IRRIGATION AND DRAINAGE 153 



The effect on evaporation of a layer of dry, granular soil when 

 placed above moist soil has been shown by a series of experiments. 

 The soil received an irrigation of 6 inches in depth over the sur- 

 face, and in the tanks which had no mulch, over a third of this 

 amount was evaporated in thirty-two days, while less than 1 per 

 cent was evaporated in the tanks which were protected by a 9-inch 

 mulch. Similar experiments carried on at Wenatchee, Washing- 

 ton, in June, 1908, showed the following losses in twenty-one days : 

 no mulch, 14I per cent of water applied ; 3-inch mulch, 4 per cent ; 

 6-inch mulch, 2 per cent ; 9-inch mulch, 1 per cent. 



From these tests it is evident that Western orchardists can 

 prevent the greater part of the evaporation losses by cultivating 

 orchards to a depth of at least 6 inches as soon as practicable 

 after each irrigation. 



Percolation losses. In the preceding paragraphs attention has 

 been called to the large amount of water that is vaporized from 

 warm, moist soils, but the loss considered here is of a different 

 character. In all modes of wetting the soil, but more particularly 

 when deep furrows are used as distributors, a part of the water is 

 liable to sink beyond the deepest roots. As a rule, the longer the 

 furrow the greater the loss from this cause. Where the furrows 

 were about an eighth of a mile long it was found, after an irriga- 

 tion, that in some parts of the orchard the soil was wet to depths 

 of between 20 and 26 feet, while in other parts the moisture had 

 not penetrated beyond the third foot. 



One of the best ways of finding out whether much water is lost 

 by deep percolation is to dig cross trenches as deep as the feed- 

 ing roots go. The moisture which passes the deepest roots in its 

 downward course may be considered wasted. 



Winter irrigation. When water is used before or after the regu- 

 lar irrigation period, or, what is in many cases the same, before or 

 after the growing season, it is termed winter irrigation. Over a 

 large part of the arid region the growing season is limited by low 

 temperatures to one hundred and fifty days or less, and when the 

 flow of streams is utilized only during this period, much valuable 

 water runs to waste. 



It was for the purpose of utilizing some of this waste that the or- 

 chardists of the Pacific coast states and Arizona began the practice 



