2 CIRCULAR 912, IT. S. DEPARTMENT OF AGRICULTURE 



operation of farmers on the northern Great Plains in 1916. The 

 testing of species is only one of the lines of work with trees and shrubs 

 that has been carried on by that station. This publication gives the 

 results of the testing program at Mandan and on farms during the 

 period 1913-50. 



CLIMATE 



The climate of the northern Great Plains is classed as semiarid. 

 The amount and distribution of the seasonal and annual precipitation, 

 however, vary from year to year. 



The monthly, growing-season (April-September), and annual 

 precipitation at the Mandan station for the years 1914-50 are given in 

 table 1. Precipitation during the growing season ranged from 2.96 

 inches to 21.36 inches, with a mean of 12.34 for the 37-year period. 

 This mean is about 78 percent of the average annual precipitation 

 of 15.91 inches. The annual precipitation ranged from 6.43 inches 

 to 24.06 inches. The years 1934 and 1936 were severe drought years; 

 precipitation was 8.13 inches in 1934 and 6.43 inches in 1936. 



The trees and shrubs used up all available moisture in the area 

 occupied by their roots during the growing season of almost every 

 year. The average precipitation of 3.57 inches, or 22 percent of the 

 total annual precipitation, which fell during October through March, 

 was not sufficient to replenish the water supply exhausted by the 

 trees and shrubs during the preceding growing season (April-Sep- 

 tember). After a few years of growth, therefore, the trees and shrubs 

 had to rely largely on current precipitation, although in some plantings 

 at least water was available in the soil immediately below that occupied 

 by roots. Soil moisture determinations made in old tree and shrub 

 plantings in 1936 showed that the available moisture at depths of 8 

 to 10 feet was identical to that found in 1917, 1918, and 1919. 



The mean monthly and the maximum and minimum daily temper- 

 atures; the mean, high, and low monthly evaporation from a free 

 water surface during the growing season; and the mean monthly 

 wind velocity for the years 1914-50 at the Mandan station are given 

 in table 2. Yearly temperatures show a range of 161° F. Rate of 

 evaporation during the growing season was nearly three times the 

 total precipitation during the same period. 



The average frost-free period for the years 1914-50 was 137 days. 

 The longest period was 172 days and the shortest was 115 days. 

 Dates of the last killing frost in the spring ranged from April 19 to 

 May 29, and of the first killing frost in the fall from September 9 

 to October 8. 



Climatic data l for the northern Great Plains show temperature 

 extremes ranging from —63° F. to 121°, frost-free periods ranging from 

 89 days to 161 davs, and annual precipitation ranging from 9.39 inches 

 to 24.20 inches. 



From the US DA Yearbook for 1941, Climate and Man. 



