Scientific Investigations Report 2006-5252

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5252

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Introduction

Description of Study Area

Lake Mead is the largest reservoir by volume in the United States and was formed when Hoover Dam was completed in 1935 (fig. 1). It took until July 1941 for water to fill Lake Mead, which has a maximum surface elevation of 1,229 ft, a maximum surface area of 162,700 acres, and a maximum available capacity of 27,377,000 acre-ft (Bureau of Reclamation, 1967). At the maximum elevation of 1,229 ft Lake Mead extends 65.9 mi upstream of Hoover Dam and has a maximum width of 9.3 mi (LaBounty and Horn, 1997, p. 95). The average lake elevation from 1942 (first complete calendar year of full pool) to 1995, based on monthly end-of-month elevations, is 1,169.9 ft (retrieved at URL: http://www.usbr.gov/lc/riverops.html), which corresponds to a lake surface area of 125,600 acres.

The drainage area of Lake Mead at Hoover Dam is about 171,700 mi2 (Tadayon and others, 2000, p. 90). Ninety-seven percent of the inflow into Lake Mead is from the Colorado River (13.12 million acre-ft/yr, Tadayon and others, 2000, p. 80) with the remaining 3 percent from the combined flow of Las Vegas Wash (148,000 acre-ft/yr; Jones and others, 2000, p. 94), Muddy River (11,700 acre-ft/yr; Jones and others, 2000, p. 55), Virgin River (176,000 acre-ft/yr; Jones and others, 2000, p. 55), and ephemeral streams. The average annual release from Hoover Dam from 1935 to 1999 was about 10.1 million acre-ft (Tadayon and others, 2000, p. 99). Flow of the Colorado River at Diamond Creek (130 mi upstream of Hoover Dam) in calendar year 1999 was 12.69 million acre‑ft, while the release from Hoover Dam was 11.04 million acre-ft (Tadayon and others, 2001, p. 110 and 128). Retention time for Lake Mead averages 3.9 years, depending on release and flow patterns.

Lake Mead is in an environment with a warm, arid climate. From 1961 to 1990, the average maximum air temperature was 105.9°F for Las Vegas, Nev., and 107.8°F for Overton, Nev. (fig. 1), whereas the average minimum air temperature was 33.6°F and 28.0°F, respectively (table 1). Average annual precipitation (1961–90) was 4.13 in. for Las Vegas and 3.31 in. for Overton (table 1). This warm, arid environment is conducive for high rates of evaporation. Sparsely vegetated, gentle to moderately sloping alluvial fans and steep, barren, rocky cliffs surround the lake. Generally, the adjacent hills rise to low or moderate height above the lake surface. The vast majority of lake is exposed to winds from the southwest to southeast.

Background

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 series of reservoirs along the Colorado River ensures a reliable supply of water for municipal and agricultural uses; allows for the generation of hydroelectric power; reduces the occurrence of uncontrolled floods within the basin; provides ecological habitat for numerous aquatic, riparian, and avian species; and provides a source of water-based recreation for millions of people.

The demand for Colorado River water has increased greatly as the population of the Southwestern United States has grown and as the maintenance and the development of ecological habitats has become a priority. Recent (2000) estimates of demand for Colorado River water in the Lower Colorado River Basin (parts of Utah, Arizona, Nevada, and California) are for irrigation of 2.7 million acres of land and municipal supply for 18 million people (Paul Matuska, Bureau of Reclamation, Lower Colorado River Regional Office, written commun., 2000). In contrast, Stanley (1960, p. 84) estimated that in 1949 Colorado River water was used to irrigate 500,000 acres of crops in Arizona and California and 300,000 acres of crops in Mexico and was used as municipal supply for 3.5 million people in California. Bureau of Reclamation (2000, p. 1) reported that in 1999 a total 10.95 million acre-ft of Colorado River water was used by the lower basin States (7.98 million acre-ft) and Mexico (2.97 million acre-ft).

Accurate estimates of water-budget components of a reservoir system are required to manage increased demands for available water supplies. Budget components such as withdrawals, diversions, return flows, change in storage contents, and tributary inflows usually can be quantified with direct measurements. Natural losses from the reservoir system, such as open-water evaporation, seepage, and evapotranspiration from riparian vegetation, are quantified using methods that are more indirect. These components typically are estimated with a variety of methods such as empirically derived relations developed with data collected at other locations, calculations using data and results from regional studies, statistically distributing water-budget residuals to selected components, or applying reasonable assumptions about the hydrodynamics of a reservoir system.

Open-water evaporation is a significant loss of water from Lake Mead. From 1953 to 1994, evaporation of water from Lake Mead was estimated to be 6.4 ft/yr, or about 791,000 acre-ft/yr (based on an average surface area of 125,600 acres). From 1955 to August 1995, the U.S. Geological Survey (USGS) computed Lake Mead evaporation using a mass-transfer method. Data required for using this method are wind speed and water temperature, collected at a floating platform in Boulder Basin, and air temperature and vapor pressure, collected at an airport in Las Vegas, Nev., and reported by the National Weather Service (Harbeck, 1958a). In September 1995, the Bureau of Reclamation removed the floating platform from the lake and the wind-speed and water-temperature data required for the evaporation computation were no longer available. Alternative data sources (such as National Park Service data) were investigated, but due to the empirical nature of the method, were determined to be inadequate for estimating evaporation rates.

As a result, the Bureau of Reclamation proposed a cooperative study with the USGS to develop a method to estimate Lake Mead evaporation rates and to evaluate previously reported rates. The method developed by the USGS would be transferable to other large open-water areas managed by the Bureau of Reclamation or other agencies in the arid Southwest.

Purpose and Scope

This report presents the results of a study to refine estimates of evaporation from Lake Mead. Previously, estimated evaporation from Lake Mead (1955–95) may not be representative because of the methods used, and because some data used in the mass-transfer method were collected in the Las Vegas Valley rather than at the surface of the lake. In this report, previously used methods of computing evaporation are described, evaluated, and compared to those used in the current study for data collected for 1997–99, and to alternative methods of computing evaporation using modern (2005) equipment and techniques. Additionally, selected data collected during previous and current studies are summarized, including monthly and annual Lake Mead evaporation rates for 1952–95 and 1997–99.

To better refine estimates of evaporation from Lake Mead, atmospheric and water properties were collected from 1997 to 1999 at four floating platforms, which were installed in water of varying depth and fetch to sample differing environmental conditions at the lake. Near-continual data collected at Overton Arm, Boulder Basin, Virgin Basin, and near Callville Bay (fig. 1) were used with an energy-budget method to determine daily, monthly, and annual evaporation rates for 1997–99.

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