B. P. I.— 586. 



DRY FARMING IN RELATION TO RAINFALL 

 AND EVAPORATION. 



INTRODUCTION. 



This bulletin contains a discussion of dry farming in relation to 

 the amount and character of the rainfall and evaporation in the 

 western United States. Usually in discussing dry-farming conditions 

 the annual precipitation is the only feature of the rainfall that receives 

 attention. But there are other factors in connection with the rain- 

 fall that have much to do with the successful production of crops. 

 The seasonal distribution of the rain, the rate at which the rain falls, 

 and the amount of rain that is lost through run-off from the surface 

 all have an important part in determining the percentage of the total 

 rainfall that is really available for the use of crops. Finally, the 

 amount of evaporation which takes place during the growing season 

 determines to some extent the amount of rain that is needed to pro- 

 duce a crop, and this varies greatly in different localities. Prospec- 

 tive settlers in these regions are apt to give very little attention to 

 climatic features other than the total rainfall. They often do not 

 even assure themselves that the figures given for a region represent 

 the normal rainfall and not simply the rainfall for a single year. 

 They ignore almost completely the seasonal distribution of the rain, 

 the frequency of torrential rains, the loss of water through surface 

 run-off, the occurrence of hail, and the amount of evaporation. It is 

 the object of this paper to bring the importance of these factors to the 

 attention of prospective settlers in regions of limited rainfall. 



Tables are also appended showing the normal rainfall for prac- 

 tically every station in these regions where precipitation records are 

 available. Accurate information regarding, the precipitation in 

 many sections of the West is now available as the result of the 

 extended observations of the United States Weather Bureau. These 

 records, which are often complete for many years and so become of 

 great value to the prospective settler, have been used in computing 

 the rainfall tables which are given in the concluding pages of this 

 bulletin. The precipitation tables are also supplemented by state 

 maps which show at a glance the distribution of the rainfall in the 

 State. Lines showing points of equal annual rainfall have been 



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