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11.2 Collection of Hydrologic Data 359
returned to the atmosphere by evaporation and transpiration. Where melting of the winter’s
snows produces a major part of the annual runoff, or where spring thaws cause serious
floods, accurate methods for determining the annual snowfall on a watershed are essential
requirements. Otherwise, storm rainfalls monitored by recording gauges form the basis for
predicting the flood stages of river systems and for estimating the expected rate of runoff
from areas yet to be provided with drains or sewers.
Much larger volumes of water are returned to the atmosphere over continents by tran-
spiration than by evaporation. In the well-watered regions of the United States, annual
evaporation from free water surfaces more or less equals annual precipitation; in the arid
and semiarid portions, it exceeds precipitation manyfold.
About half the annual rainfall normally enters Earth’s crust and about half the water
stored in its interstices lies within half a mile of the surface and can normally be drawn on
for water supply.

11.2.1 Rain Gauging
In the United States, a Weather Service was organized in the Army Signal Corps in 1870,
and the Weather Bureau was established in 1891. The bureau maintains more than 10 stan-
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dard rain gauges, or one for about 250 mi (647.3 km ). Some two-thirds of the rain gauges
are nonrecording, and under cooperative observation. The more informative recording rain
gauges have multiplied more rapidly since 1940, with radar storm-tracking stations being
added after the Second World War. The average length of record of nonrecording stations
is close to 100 years; only a few go back two centuries or more.

11.2.2 Snow Surveys

Snowfall is identified routinely at rain gauging stations. Snow surveys are special under-
takings that keep an inventory of the important frozen or winter storage of water in high
mountains that is or may be released to lowlands during the summer. Snow surveys began
in 1910. They are currently the responsibility of the Soil Conservation Service, which was
organized in 1939. More than a thousand snow courses are now traversed annually in
cooperation with states and hydraulic or agricultural interests.

11.2.3 Stream Gauging
The yield of upland catchments of moderate size was first measured in the 1860s by cities
in need of water. Before that time the Corps of Engineers had gauged only the great rivers
of the country. The Geological Survey, created in 1879, established a nationwide stream gaug-
ing network in 1894. Its program is supported by state as well as federal funds. Over the years,
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more than 10,000 stations have been operated. Their density per 1,000 mi (2,590 km )
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varies from 0.5 (Nevada) to 11 (New Jersey) or 0.2 to 4.3 per 1,000 km . All too few have
been assigned to small, and consequently flashy, streams draining areas of less than 1,000 mi 2
2
(2,590 km ). Because the total length of United States surface streams is about 3 million
miles (4.8 million km), there is one gauging station in about 400 miles (640 km).
11.2.4 Groundwater Studies
Responsibility for reporting on groundwater was given the Geological Survey in 1895, but
no significant scale of operation was achieved until the 1920s. States share the expense of
this program, too. Fluctuations of water levels are measured now in about 10,000 observation