Water use on the Snake River plain, Idaho and eastern Oregon

Amounts of water withdrawn and consumptively used for irrigation and other uses on the Snake River Plain were estimated to help determine hydrologic effects of groundand surface-water use. Irrigation is the largest off stream use of water on the plain. Surface-water irrigation began in the 1840's and increased rapidly through the early 1900's. Use of ground water for irrigation accelerated after World War II. In 1980, about 3.1 million acres were irrigated: 1.0 million acres were irrigated with about 2.3 million acre-feet of ground water; 2.0 million acres were irrigated with about 12.7 million acre-feet of surface water; 0.1 million acres were irrigated with combined surface and ground water. Historical changes in aquifer storage and spring discharge are closely related to changes in irrigation practices. Industries, including aquaculture, withdrew an estimated 1.97 million acre-feet of water. Withdrawals for other offstream uses, including public and rural supplies, totaled 132,000 acre-feet. Hydroelectric power generation is an economically important instream use of water. In 1980, about 52 million acre-feet of surface water was used to generate 2.6 million megawatt-hours of electricity. Native vegetation evapotranspired an estimated 3.7 million acre-feet, and 900,000 acre-feet evaporated from bare ground and open water. WELL-NUMBERING SYSTEM The well-numbering system (fig. 1) used by the U.S. Geological Survey in Idaho indicates the location of wells within the official rectangular subdivision of the public lands, with reference to the Boise base line and meridian. The first two segments of the number designate the township and range. The third segment gives the section number; three letters, which indicate the ¥4 section (160-acre tract), Vi-Vi section (40-acre tract), and V4-V4-V4 section (10-acre tract); and the serial number of the well within the tract. Quarter sections are lettered A, B, C, and D in counterclockwise order from the northeast quarter of each section (fig. 1). Within quarter sections, 40-acre and 10-acre tracts are lettered in the same manner. For example, well 8S-24E-31DAC1 is in the SWV4NEV4SEV4 sec. 31, T. 8 S., R. 24 E., and is the first well inventoried in that tract. FIGURE 1. Well-numbering system. INTRODUCTION This report is one in a series resulting from the U.S. Geological Survey's Snake River Plain RASA (Regional Aquifer-Systems Analysis) study that began in October 1979.