DRAINAGE OF IRRIGATED LAND. 3 
employment of one trained in this work to make the preliminary 
studies and to design the system. From a drainage standpoint, the 
average landowner rarely has any extensive knowledge of the con- 
ditions on his farm, especially of those beyond the plow depth, which 
latter are, in this form of reclamation, of the highest importance. 
SOURCE AND AMOUNT OF DAMAGING WATER. 
The most important factors affecting the design of a system of 
drainage for irrigated land are the source and movement of the 
damaging water. The water does not come directly from precipita- 
tion, as is the case in humid regions, although precipitation may have 
a bearing on the problem and may need to be considered. Percolating 
irrigation water usually is the cause of the injury, and this may have 
its movement downward through the soil of the tract being irrigated, 
laterally through pervious strata extending back under higher lands, 
or upward from pervious strata having considerable depth and con- 
necting with distant sources at a higher elevation. Under thejatter 
conditions the water is under pressure. The water may represent 
waste from the irrigation of the injured tract itself, adjacent lands, 
or distant lands; again, it may represent direct loss from irrigation 
ditches, laterals, canals, and reservoirs. In any case, a successful 
drainage system can not be designed until the source of the damaging 
water is known, and until the movement of the underground water 
has been studied. 
The most difficult problem in connection with the drainage of 
irrigated lands is the determination of the quantity of water that will 
be developed and for which it will be necessary to provide an outlet. 
For tracts up to a few hundred acres in area and having average soil 
and subsoil, the simplest method, and one which has proved reliable, 
is to determine the irrigation supply and to provide a drainage 
capacity of one-third that amount of water. As the size of the 
tract increases, however, this coefficient should be decreased. If the 
subsoil be clay, provision for one-fifth the irrigation supply will suffice 
for small tracts. In areas of a square mile or more, it is usually 
sufficient to provide for a run-off of from 1J to 2 J cubic feet per 
second for each square mile, depending upon the porosity of the soil 
and the duty of the irrigation water. 
The foregoing bases do not apply to the drainage of lands underlain 
by gravel. In such lands it is the area that is contributing the damag- 
ing water, not the area to be drained, that must be taken into con- 
sideration. Oftentimes the drainage discharge from gravelly lands 
is several times greater than the irrigation supply of the injured 
tract. 
If the height of the ground-water table be variable, it is possible 
to make a close estimate of the necessary capacity by ascertaining 
