EXPERIMENTS WITH DURUM WHEAT. 29 
The climate is classed generally as semiarid. The average annual 
precipitation over most of this area varies from 14 to 20 inches. 
Only in the extreme southern part and along the eastern border is 
this quantity exceeded. In general, the annual rainfall decreases 
from east to west across the Great Plains with the increase in alti- 
tude. Figure 11 shows the distribution of precipitation in the Great 
Plains area of the United States by means of lines drawn through 
points having the same average precipitation. 
Crop production is governed largely by the quantity and distri- 
bution of the annual rainfall. Half to two-thirds of it comes during 
the growing season, the four months from April to July, inclusive. 
Wind movement is fairly constant, and summer temperatures are 
rather high. These factors cause rapid loss of water by evaporation. 
Both hot winds and hail sometimes occur, though hot winds are not 
known at the higher altitudes. Wet weather in midsummer is con- 
ducive to rust infection, which frequently occurs locally and some- 
times extends to a general epidemic throughout the area. 
The length of the growing season varies with the altitude and 
latitude. The frost-free period decreases from about 130 days in 
the lower portions to about 90 days at the higher altitudes. In 
general, early varieties have an advantage in escaping hot winds, 
drought, and frost. A brief popular classification and description 
of the important groups of hard spring wheat and the leading varie- 
ties in each group is already available (Ball and Clark, 1915 b). 
Plants, spikes, and kernels of representative varieties of the chief 
groups of wheat grown in this area are shown in figures 4, 5, 6, 7, 
and 9. 
Experiments conducted at 15 stations in the Great Plains area 
are discussed in the following pages. The name of each station, the 
altitude in feet above sea level, the normal or average rainfall, and 
the period of years on which it is based, together with the soil type 
on which each is located, are all shown in Table II (p. 15). Figure 
12 is an outline map of this area, showing the location of each sta- 
tion and, by means of graduated columns, the average yield of the 
best variety of durum wheat and of the best variety of each group 
of common wheat during the years indicated. Thirteen of these 
stations are in the United States and two are in the Dominion of 
Canada. Durum wheat has been under test at the two Canadian 
stations since 1895. None of the experiments in the United States 
began earlier than 1902, as most of the stations were established 
later than that date. For the most part they are substations of the 
State agricultural experiment station or independent field stations 
of the United States Department of Agriculture. 
