148 Irrigation and Drainage 



tillage to conserve soil moisture is chiefly concerned 

 with saving moisture which has penetrated the ground 

 to a depth exceeding 2.5 to 3 or more inches. The 

 moisture which is caught and held by the soil closer 

 to the surface than stated must usually be taken up 

 directly by the surface feeding roots, or it must be 

 lost by surface evaporation. 



When the snows and frosts of winter have melted, 

 and the earliest spring rains have come, the soil is 

 usually left so moist as to be fully saturated with 

 water to a depth exceeding 1, 2, and even 3 feet, 

 according as the snows or rains have been copious or 

 light. At the same time, the texture of the surface 

 soil has been so changed as to place it in the very 

 best possible condition for rapidly conveying the deeper 

 soil-water to the surface, where, if the sun shines and 

 a brisk, dry wind is blowing, it will be lost with great 

 rapidity, sometimes in single exceptionally favorable 

 days amounting to 2, 3, and even 4 pounds per square 

 foot per day, equivalent to more than 40, 60 and 80 

 tons per acre. 



But these high rates of loss are not maintained, 

 fortunately, for long periods of time, even when there 

 has been no effort made to prevent them. We have, 

 however, measured losses during seven days amounting 

 to 9.13 pounds per square foot, or at a daily rate of 

 1.3 pounds; and in four days a rate as high as 1.77 

 pounds per square foot. Under extremely favorable 

 conditions, and where the surface of the soil was kept 

 continuously wet, we have measured a mean daily loss 

 by evaporation as great as 2.37 pounds for fine sand, 



