Scientific Investigations Report 2006–5043
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5043
The BOR annually reports the diversion, consumptive use, and return flow of LCR water from Hoover Dam to the northern boundary of Mexico for individual users and for Arizona, Nevada, and California. Some of this water is diverted naturally or consumed by phreatophytes on the flood plain of the LCR, an area that historically experienced natural flooding prior to the construction of dams.
LCRAS is an accounting model originally developed by the USGS and BOR that provides hydrologic information needed for legal compliance and water management. LCRAS‑generated information includes estimates of ET from irrigated areas and phreatophytes, and estimates of evaporation from the channel and reservoirs of the LCR.
This study was initiated to gain a better understanding of the quantity and accuracy of methods used to estimate water loss by phreatophytic ET along a selected reach of the LCR at HNWR. Specific objectives of the study include (1) comparing ET rates estimated by the USGS using field measurements, the energy-budget method, and satellite imagery with ET rates estimated by BOR using a vegetation coefficient and reference ET method, (2) improving the accuracy of the current phreatophytic vegetation ET coefficients used by BOR, and (3) estimating ET from phreatophytes at HNWR.
The LCRAS method computes daily ET for 11 nonaquatic phreatophyte groups and barren areas. Computed ET is the product of a daily 
and average daily 
, the latter parameter obtained from nearby AZMET or CIMIS stations. 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.
Three micrometeorological data-collection stations were established near Topock Marsh in the HNWR for 1 or more years to document daily and seasonal fluctuations in ET. Micrometeorological and soil data needed to solve the energy budget using the Bowen-ratio method were collected at each station. Stations were in high-density saltcedar, medium-density mixed vegetation, and low-to-medium density arrowweed.
Bowen-ratio ET rates for each ET station were compared to LCRAS ET rates for LCRAS vegetation groups associated with each ET station for part of 2002 and all of 2003. Generally, the LCRAS method yielded significantly higher estimated phreatophytic ET than estimates using the Bowen-ratio method.
Bowen-ratio monthly ET for the SC station was compared to monthly 
for sc_high group from June 2002 through December 2003. With the exception of November and December 2003, monthly rates were consistently higher than Bowen-ratio estimated ET rates by an average of 55 percent.
Bowen-ratio monthly ET for the MV station was compared to monthly for ms/aw and sc/ms/aw groups from June 2002 through December 2003. With the exception of December 2002, and December and February 2003, monthly rates were consistently higher than Bowen-ratio estimated ET rates for both LCRAS groups by an average of about 84 percent (ms/aw) and 105 percent (sc/ms/aw).
Bowen-ratio monthly ET for the AW station was compared to monthly for aw and low_veg groups from January through December 2003. Monthly rates were consistently higher than Bowen-ratio estimated ET rates for both LCRAS groups by an average of 97 percent (aw) and 90 percent (low_veg) higher.
Modified coefficients were computed for each vegetation group (SC, MV, and AW) by dividing available daily Bowen-ratio ET by daily LCRAS reference ET, 2002–04. A modified daily curve was developed by computing the average daily coefficient for two periods of the year that correspond to growth stages of plants (generally, dormancy during the cool season and vigorous growth during the summer) and representing these periods as a constant value. Starting and ending days for these periods were not fixed, but varied for each vegetation group, depending on the change in average values over a period of time. Linear interpolation was used to calculate daily coefficients for the two transition periods between the dormant and growth stage. The modified ranges for the SC, MV, and AW stations are 0.22–0.76, 0.30–0.53, and 0.21–0.56, respectively.
Daily and monthly ET for each station were computed with the modified and compared to Bowen-ratio ET. For the SC station, daily ET had a correlation coefficient of 0.92 for a period of 754 days, with large differences for many days. Monthly ET for 24 months had a correlation coefficient of 0.99. Total ET computed with the modified for two 12‑month periods also was computed and compared to Bowen-ratio estimated ET; estimates using modified coefficients were 3.7 and 2.6 percent higher than total Bowen-ratio ET.
For the MV station, daily ET had a correlation coefficient of 0.89 for a period of 754 days, with large differences for many days, whereas monthly ET had a correlation coefficient of 0.97 for 23 months. For the AW station, daily ET had a correlation coefficient of 0.89 for a period of 531 days, with large differences for many days, whereas monthly ET had a correlation coefficient of 0.98 for 17 months.
Using remote-sensing techniques, the area of Havasu National Wildlife Refuge was subdivided into three vegetation groups with similar relative density to vegetation at the SC, MV, and AW stations, plus barren soil. The acreage within the boundaries of these four groups, or ET units, was computed and multiplied by the associated annual ET computed with the modified method (adjusted for annual precipitation) to compute total LCR water use by phreatophytes. A total of 25,769 acre-ft 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-ft (excluding marsh ET). The relatively high LCRAS-estimated ET for this reach of the river is due primarily to higher phreatophytic ET rates estimated using the LCRAS method and original ET coefficients, and to a lesser degree because the LCRAS-estimated ET was not adjusted for annual precipitation.
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
Page Contact Information: Publications Team
Page Last Modified: Thursday, 01-Dec-2016 18:55:57 EST
