46 DRY LAND FARMING 



creases (1) with an ascending latitude, and (2) with in- 

 crease in the elevation. It is manifest, therefore, that 

 the evaporation will be much greater in warm latitudes 

 and depressed elevations because of the greater heat, than 

 in elevations the opposite, and that it will be much 

 greater in areas not tempered with humidity from the 

 ocean than in areas so tempered. Where winds are con- 

 stant, and the movement of the same is relatively high, 

 evaporation will be proportionately increased. Such a 

 condition occurs in certain portions of the Great Plains 

 area. The influences named, in conjunction with vari- 

 ations in the precipitation, increase greatly the diffi- 

 culty of formulating definite rules for the tillage of areas 

 with practically the same average precipitation. The 

 Bureau of Plant Industry found as the result of three 

 years test at Dickinson, N. D., ending with 1909, that the 

 average annual evaporation from a water surface was 

 31.4 inches, whereas from a two years test at Fallen, 

 Neb., it was 51 inches. It is very evident, therefore, that 

 a certain amount of rainfall at Dickinson will be more 

 helpful to growing crops than a similar precipitation at 

 Fallon, other things being, generally speaking, equal. 

 It has been estimated that where the evaporation is 45 

 inches and where the summer rainfall is 18 inches, the 

 larger portion of it falling in the summer, about one-half 

 of it may be saved to the crops. 



The normal temperature in dry areas greatly influ- 

 ences not only the crops that may be grown but the yields 

 of the same. The temperature is, of course, influenced 

 much by the elevation and by distance from the ocean. 

 Naturally, increase in the elevation lowers the mean tem- 

 perature, and increasing distance from the ocean results 

 in still greater contrast between the degree of the heat 

 of the days, as compared with that of the nights. In much 

 of the dry area, the nights are relatively cool, and this 

 is greatly favorable to increasing plumpness of the grain 



