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Sea Level: In this report, sea level refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)-a geodetic datum derived from a general adjustment of the first-order leveling networks of the United States and Canada. Horizontal coordinate information is referenced to North American Datum of 1983 (NAD 83). Altitude, as used in this report, refers to distance above the vertical datum. Acronyms and Symbols Acronym and Symbol Definition AW Arrowweed evapotranspiration station AZMET Arizona meteorological network BOR Burea of Reclamation C d Denominator constant for reference type and calculation tme step CIMIS California irrigation management information system C n Numerator constant for reference type and calculation time step C p Specific heat of dry air at a constant pressure C s Specific heat of dry soil C w Specific heat of water cw Cottonwood and willow phreatophyte group

The U.S. Geological Survey, in support of the U.S. Department of Energy, Yucca Mountain Site Characterization Project , collects , compiles, and summarizes hydrologic data in the Yucca Mountain region. The data are collected to allow ABSTRACT 1 Characterization Project reports of groundwater data. Parts of the text are taken from La Caniera and Westenburg (1994).

Lake Mead is one of a series of large Colorado River reservoirs operated and maintained by the Bureau of Reclamation. The Colorado River system of reservoirs and diversions is an important source of water for millions of people in seven Western States and Mexico. The U.S. Geological Survey, in cooperation with the Bureau of Reclamation, conducted a study from 1997 to 1999 to estimate evaporation from Lake Mead. For this study, micrometeorological and hydrologic data were collected continually from instrumented platforms deployed at four locations on the lake, open-water areas of Boulder Basin, Virgin Basin, and Overton Arm and a protected cove in Boulder Basin. Data collected at the platforms were used to estimate Lake Mead evaporation by solving an energybudget equation. The average annual evaporation rate at open-water stations from January 1998 to December 1999 was 7.5 feet. Because the spatial variation of monthly and annual evaporation rates was minimal for the open-water stations, a single open-water station in Boulder Basin would provide data that are adequate to estimate evaporation from Lake Mead.

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