6 BULLETIN 1340, U. S. DEPARTMENT OF AGRICULTURE 
Volcanic soils. — These are derived from volcanic debris. They 
are very limited in the Great Basin and are confined principally to 
that part located in south-central Oregon. The soil is usually 
shallow but productive. The surface is irregular and difficult to 
irrigate. Most of these soils are basaltic. 
CLIMATIC CONDITIONS 
The prevailing climatic conditions of the Great Basin are low 
annual precipitation, abundant sunshine, low humidity, occasional 
high winds, a comparatively short crop-growing season, and com- 
paratively low night and high day temperatures. 
Climatological data have been collected from typical localities 
within the Basin where irrigation is practiced, and those pertaining 
to precipitation, temperature, frost-free period, evaporation from a 
water surface and wind movement have been averaged for the num- 
ber of years of record at each station and are shown graphically in 
Figures 1, 2, and 3. 
WATER SUPPLY 
By far the larger part of the water supply of the streams in the 
Great Basin is derived from precipitation on the mountain ranges 
and elevated table-lands chiefly in the form of snow. The first waim 
weather of spring melts the snow at the lower elevations and as the 
season advances the run-off from higher elevations is increased. 
This continues until the latter part of May or the first week in June, 
when the run-off begins to decrease and continues to decrease until 
August 1 or later. During the late summer, fall, and winter months 
the streams remain low but fairly uniform in flow, barring the occur- 
rence of storms. The prevailing characteristics of the flow of Basin 
streams are shown in Figure 4. These hydrographs, giving the mean 
monthly flow in acre-feet of each of three typical streams, show great 
differences between the volumes carried in the flood period and 
those of the late summer period. They also show the necessity for 
storing a part of the flood flow for use later in the same season to meet 
"the water requirements of crops. For years the need for water 
storage has been keenly felt throughout the Great Basin. With few 
exceptions, the flood flow is unutilized and the only water available 
for crops during the latter part of the crop-growing season is derived 
from the scanty summer flow. 
This condition has also induced many farmers to apply excessive 
quantities of water to their fields in the spring when the streams are 
high, in the belief that a part would be retained in the soil to nourish 
crops when little could be diverted from the streams. While this 
custom results in some benefits to those who practice it, from the 
standpoint of water conservation it has little to recommend it, inas- 
much as the greater part of the excessive quantities of water applied 
in the spring percolates through porous soils, is diverted and reused, 
or collects in low-lying places, and damages soils and crops by water- 
logging and alkali. 
A better and more economical plan is for each community of 
farmers receiving water from the same source or sources to unite into 
one organization such as an irrigation district, and, by the sale of 
bonds or otherwise, raise sufficient money to provide storage for a 
