Uncertainty in annual streamflow and change in reservoir content data from selected surface-water gaging stations on the lower Colorado River

The lower Colorado River is an important water resource for metropolitan populations, agriculture, and industry in California, Arizona, and Nevada. The Bureau of Reclamation (BOR) manages the river, releasing water stored in Lakes Mead, Mohave, and Havasu, and in other smaller reservoirs as needed so that it can be used by diverters. To help guide river management, streamflow and reservoir content are monitored at strategically located gaging stations along the lower Colorado River, its tributaries, and its diversions. The data obtained from these gaging stations, however, contain uncertainty and are only estimates of the ‘‘true’’ streamflow and reservoir content. As part of a cooperative project with the BOR, the U.S. Geological Survey (USGS) estimated the standard error of the annual discharge for calendar years 1995–99 at 14 streamflow-gaging stations and the standard error of the change in reservoir content at 2 reservoir-content gaging stations (table 1 and fig.1; Anning, 2002). These standard error estimates provide a measure of the random uncertainty for the annual data.