SOURCES OF CONTAMINATION 225 



by irrigation. This area will be greatly enlarged in the 

 future. The 25 or 30 per cent of the earth's surface which 

 receives too little rainfall to allow farming, without ir- 

 rigation includes some of the richest known farming land. 

 The southwestern parts of Africa, South America, and 

 Australia; the northern part of Africa; the northern and 

 western parts of North America and Asia; and parts of 

 eastern, southern, and western Europe are all too dry to 

 permit of successful farming without the use of more water 

 than falls naturally on the land. The successful farming 

 of these areas is possible only through irrigation. There 

 is much more land needing irrigation than there is water 

 to supply the need. For this reason, it is important to be 

 able to utilize all available water. Even water that would 

 not be used if sufficient pure water could be had must 

 be utilized. It becomes necessary therefore to know just 

 what are the danger limits of alkali in irrigation water. 

 If the farming of certain lands requires irrigation with 

 water that will render the land unproductive, it is highly 

 desirable to prevent the erection of expensive structures 

 for diverting the water and laborious operations in bring- 

 ing the land into a state of cultivation. 



Sources of Contamination. Much valuable informa- 

 tion has been gathered in the past on the different phases 

 of the alkali-irrigation-water problem. It has been ob- 

 served that most of the contamination of irrigation streams 

 is due to seepage and drainage waters which find their 

 way back into the rivers and canals. Observations by 

 the U. S. Geological Survey and the U. S. Department of 

 Agriculture show that 65 per cent of the Gila River 

 water (27) and 30 to 40 per cent of the Salt River water (3) 

 (32) found its way back into the rivers after being used 

 for irrigation. 



