14 DRY FARMING IN RELATION TO RAINFALL AND EVAPORATION. 



crop on account of the low rainfall in the autumn months and for 

 the purpose of supplementing the low annual rainfalls 



In the Great Plains the monthly distribution of the rainfall is 

 quite different from that of the Intermountain region, the greatest 

 monthly rainfall occurring in June, July, and August. Summer till- 

 age for moisture conservation is not so well adapted to this region. 

 The frequent rains during the summer months repeatedly pack the 

 surface mulch on the summer- tilled land. This requires frequent 

 cultivation in order to avoid the loss of water by evaporation, and so 

 increases the expense of maintaining the mulch. Owing to the high 

 evaporation in the Great Plains during the summer months, very 

 little water will be stored in the summer-fallowed land unless the 

 mulch is carefully kept up by thorough cultivation after every rain. 

 The higher annual precipitation in the Great Plains and the increased 

 cost of maintaining a good mulch on summer-tilled land have thus 

 combined to make summer fallowing in this region much less popu- 

 lar than in the Great Basin country. Furthermore, the blowing of 

 the dry surface mulch of the summer fallow sometimes becomes very 

 serious in the Great Plains owing to high winds. 



For the above reasons annual cropping is largely used in preference 

 to summer tillage in the Great Plains, although in some parts of the 

 area summer-tillage methods are considered necessary to insure 

 sufficient moisture, even at the cost of the increased labor necessary 

 to maintain the surface mulch. Spring grains are generally used 

 under the annual cropping method, since the crop escapes the dry 

 fall and winter, and the land, having been recently worked, is in the 

 best condition to absorb the summer rainfall. The shade of the 

 growing crop partially protects the ground from excessive loss of 

 water by evaporation, while the small summer showers that are use- 

 less on summer- tilled land always help the growing crop. Thus we 

 have a radically different system of farm practice in the Great Plains 

 from that found in the Intermountain country, owing in part to the 

 difference in the distribution of the monthly rainfall of the two 

 regions and in part to the greater rainfall in the Great Plains. 



THE RELATION OF THE CHARACTER OF RAINFALL TO ITS USE- 

 FULNESS. 



The way in which the rain comes has a great deal to do with its 

 usefulness. Summer-fallowing methods, for example, are not well 

 adapted to regions in which the rainfall comes largely in the form of 

 little showers from one-tenth to one-half inch. Rains of this kind 

 penetrate the soil only a few inches, and the water is practically all 



a See "Arid Farming in Utah," by Widtsoe and Merrill, Bulletin 91, Utah Agricul- 

 tural Experiment Station; also "Dry Farming in the Great Basin," by C. S. Scofield, 

 Bulletin 103, Bureau of Plant Industry, U. S. Department of Agriculture, 

 J88 



