200 



DRY FARMING CONGRESS. 



tion 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 tempera- 

 ture of the evaporating surface, the dryness of the air and upon the veloci- 

 ty 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 becomes 

 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 crop and the soil for 

 moisture. 



Settlers looking into the possibilities of a new country inquire only 

 regarding the rainfall. The evaporation is not considered. This is doubt- 

 less largely due to the unfortunate fact that evaporation data are not yet 

 generally 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 rainfall 

 the settler has a proportionately better chance to mature a fair crop. 



There is an oft-repeated adage that there are only two sure things in 

 this world — death and taxes. To this list we may, however, add a third — 

 evaporation. It is always with us. It is unceasing in its activities, re- 

 lentless in its watchfulness, ready at any moment to remove the water 

 from unprotected soils. 



The next slide will give the evaporation at all the stations which I 

 have been able to find in the United States. Determinations have been 

 mostly made in the west. And the figures on this diagram represent the 

 total evaporations from April to September inclusive; that is, the seasonal 

 evaporation; the evaporation during the six summer months, from a freely 

 exposed tank set in the soil. You will notice, for example, that here in 

 New England the evaporation for the six months runs 28, 29, 27, 32 and 

 35 inches, while through northern Ohio, through Michigan and Wisconsin 

 it is practically the same, in Iowa practically the same, and a little higher 

 in North Dakota. These, by the way, represent two of the co-operative 

 stations — this evaporation as we have measured it during the past year in 

 North Dakota. Here is the evaporation at another one of the stations, 

 South Dakota, 32 inches. You will notice that it is higher, for the same 

 period. Here is an evaporation at North Platte, Nebraska, 41 inches, higher 

 than at Hymore, South Dakota. The evaporation at Hayes-, Kansas, 45 

 inches. These re all co-operative stations. The evaporation at Amarillo, 

 Texas, during the same period is 54 inches. 



In other words, with the same rainfall in North Dakota and at Amarillo, 

 during the growing season, the man at Amarillo would be working under 

 conditions which are practically twice as severe as the man in North 

 Dakota. Under those conditions why are we justified in talking alone 

 of precipitation? What does precipitation alone mean in connection with 

 such figures as- those? If we assume that the precipitation is in propor- 



