4 BULLETIN" 1340, IT. S. DEPARTMENT OF AGRICULTURE 
The soil moisture requirement of a crop is the amount of soil 
moisture expressed as a percentage of the dry weight of a unit of 
soil required for the proper growth of the crop. 
The irrigation requirement of arable land is the amount of water, 
including the natural precipitation, required for profitable crop 
production under the physical and normal climatic conditions of the 
locality. 
THE GREAT BASIN 
The Great Basin (pi. 1), as Gilbert 4 has stated, "is not, as the 
title might suggest, a single cup-shaped depression gathering its 
waters at a common center, but a broad area of varied surface natu- 
rally divided into a large number of independent drainage districts." 
It includes within its confines about 95 per cent of the area of 
^Nevada, all of western Utah, a long strip in California bordering 
on Nevada, and small portions of Oregon, Idaho, and Wyoming. It 
is 572 miles from east to west, 717 miles from north to south, exclu- 
sive of the Salton Sea Basin, which is now irrigated by the waters 
of the Colorado River, and contains in round numbers 138,789,000 
acres. On the west the Sierra Nevada Mountains act as an enormous 
retaining wall in separating this territory from the central plain of 
California, and the Wasatch and Uintah ranges on the east serve 
a like purpose in separating it from the Colorado River drainage, 
and a divide at the north prevents its waters from joining those of 
the Columbia River. The divide between the Great Basin and that 
of the Colorado River, toward the south, is less clearly defined, as 
scant rainfall has prevented a deeply eroded ridge. 
The aridity of its arable lands and the drainless character of its 
lakes and sinks distinguish this region from other parts of the coun- 
try the streams of which drain either directly or indirectly into the 
ocean. Geologists are of the opinion that aridity has been the pre- 
vailing characteristic of the Great Basin for countless ages. It was 
only during a part of the Pleistocene period that semiarid condi- 
tions seem to have prevailed. At the time when the mastodon 
roamed over this high plateau the climate underAvent a change. 
Due to an increase in precipitation, and possibly a decrease in evapo- 
ration, the run-off from the several watersheds exceeded the quantity 
of water evaporated and resulted in the natural storage of enormous 
quantities of it. Most of this excess water was stored in two lakes, 
one of which has been called Bonneville, in honor of the army 
captain and traveler of that name, and the other Lahontan, from 
another explorer of the region, Baron LaHontan. Lake Bonneville, 
of which Great Salt Lake is but a remnant, was 346 miles long, 145 
miles wide, and had a surface area of 12,640,000 acres. Its highest 
shore line was about 1,000 feet above the present surface of Great 
Salt Lake, and when it was at this elevation that part of its annual 
inflow which was not evaporated was discharged into Snake River 
and later into the Pacific Ocean through the Columbia River. Lake 
Lahontan, on the other hand, was never filled. It reached an ele- 
vation about 525 feet above the present level of Pyramid Lake in 
western Nevada when the yearly loss from evaporation began to 
4 Lake Bonneville, Monograph No. 1, Geological Surrey, U. S. Department of the Inte- 
rior, by G. K. Gilbert, 1890. 
