62 



THE IRRIGATION AGE. 



off were caught and used in irrigation, nearly one- 

 third of the land could be irrigated, but in that region 

 the rainfall itself is usually sufficient for agriculture, 

 and irrigation is only needed as supplementary to 

 the rainfall. 



EFFECT OF TOPOGRAPHY. 



Where the runoff is from 2 to 5 inches the total is 

 about one-seventh of the amount necessary for irriga- 

 tion; that is, a catchment of seven acres, if all is 

 utilized, will irrigate one acre. Where the runoff is 

 from to 2 inches, some interesting conditions are 

 found, which must be more fully explained. Here the 

 topographic conditions are controlling, and very large 

 districts exist where there is no runoff, and other 

 districts where the runoff is very slight, it being found 

 only in storm-water streams and very infrequent 

 springs. We may divide this district where the run- 

 off is from to 2 inches into three portions, which ap- 

 pear to be nearly equal. As determined by topo- 

 graphic conditions and diminished rainfall, one-third 

 will have no runoff, another third will have a runoff 

 of 1 inch, and the remaining third a runoff of 2 inches. 

 If all of this water could be caught and used upon 

 the land in irrigation, then on the land where there is 

 no runoff there would be no irrigation; on the second 

 portion, where the runoff is 1 inch, one acre in 

 twenty-four could be irrigated; on the third portion, 

 where the runoff is 2 inches, one acre in twelve could 

 be irrigated. 



ABSOLUTE CATCH AND POSSIBLE CATCH. 



So far we have considered the problem only on the 

 supposition that all of the water can be caught, but 

 this is not possible. This total runoff we will call the 

 absolute catch, and we will now proceed to find the 

 possible catch, the practical catch, and the crude 

 catch, as we have defined the absolute duty, the pos- 

 sible duty, the practical duty and the injurious duty 

 of water. 



The water which is used in irrigation must be 

 caught. The season of irrigation is short as com- 

 pared with the remainder of the year. All the water 

 which is not used as it is caught must be stored in 

 reservoirs; but from these reservoirs a part of the 

 stored water will evaporate, the amount varying in 

 different latitudes and altitudes from 50 to 100 inches 

 annually. That is, if a reservoir has a surface of 

 10 acres, then 10 acres of water 50 to 100 inches in 

 depth will be evaporated annually. It is not possible, 

 therefore, to hold all the water stored; but if we sup- 

 pose that the catch of the waters be perfect, which 

 can be effected only at an enormous and impractic- 

 able expense, then the possible catch is that which is 

 used immediately after being caught and that remain- 

 ing in the reservoir after evaporation. This possible 

 catch will be 30 per cent, less than the absolute catch. 



Space does not here permit of a discussion of the 

 facts which lead to this conclusion, and only the 

 simple statement is made. The possible catch, then, 

 is 70 per cent, of the absolute catch. 



THE PRACTICAL CATCH. 



The possible catch is impracticable by reason of 

 excessive cost; and we have a further reduction to 

 make. Where the catch is concentrated by greater 

 rainfall and most favorable conditions of topography, 

 the practical catch is comparatively large; where the 

 rainfall is less and the topographic features more 

 unfavorable, the practical catch is very small. It will 

 never pay to impound the* storm waters of sand 

 deserts; it will never pay to impound the storm 

 waters of bad lands; it will never pay to impound the 

 storm waters of land of volcanic scoria; and there are 

 many other minor conditions of storm-water catch- 

 ment which are inhibitory. Storm waters can rarely 

 be caught at a practical expense where the rainfall is 

 less than 12 or 15 inches. All such waters must there- 

 fore be neglected as not entering into the problem of 

 catch. But where there are mountain lands which 

 condense an amount of water greater than the adja- 

 cent lowlands, and where at the same time the catch- 

 ment surfaces are topographically advantageous, very 

 large quantities of storm waters may be secured; but 

 to a large extent they are added to perennial streams 

 and their catch therefore depends upon the control of 

 perennial streams. 



LIMITATIONS UPON CULTIVATION. 



There is still another important factor to examine 

 in this connection. Rains fall in intermittent storms. 

 Some showers are gentle and furnish small amounts 

 of water; some great storms furnish large quantities 

 of water. The maximum storms which cause de- 

 structive floods fall at intervals of years. Now it will 

 never be practicable to catch all the water of maximum 

 storms, because of the great expense of constructing 

 the necessary works. On every stream where works 

 are constructed for diverting the water, spill-ways 

 are necessary to provide against the maximum 

 storms. Practically it will be found economic to 

 waste all the water of storms which much exceed the 

 mean, and these are the rains which serve greatly to 

 increase the run-off. 



We have, then, to deduct from the possible catch 

 that portion of the storm-water streams which must 

 be neglected, and that portion of the great storm 

 floods which will be spilled, which together amount 

 to about 30 per cent. Thus the possible catch will be 

 reduced to the practical catch. The reason for reach- 

 ing this conclusion cannot be entered upon here for 

 want of space. It is thus found that there are inex- 

 orable conditions which limit the amount of land 

 which can be practically cultivated in the arid region. 



