134 ENGINEERING FOR LAND DRAINAGE. 



the drains are laid 3 feet deep we have a soil res- 

 ervoir of that depth which, if it is very dry, will hold 

 one third of its depth in water. But this condition is 

 rarely if ever met with in practice, but if the soil is 

 well drained it will take 2 inches of rain to saturate it, 

 the amount, however, depending upon the soil. The 

 evaporation, which in the summer begins at once, and 

 also the absorption and evaporation of moisture by 

 plants, make a large draft upon water of the soil. 



Most of the data on evaporation and drainage are 

 taken from observations made in England. This does 

 not apply to our soil and climate for the reason that 

 the atmosphere of England is much more humid and 

 the evaporation from plants and soil much less than it 

 is in this country. It has been shown by good authority 

 that evaporation from soil and water in this country 

 is fully twice as great as that in England, and it fol- 

 lows that a like difference should exist in evaporation 

 from plants. 



An experiment made in 1889 at Uniontown, Ala., 

 for the purpose of finding the percentage of the rainfall 

 which passes oflf throusfh tile drains is stated briefly as 

 follows: 



The tract of land consisted of three acres drained by 

 lines of tile 3 feet deep and 30 feet apart. The meas- 

 urements recorded were the outflow of excessive rainfall 

 April 13th and 14th, when 3. 39 inches fell in twenty- four 

 hours upon land denuded of vegetation by the prepara- 

 tion for spring crops. From the measurements made it 

 appears that the greatest discharge from the drains was 

 about eighteen hours after the first heavy rainfall, at 

 which time the drains were discharging } of an inch 



