THE WATER OF SOILS. 251 



New Mexico and other states of the arid region. The Gila, 

 Pecos and upper Rio Grande are cases in point, and to a cer- 

 tain extent the Colorado of the West. 



Muddy Waters. In the latter as well as other streams of 

 Arizona, there is another point which sometimes creates diffi- 

 culties to the irrigator, together with some current expense. 

 It is the amount of silt or mud carried by the water, which 

 while it is a benefit to the land over which it is spread, (" warp- 

 ing ") as in the classic case of the Nile, often clogs the irriga- 

 tion ditches to such an extent as to cause considerable incon- 

 venience and expense in cleaning them out. This is especially 

 the case in the streams draining pasture lands that have been 

 overstocked, and where the destruction of the natural herbage 

 allows the rain water to run off rapidly, at first forming run- 

 lets and then gullies and ravines that originally were simply 

 cow-paths leading toward the watering places. 1 The devasta- 

 tion of lands thus caused in Arizona is almost as great as that 

 which has occurred in the Cotton states, as mentioned above 

 chap. 12, p. 217. 



These variations in the character of the irrigation water 

 must of course be watched by the farmer who does not receive 

 directly from mountain streams, or from deep artesian wells 

 water known to have a constant content of saline matter. 



THE DUTY OF IRRIGATION WATER. The amount of water 

 thought to be needed for the production of satisfactory crops 

 varies widely in different regions, ranging all the way from 

 about two feet to as much as eight annually, within the United 

 States; while in the sugar-cane fields of the Hawaiian Islands 

 as much as three inches per week, or over twelve acre-feet in 

 the course of the year, have been thought to be beneficial, if 

 not absolutely required for the best crop results. 



As has been stated above (chap. 12. p. 215), the rainfall 

 limit below which irrigation becomes, if not absolutely essen- 

 tial, at least a highly desirable condition for the safety of 

 crops, is usually assumed to lie at about 20 inches (500 mili- 

 meters). This general statement is, however, subject to ma- 

 terial modification according to the manner in which the rain- 

 fall is distributed. Thus in central Montana with 24 inches 

 of rainfall distributed throughout the year, irrigation is indis- 



1 Hull. Arizona Kxp't Station Nos. 2, 38. 



