158 



THIRD ANNUAL SESSIONS 



fall. This rain came so rapidly that it seemed to compact and 'sleek' 

 the surface of the summer fallowed land and the rain then ran rapidly 

 off the surface. The stubbie plots, on the other hand, which were very 

 dry and showed many surface cracks, took up during this rain an amount 

 of moisture equal to about 1.5 inches of rainfall, approximately 60 per 

 cent, of the total precip'tation. 



Evaporation an Important Factor 



"In considering the problems of dry farming, it must be remembered 

 that the amount of water required to produce a crop is dependent pri- 

 marily upon the amount of evaporation. Evaporation is in turn depend- 

 ent upon a number of factors, the pr'ncipal ones being the temperature 

 and moisture content of the air and the wind velocity. The evaporation 

 increases as the temperature of the air or the velocity of the wind in- 

 creases. The evaporation decreases as the amount of moisture in the air 

 increases. High temperature, high wind, and dry air together result in 

 the loss of a large amount of water from the crop and the soil. 



Soil Treatment. 



"The loss from the soil can be controlled to some extent by surface 

 cultivation. The loss from the leaves of the crop is not under our con- 

 trol. The more severe the conditions which determine the evaporation, 

 the greater is the amount of moisture lost through the leaves of the 

 plant. A knowledge of the amount of evaporation in different dry farm- 

 ing sections thus becomes of fundamental importance in selecting a dry 

 farm. 



Evaporation. 



"The evaporation at the government experimental stations is deter- 

 mined by measuring the loss of water from a tank eight feet in diameter 

 and two feet deep buried in the soil with its top four inches above the 

 surface, and kept filled with water to approximately four inches from the 

 top of the tank. Such measurements do not, of course, represent the 

 evaporation from the soil, but they serve to give us a valuable compara- 

 tive measure of the evaporation in different parts of the country. The 

 accompanying evaporation chart (Pig. 2), shows the evaporation during 

 the summer months for the year 1908 at Edgeley, North Dakota; North 

 Platte, Nebraska, and Amarillo, Texas. 



"The amount of evaporation for ten day periods during each month 

 is represented by the height of the vertical lines (Pig. 2). The amount of 

 evaporation in inches can be found by comparing the lengths of the lines 

 with the scale at the left of the diagram. It will be noted that there is 

 a marked variation in the evaporation for different ten-day periods 

 throughout the season. The maximum of evaporation in Texas occurred 

 in June, in North Dakota the last part of July, while at North Platte, 

 Nebraska, the highest evaporation occurred in September which was an 

 extremely dry hot month. It is of special interest to note the manner in 

 which the evaporation increases as one proceeds south through the Great 



