UNITED STATES DEPARTMENT OF AGRICULTURE 



Washington, D. C. 



Contribution from the Bureau of Public Roads 

 THOMAS H. MacDONALD, Chief 



PROFESSIONAL PAPER 



jTU&'^J'L 



August 6, 1920 



CAPILLARY MOVEMENT OF SOIL MOISTURE, 



By Walter W. McLaughlin, Senior Irrigation Engineer. 



CONTENTS. 



Page. 



Object 2 



rian of experiments 2 



Rate and extent of movement of soil 



moisture by capillarity 13 



Effect of gravity on the movement of 



soil moisture by capillarity 39 



Evaluation of empirical curves 47 



Page. 



Open versus covered flumes 54 



Effect of temperature on soil-moisture 



conditions 56 



The capillary siphon 58 



Capillary movement of moisture from 



a wet to a dry soil 63 



References 69 



The irrigation engineer has long felt the need of more detailed 

 information as to the importance of capillarity as a source of loss 

 of water from irrigation works and the part it plays in distributing, 

 within the soil, water applied in irrigation. It has long been recog- 

 nized that impounding reservoirs and conveying channels lose more 

 water than can be accounted for by direct percolation and evapora- 

 tion. Whether this loss was the result of capillary action alone or 

 in combination with the transpiration from plant growth along canal 

 banks has been only a matter of conjecture. Where the water ap- 

 plied to soil by irrigation goes and how it ultimately distributes itself 

 within the soil have been questions of speculation. 



It has been observed that the percentage of moisture determined in 

 the field in the usual way has not always given a true basis upon 

 which to determine the necessity of applying water by irrigation. 

 In some instances, the percentages of moisture determined have been 

 above the wilting point and yet plants were wilted. 



This condition has caused the irrigation engineer to speculate 

 upon the probability of the rate of movement of soil moisture from 

 one point to another by capillarity, as well as the extent to which the 

 moisture may move. 



The irrigator is always confronted by questions of methods of 

 irrigation, duration of irrigation, and frequency of irrigation. The 



147697°— Bull. 835—20 1 



