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 and part of Mexico. Water is diverted or pumped from the lower Colorado River mainstream and reservoirs and is the principal source of water in parts of Arizona, southern Nevada, and southern California.
In 1984 the U.S. Geological Survey, in cooperation with the Bureau of Reclamation, 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. LCRAS is modified as technology improves and additional hydrologic data become available. Currently, LCRAS includes daily evapotranspiration (ET), which is the product of daily reference ET and a daily ET coefficient for 11 nonaquatic phreatophyte groups and barren areas.
This study was done to improve the accuracy of methods used to estimate phreatophytic ET. Specific objectives of the study include: (1) comparison of Bowen-ratio ET estimates with estimates calculated using the current LCRAS method, (2) improvement of the current ET coefficients, and (3) estimation of total phreatophytic water use at Havasu National Wildlife Refuge (HNWR).
ET data were collected at three Bowen-ratio stations at HNWR, 2002–04. A saltcedar (SC) ET station was installed in a large area of medium-to-high density, homogeneous saltcedar. A mixed-vegetation (MV) ET station was installed in an area of medium-density phreatophytes that consist of mixed vegetation. An arrowweed (AW) ET station was installed in homogeneous, low-to-medium density arrowweed.
Monthly Bowen-ratio ET was compared to monthly LCRAS ET for phreatophyte groups similar to vegetation growing at the ET stations. Generally, the LCRAS method yielded higher estimated phreatophytic ET than Bowen-ratio method estimates.
Monthly LCRAS ET for the sc_high group averaged 55 percent greater than Bowen-ratio ET for the SC station. Monthly LCRAS ET for the ms/aw group and sc/ms/aw group was about 84 percent and 105 percent greater, respectively, than Bowen-ratio ET for the MV station. Monthly LCRAS ET for the aw group and low_veg group were about 97 percent and 90 percent greater, respectively, than Bowen-ratio ET for the AW station.
Barren ET was not measured directly as part of the study, but LCRAS barren ET rates were compared to reported ET rates of similar environments. Estimated monthly LCRAS ET for bare-soil areas seems reasonable from May to November, but seems high for January, February, and March.
As part of this study, daily ET coefficients were computed for each ET station from available daily Bowen‑ratio ET for each ET station and corresponding daily reference ET. Modified ET coefficient curves were derived and daily and monthly ET were computed using the modified ET coefficients. Daily modified ET coefficients also were developed for barren soils.
Daily ET for the SC station, computed using modified ET coefficients, was compared to Bowen-ratio ET and had a correlation coefficient of 0.92 for 754 days (May 23, 2002–June 14, 2004), with large differences for many days. Monthly ET compared for 24 months (June 2002–May 2004) had a correlation coefficient of 0.99.
Daily ET for the MV station, computed using modified ET coefficients, was compared to Bowen-ratio ET and had a correlation coefficient of 0.89 for 754 days (May 23, 2002–June 14, 2004), with large differences for many days. Monthly ET compared to Bowen-ratio ET for 23 months (June 2002–May 2004; excluding March 2004) had a correlation coefficient of 0.97.
Daily ET for the AW station, computed using modified ET coefficients, was compared to Bowen-ratio ET and had a correlation coefficient of 0.89 for 531 days (January 1, 2003–June 14, 2004), with large differences for many days. Monthly ET compared to Bowen-ratio ET for 17 months (January 2003—May 2004) had a correlation coefficient of 0.98. Estimated ET for barren soil, computed using the modified coefficients, was less than 1.0 foot per year, 2002–04.
Phreatophytes within HNWR use Colorado River water in the shallow aquifer adjacent to the river. Using remote-sensing techniques, the phreatophyte areas of HNWR were subdivided into three ET units based on relative vegetation densities of high, medium, and low and one unit for barren. The acreages within the boundaries of these ET units were computed and multiplied by the associated annual ET for 2004 (computed with the modified coefficient method and adjusted for annual precipitation). A total of 25,769 acre-feet of phreatophytic water use was estimated for HNWR, which is about two-thirds of the previously reported LCRAS-estimated phreatophytic water use of 40,137 acre-feet (excluding marsh/wetland ET).
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
Persistent URL: https://pubs.water.usgs.gov/sir20065043
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