LAND DRAINAGE 659 



bottom of the open ditch may go below this level, it is 

 often advisable to insert check gates to hold the water 

 level when it has been lowered to the desired depth. 



561. Drainage of irrigated and alkali lands. — Exces- 

 sive irrigation and the occurrence of underground seep- 

 age has resulted in the water-logging of extensive tracts 

 of arid and semiarid land, and in the serious accumulation 

 of alkali salts in the surface soil. An effective remedy 

 for this condition is the installation of a thorough system 

 of drains, 1 preferably underdrains, coupled with heavy 

 irrigation by means of which the excess salt is leached out 

 in the drainage water. The most seriously alkaline land 

 is now being effectively reclaimed by drainage, for the 

 production of alkali-sensitive crops. 



For this purpose drains are installed deeper than is the 

 custom in humid regions, in order to reduce the capillary 

 rise of moisture to the surface of the soil, where the alkali 

 salts are deposited in injurious amounts. The drains 

 are often placed at depths of from four to six feet. Special 

 care is also taken to intercept the underground seepage. 

 Sometimes the seepage water from leaky canals and reser- 

 voirs and from over-irrigation may pass long distances 

 in porous gravel strata and rise to the surface of the land 

 on encountering some impervious obstruction. In such 

 cases wells may be sunk many feet to the water-bearing 

 stratum, and the water thus conducted away in drains 

 far enough below the surface to avoid injury to the soil. 



Many special problems are encountered, such as the 

 occurrence of hardpan — usually a stratum cemented by 

 alkaline carbonates — and the development of a serious 



1 Elliott, C. G. Development of Methods of Draining Irri- 

 gated Lands. U. S. D. A., Office Exp. Sta., Ann. Rept., 

 pp. 489-501. 1910. 



