250 Irrigation and Drainage 



In the third place, the temperature of the surface 

 foot of soil in the daytime of midsummer, with its 

 contained moisture, is usually as high as 68 to 

 75, and to lower its temperature 1 F. requires the 

 absorption by water added of from 25 to 40 heat units, 

 according as the soil varies from a nearly pure sand, 

 weighing 110 pounds per cubic foot, and containing 

 4 per cent of water, to a humus soil, containing 30 

 per cent of water and 50 pounds of dry matter per 

 cubic foot. 



One heat unit is taken as the amount needed to raise 1 

 pound of water at 32 to 33 F. With the relations stated, it 

 appears that 4 inches of water having a temperature of 45 F. 

 applied to a field having a soil temperature of 75 might lower 

 the surface foot to 65 or 61.7, according to the specific heat of 

 the soil ; and with a soil temperature of 68, the lowest tem- 

 perature the 4 inches of water could produce would range be- 

 tween 60 and 57.6. But this assumes that the water is applied 

 at once, with no opportunity for warming until it is brought into 

 contact with the soil, which, of course, cannot be the case. If 

 the irrigation water has a temperature of 50 F., then the lowest 

 degree 4 inches of water could force upon the surface foot of 

 soil would be some amount above 66.7 to 63.7 when the origi- 

 nal soil temperature was 75, or 62 to 59.9 if the initial soil 

 temperature were 68 F. 



The results summarized on page 214 indicate that the mean 

 amount of water used in single irrigations is at the rate of 2.02 

 inches once in 10 days. Hence, were the coldest water used in 

 this quantity, the greatest depression of the temperature of the 

 surface foot could not exceed 6.7 F. This assumes that neither 

 the water nor the soil receives any heat during the time the 

 water is being applied. It is clear, therefore, that where good 

 judgment is exercised in the application of either well or spring 

 water, it may be used without in any serious way interfering with 

 normal growth. The chief danger will, of course, lie in the ap- 



