EVAPORATION AS INFLUENCING AGRICULTURE. 



21 



the additional amount of rain required for a grain crop in Texas as 

 compared with Montana. Furthermore, the evaporation in Texas 

 during the six summer months exceeds that in Montana by about 

 20 inches. Therefore, if the summer evaporation at one place in the 

 Great Plains exceeds that at another by 3 inches a corresponding 

 increase in the summer rainfall of approximately 1 inch at the first 

 place is needed to offset the increased evaporation. A rainfall of 21 

 inches in the Panhandle of Texas is therefore really no better for 

 growing crops than 15 inches in Montana or Dakota, owing to the 

 greatly increased evaporation on the Texas plains. 



The influence of the amount of evaporation in different sections of 

 the Great Plains upon the effectiveness of the rainfall in crop pro- 

 duction is illustrated in the accompanying chart (fig. 5) . The heavy 

 curved line marked "20" passes through points in the Great Plains 

 States having an annual average rainfall of 20 inches. From what 

 has already been said, it is evident that the conditions determining 

 crop production become more and more severe as we pass southward 

 along this line, owing to the increase in the evaporation. It would 

 therefore be necessary to move more and more to the eastward from 

 the line representing an annual rainfall of 20 inches as we go south- 

 ward through the Great Plains, in order to find conditions that are 

 equally favorable for crop production so far as rainfall is concerned. 

 The broken line which is coincident with the line of 20 inches annual 

 rainfall in the northern part of North Dakota and which becomes 

 more and more separated from it as we move southward through the 

 Great Plains passes through points having a rainfall equivalent to 

 20 inches in North Dakota. The actual rainfall along this line 

 increases as we go south through the area until the lowlands of Texas 

 are reached, but owing to the increased evaporation it is no more 

 effective in growing crops than is 20 inches in North Dakota or 

 Montana. 



To the west of the line of 20 inches annual rainfall will be found 

 a second line which passes through points having an average annual 

 rainfall of 15 inches. In connection with this line of 15 inches annual 

 rainfall there is given on the map a second broken line which passes 

 through points having a rainfall equivalent to 15 inches in North 

 Dakota or Montana. It will be noted that through southwestern 

 Nebraska and northwestern Kansas this broken line is practically 

 coincident with the line of 20 inches annual rainfall. In other words, 

 15 inches of rainfall in Montana or Dakota are equivalent to 20 

 inches of rainfall in southwestern Nebraska or western Kansas. In 

 southwestern Kansas, eastern Oklahoma, and in Texas this line lies 

 wholly to the east of the line representing 20 inches annual rainfall. 

 Throughout this latter section, then, more than 20 inches of rainfall 



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