222 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



The artificial drainage of irrigated land is sometimes nearly as 

 expensive as the construction of the irrigation system. And, in 

 many cases, it is no less essential. Where the drainage system, either 

 natural or artificial, functions properly there should be no excessive 

 accumulation of alkali salts in irrigated land unless the soil be- 

 comes so impermeable that the irrigation water can not get through 

 it. Undoubtedly there are instances at present, and there have been 

 many in the past, where irrigated land has become impermeable and 

 unproductive as a result of the use of water containing much more 

 sodium than calcium and magnesium. Such impermeability may 

 develop in the ordinary course of careful irrigation and without the 

 appearance of the usual symptons of a alkali injury such as the 

 swamping of the land or the accumulation of salts beyond the toxic 

 limits of plants. More often this condition of impermeability sets 

 in after the land has been swamped with salty water as a result of 

 excessive irrigation with inadequate drainage and an attempt is 

 made to reclaim the injured land by improving the drainage con- 

 ditions. 



THE PERMANENCE OF IRRIGATION AGRICULTURE. 



Irrigation has been practiced since the earliest historic times. 

 The Garden of Eden and the gardens of Babylon were located in the 

 desert valley of the Tigris and the Euphrates and were watered arti- 

 ficially. The valley of the Nile has been irrigated for thousands of 

 years and its irrigated area is being extended almost continually. 

 In the Transcaspian region of southwestern Asia there are numer- 

 ous places where irrigation prospered for some years and was then 

 abandoned. 



In our own country, where irrigation was begun about three- 

 quarters of a century ago, there are places where it has continued to 

 be successful on the same land and many other places where, after 

 a few years, the land has become unfit for crop production and it has 

 been necessary to conduct the water to other lands or let it go 

 unused. 



It is clear that one must be cautious in generalizing- about the 

 permanence of irrigation. We lack essential information particu- 

 larly concerning many of the cases where it has been abandoned. 

 We know that many of the failures may be traced directly to the 

 excessive accumulation of alkali salts. In other places the out- 

 standing feature has been high ground-water due to inadequate drain- 

 age with alkali troubles less conspicuous. No doubt changing eco- 

 nomic conditions have been the determining factors in still other 

 cases. 



In the case of Egypt, which is probably the best example of long 

 continued and successful irrigation, there is reason for believing 



