As part of the U.S. Geological Survey's
National Water-Quality Assessment Program in
the upper Snake River Basin study unit, land- and
water-use data were used to describe activities that
have potential effects on water quality, including
biological conditions, in the basin. Land-use maps
and estimates of water use by irrigated agriculture
were needed for Gooding, Jerome, Lincoln, and
Minidoka Counties (south-central Idaho), four of
the most intensively irrigated counties in the study
unit. Land use in the four counties was mapped
from Landsat Thematic Mapper imagery data for
the 1992 water year using the SPECTRUM computer
program. Land-use data were field verified in
108 randomly selected sections (640 acres each);
results compared favorably with land-use maps
from other sources. Water used for irrigation during
the 1992 water year was estimated using land-use
and ancillary data. In 1992, a drought year, estimated
irrigation withdrawals in the four counties
were about 2.9 million acre-feet of water. Of the
2.9 million acre-feet, an estimated 2.12 million
acre-feet of water was withdrawn from surface
water, mainly the Snake River, and nearly
776,000 acre-feet was withdrawn from ground
water. One-half of the 2.9 million acre-feet of water
withdrawn for irrigation was considered to be lost
during conveyance or was returned to the Snake
River; the remainder was consumptively used by
crops during the growing season.
