20 DRY FARMING IN RELATION TO RAINFALL AND EVAPORATION. 



half the rainfall and so save the remainder of the rainfall for the use 

 of the crop. In establishing a mulch it is necessary to sacrifice all 

 of the moisture in the surface layer of soil that is used to form the 

 mulch. Any rain, then, that simply wets the surface mulch is of no 

 value in storing moisture in summer- tilled land, since it must all be 

 evaporated in order to establish the mulch again. 



EVAPORATION AS INFLUENCING AGRICULTURE IN THE GREAT 



PLAINS. 



The increase in evaporation is very marked as we proceed south- 

 ward through the Great Plains, the seasonal evaporation in the Pan- 

 handle of Texas being 54 inches, or about double that in North 

 Dakota. The influence of this increased evaporation upon the agri- 

 cultural operations is seen by comparing the agricultural operations 

 along the line of 20-inch annual rainfall in the two States. This line 

 in North Dakota passes through the head of the Red River valley, 

 where the rainfall has been ample for the successful growing of wheat 

 upon the same land year after year. In Texas the 20-inch line passes 

 through a country which is still largely grazing land and where agri- 

 culture is confined to the most drought-resistant crops, such as kafir 

 and milo. It is evident, then, that in dry-farming sections having 

 equal rainfall the North has a decided advantage over the South- 

 owing to the lower evaporation. 



Dr. H. L. Shantz, of the Bureau of Plant Industry, finds that the 

 distribution of a native grass is a good index of the rainfall required 

 in different sections of the Great Plains. Short grass, which consists 

 chiefly of buffalo grass and grama grass, is well suited for this purpose, 

 since it occurs from Montana to Texas. In each section the grass 

 grows as far west as the opposing influences of rainfall and evaporation 

 will permit. Its growth to the east is checked by competition with 

 the prairie grasses. Thus we have a strip of short grass extending 

 from Montana to Texas, limited on the west side by drought and on 

 the east by competition with other grasses. Therefore the increase in 

 the annual rainfall as we go from north to south in the short-grass 

 region represents the additional amount of rain needed to offset the 

 increased evaporation. In Montana, short grass unmixed with other 

 grasses, occurs in districts having an annual rainfall of approximately 

 14 inches; in Colorado, in regions having a rainfall of 17 inches; and 

 in the Panhandle of Texas, in regions having an annual rainfall of 21 

 inches. 



The same type of native grass requires in Texas 7 inches more of 

 rain a year than in Montana. This gives us at once a measure of 



a The authors are indebted to Doctor Shantz for this preliminary account of his 

 investigations on the distribution of the vegetation of the Great Plains, which will 

 shortly appear as a bulletin of the Bureau of Plant Industry. 

 188 



