44 AGRICULTURE ON THE PACIFIC SLOPE 



get their water at depths of fifteen and even twenty 

 and more feet; and alfalfa is sometimes found to have 

 its main or " tap " root down to thirty and even fifty 

 feet, reaching for bottom water. Alfalfa does not do 

 well when bottom water is less than five feet below the 

 surface. 



Over-irrigation is hurtful. When by too much irri- 

 gation the ground-water level is made to rise too near 

 the surface, trees and vines may die. Or at least they 

 may not bear well, because their deep roots are killed by 

 drowning. This may cause disease in the whole plant. 



Where gravel beds lie under the soil, enormous 

 amounts of water may be lost by draining into the 

 springs and streams. The farmer must always watch 

 what becomes of his irrigation water, by digging holes 

 to find out how fast and how far it soaks down. 



How is water lost into the air ? Even after the water 

 has soaked into the ground, a very large part of it may 

 still be lost by evaporation under the hot sun and dry 

 winds of summer; for the jjQJLjtcts like a lamp wick, 

 drawing up the moisture from below as it evaporates 

 above. In this way it may waste the rain water of 

 winter as much as does the actual run-off. Water 

 eyapora/tesjastest when the air is hot and dry and when 

 there is much wind. More than a hundred inches in 

 depth may evaporate from a water surface in the Colo- 

 rado desert in a year. Twenty-five to thirty-five inches, 

 however, is a more usual figure elsewhere. 



