2 BULLETIN" 190, U. S. DEPARTMENT OF AGRICULTURE, 



Drainage practice in the arid section diJffers greatly from that in 

 the humid region. The nature of the land requiring drainage is 

 different, and the methods apphed are unlike those employed in the 

 humid section. In fact, there is httle in common between the two 

 situations, and drainage experience in the humid section avails Httle 

 in dealing with the problems of draining irrigated lands. For this 

 reason hterature on the general subject of drainage should be used 

 with caution, as the difference in conditions betVv^een the arid and 

 humid regions has been clearly recognized only within the last few 

 years. 



MANIFESTATIONS OF POOR DRAINAGE CONDITIONS. 



Injury wrought by overirrigation manifests itself in several ways. 

 In some cases the lands are literally swamped, and ponds, bogs, and 

 tule marshes are dommant features. In others the ground is merely 

 waterlogged, and the injury is shown by the wet condition of the 

 soil, by the presence of alkali salt crusts on the surface of the higher 

 spots of ground, and by any vegetation that may have survived the 

 inhibitive conditions. Very often, however, there is no visible 

 sign of wetness, and the only marks of the injury are the incrusta- 

 tions of alkah salts, the presence of highly alkali-resistant plants, or 

 the absence of aU vegetation in the worst-affected areas. 



SPECIFIC OBJECTS OF DRAINING. 



It is important, at the outset, to understand the specific results 

 which it is proposed to accomphsh by draining. These may briefly 

 be stated as follows: (1) To lower the ground-water table to such a 

 depth that the moisture and air conditions within the root zone are 

 properly balanced; (2) to provide an outlet for percolating water, 

 so that fluctuations of the ground-water table within the root zone 

 will be prevented ; (3) to effect rapid removal of the excess moisture 

 resulting from spring thaws; and (4) to provide an outlet for the 

 downward moving water used to dissolve out the injurious salts. 



PRELIMINARY INVESTIGATIONS. 



NECESSITY FOR KNOWLEDGE OF CGNDITIGNS. 



Havmg in mind the above-mentioned objects of draining, it is 

 clear that the existing conditions must be known before work is com- 

 menced, and that the system must conform to these conditions. Ran- 

 dom procedure is manifestly poor policy where the objects aimed at 

 are so definite. A little study and the application of judgment will 

 often indicate how the drainage may be satisfactorily accomplished 

 at comparatively smaU cost. On the other hand, it is quite possible, 

 in the irrigated section, to install a system having many more lines 

 than are necessary, and still meet with total failure. There are few 

 projects so small and apparently so simple as not to warrant the 



