Satellite Data

Satellite technology and digital mapping allow policymakers to track important information, such as water use. The passage below explains how technology has been used to identify how water from the Colorado River has been distributed in California, Arizona, and Nevada. The maps, then, serve as the foundation for future policy. Given the region’s historical demand for water, one can imagine that such maps are sometimes used as political instruments.

LOWER COLORADO RIVER The Colorado River is the principal source of water in the valley of the Colorado River between Hoover Dam and the international boundary with Mexico (fig. 1). Agricultural, domestic, municipal, industrial, hydroelectric-power generation, and recreation are the primary uses of river water in the valley. Most of the consumptive use of water from the river occurs downstream . . . where water is diverted to irrigate crops along the river or is exported to interior regions of California and Arizona.

WHY ACCOUNT FOR CONSUMPTIVE USE? Accounting for the use and distribution of water from the Colorado River is required by law. Water in the river has been apportioned among seven States and Mexico in accordance with documents and laws known collectively as "The Law of the River." The Colorado River Compact of 1922 and the U.S. Supreme Court Decree of 1964, Arizona v. California, hold the most significance for Arizona, California, and Nevada---the three States downstream from Hoover Dam. . . .

LOWER COLORADO RIVER ACCOUNTING SYSTEM The U.S. Geological Survey, in cooperation with the U.S. Bureau of Reclamation, developed the Lower Colorado River Accounting System (LCRAS) to estimate the annual consumptive use by vegetation of water from the lower Colorado River and to equitably distribute that use among water users between Hoover Dam and Mexico. LCRAS combines estimates of consumptive use by vegetation . . . with data . . . obtained from digital-image analysis of satellite data. . . .

EVALUATION OF THE ACCOUNTING SYSTEM Assuming that satellite imagery continues to be available, LCRAS is an effective method of calculating and distributing consumptive use by vegetation among the users of water from the lower Colorado River. A few problems associated with the measurement or calculation of some of the components have been identified. Estimates of consumptive use by vegetation depend on the accuracy of the computations of flow in the river. . . . The calculation of [water use] by vegetation depends on (1) the correct identification from satellite imagery of the type of vegetation and the . . . extent of each type of vegetation and (2) the calculated water-use rates for each type of vegetation. The most difficult and critical part of the calculation is establishing accurate water-use rates. . . . The correct identification of the type of vegetation, especially where multiple crops are grown in the same field, is important because of this variation in water use by type.

^{Source: Accounting System for Water Use by Vegetation in the Lower Colorado River Valley. http://az.water.usgs.gov/factsheets/fact.lcras/fact.lcras.html. Last modified: July 30, 2001.}