130 THE IKRIGA TION A GE. 



times when it was deficient, taken from the ground 4.044 tons per acre 

 in 1896; 4.434 tons in 1897; 4.032 tons in 1898 and 4.242 tons in 1899. 



From what has now been said it must be clear that the productive 

 capacity of the soils now cultivated in the United States is little more 

 than one half what it might be were it possible to fully control the 

 matter of soil moisture. It must be clear therefore that the maximum 

 duty of water and the extent to which tillage may take the place of 

 irrigation is one of the most fundamental questions concerning the 

 material welfare of this country. 



It may be well to put the question first in this way. To what ex- 

 tent must tillage take the place of irrigation? 



Were it desirable to irrigate all agricultural lands lying in humid 

 climates, it would not be possible to do so, on account of the insuffici- 

 ency of water for the purpose. The truth of this proposition will be 

 evident if we deal quantitatively with the problem. 



Humphreys and Abbott have placed the mean annual discharge of 

 the Mississippi at 19,500,000,000,000 cubic feet, while the catchment 

 area is placed at 1,244,000 square miles. Assuming that, these quan- 

 tities are correct, then the mean annual run-off for the whole Missis- 

 sippi basin would be 6.747 inches. But not all of this run- off is avail- 

 able for irrigation, were it desirable to so use it, for during a large 

 part of the time this water is flowing away when the season does not 

 permit of its being used, and it is impracticable to impound it and hold 

 it until it might be used. If we take the mean daily discharge of the 

 river as 3^5 of its annual amount, and allow that the whole of this is 

 available for irrigation purposes during the irrigation season, it is 

 capable of watering but about .1 of the catchment area at the rate of 

 2 inches of water once in 10 days. 



To emphasize this point still further we may bring together in 

 close review the extent of irrigation as it is today practiced in the 

 various parts of the world, and quote the statement of Wilson: "The 

 total area irrigated in India is about 25,000,000 acres, in Egypt about 

 6,000,000 acres, and in Italy about 3,700,000 acres. In Spain there are 

 500,000 acres, in Prance 400,000 acres, and in the United States 4,000, - 

 000 acres of irrigated land. This means that crops are grown on 40, - 

 000,000 acres, which but for irrigation, would be. relatively barren or 

 not profitably productive. In addition to these, there are some mil- 

 lions more of acres cultivated by aid of irrigation in China, Japan, 

 Australia, Algeria, South America, and elsewhere." 



These figures seem enormous as we read them, and so they are, 

 but they leave an exaggerated impression on the mind which needs to 

 be corrected, for very few realize the magnitude of the volume of 

 water which must be handled in raising a crop by irrigation. In order 

 that we may not mislead in this direction we wish to make the cor- 

 rection. Let us suppose that the amount of land which is actually 

 under irrigation at the present time is four times the 40,000,000 of 



