UNITED STATES DEPARTMENT OF AGRICULTURE 
dWjb BULLETIN No. 835 4W 
^Mti^^iK? Contribution from the Bureau of Public Roads %SS!^^i( 
Bureau i 
THOMAS H. MacDONALD, Chief 
JfiJP^mfl* 
Washington, D. C. 
PROFESSIONAL PAPER 
August 6, 1920 
CAPILLARY MOVEMENT OF SOIL MOISTURE. 
By Walter W. McLaughlin, Senior Irrigation Engineer. 
CONTENTS. 
Page. 
Object 2 
Plan of experiments 2 
Eate 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 1_ 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 \ 
