IKEIGATION WATER. 



17 



LIGHT. 



The whole semiarid country is a region of intense sunlight. On 

 account of the clearness of the air, the small amount of cloud, and 

 the rarity of the air caused by the high altitude, the sun's rays lose 

 much less energy before striking the earth. Although this is a sub- 

 ject not usually considered it is undoubtedly an important one — 

 how important no one knows. It is kno\\'n that plants use more 

 water when exposed to strong light. With fairly favorable condi- 

 tions of heat and moisture the quality and yield of grain depend 

 largely on the intensity and duration of light. It seems compara- 

 tively certain that this is one of the main factors responsible for the 

 uniformly high quality of grain produced in the semiarid region and 

 the large yields obtained whenever an adequate supply of moisture 

 is available.^ 



IRRIGATION WATER.2 



The extent of territory in this region that can ever be irrigated is, 

 indeed, an extremely small proportion of the whole. At best, the 

 water hi the streams is sufficient for only small patches in comparison 

 to the whole, or narrow strips along* the streams. This water is 

 supplied niauily by the precipitation in the mountains. The amount 

 of water lost by surface run-off in the semiarid region itself is com- 

 paratively small and is commonly much exaggerated. It would in 

 reality make only a thui covering over the entire surface. We see 

 water flo\\Tng hi a draw and think of its volume, but do not stop to 

 think how far apart the watercourses are, and from what a large 

 area the little stream collected the water. Of course, there is con- 

 siderable movement of water from higher to lower ground, especially 

 during driving storms, so that much more water goes into the ground 

 on one part of a field than on another. Some water also accumulates 

 in low places, where it remains tih evaporated, being thus lost to 

 -agriculture. OccasionaUy too, considerable water finds its way into 

 the streams. A considerable but unknoA\Ti quantity is also lost by 

 seepage. 



The Kepublican Kiver, which ri^es in the plains of Colorado and 

 hsLS most of its drainage basin in the semiarid region, though its 

 mouth is in a region of much heavier rainfall, has an average annual 

 discharge of only about three-fifths of an inch for its entire basin. 

 In other words, if all the water discharged by tins stream during a 

 year were spread out on the land from which it was collected there 

 would be but tliree-fifths of an hich over the entire area. It must 



1 For a further discussion of this subject, see Bulletin 36, U. S. Weather Bureau. 



2 It would be unnecessary to mention this subject here except to warn persons from accepting statements 

 concerning future irrigation on land where there is no hope for irrigation. 



215 



