236 Bulletin 37. 



ning of the summer of 1900, so moist that it could retain no ad- 

 ditional water permanently. Its degree of saturation was about 

 the same as that of the surface stratum of loam. If any water es- 

 caped the tree roots of the 16th, 17th, 1 8th and 19th feet (where 

 the soil was so moist at the beginning of the season) and percol- 

 ated downwards, it had evidently passed on through this formerly 

 dry stratum into the soil below. Continued heavy irrigation of 

 this orchard during the winter may finally so fill all the soil with 

 moisture from the 15th foot to ground water that at the end of 

 succeeding summers the increase in the degree of saturation will 

 be regular from the 15th foot downward. The line on a diagram 

 representing this condition would then be an inclined one, ap- 

 proximately, straight, instead of curved or angular as are the lines 

 representing the past conditions. 



Amount of Water Needed by an Orchard. 



The set of samples taken April 12, 1901, showed that (as 

 will be seen by referring to Table II) the gain during the past 

 winter was just about the same as the loss during the previous 

 summer — between 108 and 109 pounds per 25-foot column, or an 

 equivalent of approximately 21 inches of water over the orchard. 

 This evidently indicates about how much water should be left 

 deposited, henceforth, at the end of each winter, in this under- 

 ground bank, that the individuals depending upon the deposit 

 for a living may not suffer during the summer. 



The difference between the amount applied during the winter 

 ( 48 inches) and the gain in soil moisture (21 inches) represents 

 the evaporation from the soil, the amount used and exhaled by 

 the trees, and the amount used and exhaled by the clover grown 

 in the orchard for a green-manuring crop. The latter, judging by 

 the amount needed to grow similar crops during the winter, prob- 

 ably withdrew from the soil fully 20 inches of water. The amount 

 lost by evaporation from the soil surface was probably slight, as 

 the soil was covered with the growth of clover most of the period. 



The above amount (four feet) probably represents quite ac- 

 curately the amount that need be applied to deciduous orchards 

 in the warm valleys of southern Arizona tc grow a heavy green- 



