IBRIGATION WATER. 17 



LIGHT. 



The whole semiaricl country is a region of intense sunhght. On 

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

 the rarity of the air caused by tlie lii.uh 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 known 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 ciuality of grain produced in the semiarid region and 

 the large yields obtained whenever an adequate su):)j)ly 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 m the streams is sufhcient for only small patches in comparison 

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

 supplied mainly by the precipitation in the mountains. The amount 

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

 paratively small "and is commonly much exaggerated. It would in 

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

 water flowing in 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 till evaporated, being thus lost to 

 agriculture. Occasionally too, considerable water finds its way into 

 the streams. A considerable but unkno^^^l quantity is also lost by 

 seepage. 



The Republican River, which rises in the plains of Colorado and 

 has most of its drainage basin in the semiaritl region, though its 

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

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

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

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

 would be but three-fifths of an inch over the entire area. It must 



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

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

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



215 



