308 THIRD ANNUAL SESSIONS 



Montana — 



East Portion 12.80 18.75 



West Portion . . 12.56 22.63 15.34 



Wyoming * 10.04 18.68 14.31 



Colorado 6.43 25.59 16.42 



New Mexico 3.91 18.79 13.45 



North Dakota 13.28 20.68 17.79 



South Dakota 13.54 28.98 21.45 



Nebraska 14.32 36.04 24.42 



Kansas 15.40 44.54 27.77 



Oklahoma 19.27 42.37 31.83 



Evaporation. 



2. Evaporation table, showing total evaporation, in inches, for crop 

 season April 1 to October 1, compiled by Dr. L. J. Briggs, U. S. De- 

 partment of Agriculture, Washington, D. C. 



Evaporation is a factor of equal importance with precipitation in de- 

 termining the agricultural productiveness of a region. By the term "evap- 

 oration," is meant the number of inches of water which vaporizes or 

 evaporates from a clear water surface in a freely exposed open tank 

 during a given period. Thus the annual evaporation is the total num- 

 ber of inches of water which evaporates during the jeax, just as the pre- 

 cipitation is measured by the total number of inches of water falling 

 into the tank as rain or snow during the year. Evaporation depends 

 upon the temperature of the evaporating surface, the dryness of the air 

 and upon the velocity of the wind. The hotter the day, the greater the 

 evaporation, the dryer the day, the greater the evaporation; the harder 

 the wind blows, the greater the evaporation. 



The amount of evaporation from an open tank of water thus be- 

 comes a measure of the combined action of the different factors which 

 determine the evaporation in that locality. The higher the evaporation 

 from the tank, the greater is the demand made upon the crops and soil 

 for moisture. 



Settlers looking into the possibilities of the new country inquire only 

 regarding rainfall. The evaporation is not considered. This is doubtless 

 due to the unfortunate fact that the evaporation data are not yet gen- 

 erally available. Such records would be of great value to the settler. 

 In dry farming the most favorable region, other factors being equal, is 

 obviously the one with the lowest evaporation. The demands upon the 

 moisture in the soil are here the smallest and, in times of scanty rain- 

 fall, the settler bas a proportionately better chance to mature a fair 

 crop.. 



2. EVAPORATION DATA IN INCHES. 



Texas, 45; Nebraska, 41; Kansas, 45; South Dakota, 32; North 

 Dakota, 30; Wyoming, 37; California, 34-57; Colorado, 29-49; New 

 Mexico, 55; Arizona, 53-56; Utah, 31-34; Nevada, 43; Washington, 24. 



