Scientific Investigations Report 2006–5043
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5043
The Colorado River and its tributaries are important sources of water for parts of seven Western States (Wyoming, Colorado, Utah, New Mexico, Arizona, Nevada, and California) and part of Mexico. The section of river that flows from Hoover Dam to the International Boundary between the United States and Mexico is the southern part of the lower Colorado River (LCR; fig. 1) and is regulated by a series of dams. Water is diverted or pumped from the LCR mainstream and reservoirs and is the principal source of water for irrigation and domestic use in parts of Arizona, southern Nevada, and southern California.
Consumptive use of Colorado River water is defined as “…diversions from the stream less such return flow thereto as is available for consumptive use in the United States or in satisfaction of the Mexican treaty obligation” (U.S. Supreme Court, 1964). Consumptive use of Colorado River water is apportioned among Arizona, Nevada, California, and Mexico and primarily used for municipal or agricultural supply. Apportionments are regulated by laws, agreements, and policies that constitute the “Law of the River” as described in detail by Owen-Joyce and Raymond (1996) and the Bureau of Reclamation (2000). Some water released from dams for diversion is consumed by phreatophytes on the flood plain of the LCR, an area that was inundated by water from periodic flooding of the river prior to the construction of dams. This consumption of water by phreatophytes requires increased releases from dams to ensure that the appropriate amount of water is available for diversion at various points along the river.
The Bureau of Reclamation (BOR) water-resource managers need to understand the fate of water once it is released from Hoover Dam to effectively manage the LCR. Effective management requires an understanding of the relations among the release of water from dams, the consumptive use of diverted water, and the losses of water between points of release and points of diversion. This study was initiated to improve the accuracy of methods used to estimate water loss by phreatophytic evapotranspiration (ET) along the LCR.
In 1984 the U.S. Geological Survey (USGS), in cooperation with the BOR, developed the Lower Colorado River Accounting System (LCRAS) as a means to estimate loss of water by crops and phreatophytes along the Colorado River between Hoover Dam and Mexico (Owen-Joyce and Raymond, 1996).
LCRAS provides information that the BOR uses to manage distribution and allocation of river water (Bureau of Reclamation, 2000). That information includes: (1) estimates of ET from irrigated areas for monitoring of agricultural water use, (2) estimates of ET from phreatophytes for environmental resources assessment and management, and (3) estimates of evaporation from the channel and reservoirs of the LCR for river-system resource assessment and management.
BOR applied LCRAS, as a demonstration of technology, to estimate ET of Colorado River water by phreatophytes. From 1995 to 2003, estimates of average annual ET ranged from about 4.0 to 5.5 ft (Bureau of Reclamation, 2004).
Estimates of phreatophytic ET of the Colorado River are high compared to rates determined during recent USGS ground-water discharge studies (table 1) and ET studies on other southwestern rivers. Annual ET rates of 2.4 ft for unflooded saltcedar and 4.0 ft for flooded saltcedar were estimated during a study of an area surrounding the Middle Rio Grande, New Mexico, from May 13 to October 16, 1999 (Cleverly and others, 2002). Reported annual ET rates were from 1.1 to 1.9 ft for mesquite shrubland, 2.1 ft for mesquite woodland, and from 1.6 to 3.2 ft for mature cottonwood for studies along the San Pedro River, Arizona, completed in 1997 and 2003 (David G. Williams, Departments of Renewable Resources and Botany, University of Wyoming, Laramie, Wyoming, and Russell L. Scott, Southwest Watershed Research Center, U.S. Department of Agriculture, Agricultural Research Service, Tucson, Arizona, written commun., 2005).
This report documents results of a BOR and USGS cooperative study from 2001 to 2004. The purpose of the study was to improve estimates of ET for phreatophytes along the LCR, which are needed to provide BOR with more accurate information for environmental resources assessment and management. Specific objectives of the study include: (1) comparison of Bowen-ratio ET estimates with estimates calculated using the LCRAS method applied by BOR, (2) improvement of the ET coefficients currently being used by LCRAS, and (3) estimation of total phreatophytic water use at HNWR.
The BOR, Boulder Canyon Operations Office provided funding for the study and satellite images. U.S. Fish and Wildlife Service issued a permit to install micrometeorological stations in HNWR.
The Colorado River flows more than 1,400 mi from headwaters in Wyoming and Colorado before it discharges into the Gulf of California. The total drainage area in the United States, above the International Boundary with Mexico, is about 246,700 mi2 (table 2).
Below Hoover Dam, a segment of the LCR forms part of the Arizona–Nevada State boundary and the entire Arizona–California State boundary; farther downstream the southern part of the river forms 23 mi of boundary between Mexico and the United States (Owen-Joyce and Raymond, 1996, p. 2). The drainage area of the Colorado River at Hoover Dam is 171,700 mi2 (table 2). Significant amounts of water are diverted from the LCR system and exported to southern California, and central and southern Arizona for irrigation and municipal supply (Fisk and others, 2004). An average annual flow of 10.11 million acre-ft is released from Hoover Dam; tributary inflow to the LCR is a small percentage of flow in the river. Only 15 percent of the release from Hoover Dam reaches Mexico (table 2) and much of that flow is from irrigation spills or drains (monthly return flow reported by Fisk and others, 2004).
Flow in the LCR has been regulated since 1935, the year Hoover Dam was completed. Since that time, seven more dams have been constructed between Hoover Dam and the boundary with Mexico. In addition to the construction of dams, sections of the river channel have been stabilized to reduce the frequency of flooding. As a result, the natural flood plain is no longer inundated by periodic flooding. In the absence of periodic flooding, the river channel no longer meanders in the flood plain and the flood plain is no longer flushed. Consequently, salts accumulate in small water bodies, in the soil horizon, and in the ground water (Guay, 2001), impacting the biota of the flood plain.
Part of the LCR, from Hoover Dam to the Gulf of California, flows through deserts of the southwest. The climate of the LCR is arid and warm, with extremely hot summers.
The vegetation growing on the uplands surrounding the LCR flood plain is typical of southwestern deserts. Xerophytes grow on hills and fans and some phreatophytes grow in drainages, both of which survive on sparse local precipitation and sporadic runoff. Phreatophytes growing on the LCR flood plain have a relatively constant supply of water from the shallow aquifer fed by the LCR. Phreatophytes include grasses, a variety of salt bushes, arrowweed, baccharis, honey and screwbean mesquite, saltcedar, cottonwood, and willow and occur in densities ranging from low to high. The largest areas of phreatophytes are found on four national wildlife refuges along the LCR (Bureau of Reclamation, 2004). Outside of the wildlife refuges, contiguous areas of phreatophytes are relatively small and with few exceptions pre-existing natural vegetation is commingled with, or has been replaced entirely by, urban, semi-urban, or agricultural development.
For purposes of computing water budgets, BOR has divided the reach of the Colorado River from Hoover Dam to Mexico into four subreaches between major dams (fig. 1). These reaches are Hoover Dam to Davis Dam, Davis Dam to Parker Dam, Parker Dam to Imperial Dam, and Imperial Dam to the International Boundary with Mexico.
The area selected for this study is the 37,515 acre HNWR (fig. 2), between Davis Dam and Parker Dam on the LCR (U.S. Geological Survey, 2005, p. 105). The 4,260 acre Topock Marsh (Guay, 2001) is in HNWR on the Arizona side of the river and originally was formed by the meandering LCR and supplied by flood and backwater from the river. Water levels in the marsh are now controlled by diversion of LCR water and gated outlets that regulate return flow back to the river. Generally, the relief of the HNWR within the LCR flood plain is flat, major relief features being natural sand dunes northwest of Topock Marsh and manmade dikes and dredge piles near the river.
For more information about USGS activities in Nevada, visit the USGS Nevada Water Science Center home page .
U.S. Department of the Interior |
U.S. Geological Survey
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