DEAIISTAGE OF IKEIGATED LAND. 3 



employmeiit of one trained in this work to make tlie 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 m'igated 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 doivnward 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 the latter 

 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 tlie irrigation supply will suffice 

 for small tracts. In areas of a square mile or more, it is usually 

 sufficient to provide for a mn-off of from IJ to 2^ cubic feet per 

 second for (^acli square mile, depending upon the porosity of the soil 

 and the duty of the irrigation water. 



'r\\('. foregoing bases do not apply to the drainage of lands underlain 

 by grav('l. hi siich lands it is the area that is contributing tJio damag- 

 ing water, not the area to b(^ driiined, that must be taken into con- 

 sideration. Oftentimes th(i drainage dischaige fiom gravelly lands 

 is several times gn^ater ijian tjie irrigation siij)])ly of the injui'ed 

 truct. 



if the height of tjic, groiind-wuUu' tal)l(^ be variable, it is possible 

 to make a c1oh(; (wtiinate of tjie necessary capacity by ascertaining 



