous cup atmometer. 
