16 BULLETIN- 1340, XJ. S. DEPARTMENT OF AGRICULTURE 
per cent, and the minimum percentage required for crop growth 
about 12 per cent. 
From the close of the irrigation season of 1901 to January 1, 1911, 
irrigation experiments were conducted on the Greenville farm by the 
Utah Agricultural Experiment Station in cooperation with the Divi- 
sion of Agricultural Engineering, Bureau of Public Roads. A part 
of these investigations had for its object the determination of the 
relation existing between the quantity of water, including soil mois- 
ture and rainfall, used by various plants and the yields of these 
plants. This relationship is shown graphically in Figure 7, in which 
the mean of a large number of experiments on each of eight crops is 
given. During the 9-year period covered by these experiments the 
average annual rainfall at Logan, Utah, was about IT inches. The 
early spring rainfall was heavy and the rainfall during the growing- 
season was about 5 inches. This rainfall, which is far above the 
average of the Great Basin, coupled with the fertility of the soil and 
the quantity of soil moisture available, was sufficient to produce, os 
the diagram shows, fairly good yields when small quantities of irri- 
gation water were added. To illustrate, an equivalent of 10.25 acre- 
inches of soil water and rainfall, with the addition of 5 acre-inches 
of irrigation water, produced 6,080 pounds of dry matter per acre, 
composed of the roots and tops of sugar beets, whereas the addition 
of 30 acre-inches of irrigation water produced 10,271 pounds of dry 
matter per acre. 
In 1905 a series of experiments was carried on in several coun- 
ties of Utah by the Division of Agricultural Engineering, Bureau 
of Public Roads, in cooperation with the Utah Agricultural Ex- 
periment Station to determine among other facts the effect on yields 
of variations in the quantity of water applied, above a relatively 
small amount. The results are shown graphically in Figure 8. The 
alfalfa was grown at Richfield, Utah, on a soil composed chiefly of 
clay and sand. The oat crop was grown on clay-loam soil near the 
town of Tooele in Tooele County. The sugar beets were grown on 
sandy, gravelly loam underlaid with coarse gravel, near Provo, 
Utah County. The potatoes were grown on a black loam soil mixed 
with clay in Salt Lake County. 
Cooperative irrigation experiments on a 40-acre tract near Good- 
ing, Idaho, were begun in 1909 and continued to the close of 1916, 
by the Division of Agricultural Engineering and the Idaho Agri- 
cultural Experiment Station. The soil of the tract is a medium 
clay loam with a clay subsoil underlaid at a depth of 8 to 12 feet 
by the basaltic lava rock common to southern Idaho. The precipi- 
tation during the crop-growing season (April 1 to August 31) for 
the years 1910 to 1916 inclusive, averaged 2.91 inches and the tem- 
perature for the same 7-year period ranged from a monthly mini- 
mum of 8.3° F. to a monthly maximum of 95.9°, while the average 
yearly mean was 47.2°. 
The tract on which these experiments were made lies outside the 
Great Basin, but the soil and climate conditions are similar to those 
of the southeastern part of Idaho located within the Great Basin. 
In determining the quantity of irrigation water necessary to pro- 
duce profitable yields, one set of plots received a small quantity of 
water, another set a medium quantity, and a third set a larger 
