280 THE FUTURE OF ARID LANDS 
measures, the reuse of drainage and return flow water from irri- 
gated land and the use of sewage effluent and industrial wastes 
whenever possible. 
Bowen and Powell have discussed two major possibilities of 
additional water supplies: the induction of precipitation, and the 
demineralization of saline waters. I wish to direct my attention 
to one aspect of the third question proposed as a subject for 
discussion at this technical session: ‘“‘How practicable it is to reuse 
waste waters?’ This is the salinity factor in the reuse of waste 
waters. Three points will be considered: (1) the characteristics 
which determine the quality of water for irrigation use; (2) the 
characteristics of return flow water and drainage as they relate 
to the quality of water for agricultural use; and (3) the conditions 
under which saline waters may be used to augment the water 
supplies essential to the maintenance and extension of irrigation 
agriculture in arid lands. 
Quality of Irrigation Water 
Although the quantity of available water is the primary con- 
sideration in the development of irrigation agriculture, quality 
of water becomes a more and more critical factor as the supplies 
of surface and ground waters are depleted. Four characteristics 
determine quality of water for irrigation use: (1) the total con- 
centration of soluble salts, (2) the concentration of sodium and 
the proportion of sodium to calcium plus magnesium, (3) the 
concentration of bicarbonate, and (4) the occurrence of micro- 
elements such as boron in toxic amounts. 
In many cases, the total concentration of soluble salts is the 
best single index for evaluating the quality of irrigation water. 
The salt content of most irrigation waters ranges from 0.1 to 
5 tons of salt per acre-foot of water (approximately 70 to 3500 
ppm). The amount of salts in river waters in western United 
States varies from as low as 70 ppm in the Columbia River at 
Wenatchee, Washington, to 740 ppm in the Colorado River at 
Yuma, Arizona, 1,574 ppm in the Sevier River at Delta, Utah, 
and 2,380 ppm in the Pecos River at Carlsbad, New Mexico. The 
salt concentration in river waters may vary materially, depending 
upon the sampling site, and this factor is important in relation 
