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 dr\dng 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 cjuestionable whether in all cases shad- 

 ing has a really l^eneficial effect, though under semi- 

 arid or sub-humid conditions the benefits derived 

 from shading are increased largely. Ebermayer 

 showed in 1873 that the 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- 



