150 DRY-FARMING 
the sunshine and the air to evaporate the water 
very quickly. Such cultivation is very desirable 
for other reasons also, as will soon be discussed. 
Meanwhile, the water-dissipating forces of the dry- 
farm section are not wholly objectionable, for 
whether the land be cultivated or not, they tend to 
hasten the formation of dry surface layers of soil 
which guard against excessive evaporation. It is in 
moist cloudy weather, when the drying process is 
slow, that evaporation causes the greatest losses of 
soil-moisture. 
The effect of shading 
Direct sunshine is, next to temperature, the most 
active cause of rapid evaporation from moist soil 
surfaces. Whenever, therefore, evaporation is not 
rapid enough to form a dry protective layer of top- 
soil, shading helps materially in reducing surface 
losses of soil-water. Under very arid conditions, 
however, it is questionable whether in all cases shad- 
ing has a really beneficial effect, though under semi- 
arid or sub-humid conditions the benefits derived 
from shading are increased largely. Ebermayer 
showed in 1873 thatthe shading due to the forest 
cover reduced evaporation 62 per cent, and many 
experiments since that day have confirmed this 
conclusion. At the Utah Station, under arid condi- 
