50 LAND DRAINAGE 



in the soil, of unnecessary amounts of even capillary 

 water. 



As the curves approach that part of the chart indicating 

 a saturated condition of soil, they indicate a very low 

 temperature. 



73. Heat lost in the evaporation of gravitational water. 

 In paragraph 56 it was pointed out that evaporation 

 of water always results in the loss of heat, that the amount 

 of heat used in the evaporation of unit weight of water 

 is constant, and that the heat rendered latent in the evap- 

 oration of two tons of water from an acre of soil is suffi- 

 cient to warm an acre-foot with its normal (20 per cent) 

 content of water 2.416 F. 



King and his assistant found that from the unculti- 

 vated surface of a sandy loam, placed in cylinders and 

 exposed in an open field for a period of thirty-seven 

 days, losses by evaporation occurred at the rate of 7.24 

 tons to the acre for twenty-four hours. In this case, 

 the water-table averaged 20 inches below the sur- 

 face of the soil. The series of experiments of which 

 this was a part showed that the evaporation losses vary 

 with the kind of soil and with the time it has been under 

 cultivation. 1 



In the Michigan Agricultural College soil laboratories, 

 in experiments with loam soils repeated through a 

 period of fourteen years, the losses by evaporation from 

 uncultivated surfaces 24 inches above the water-table 

 differed very little from 10 tons to the acre for twenty-four 

 hours. 



King concluded, from experiments in the open field, 

 that from very wet soils in April and May, the evaporation 

 losses frequently amount to 28 tons to 33 tons to the 



1 Report Wisconsin Exp. Station, 1898, p. 134. 



