Scientific Investigations Report 2006–5043
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5043
Monthly ET was computed for three Bowen-ratio stations at HNWR from 2002 to 2004 and compiled for six LCRAS phreatophyte groups, including barren soil. Monthly ET for each Bowen-ratio station was compared to monthly LCRAS ET for phreatophyte groups with similar vegetation and densities.
Micrometeorological and soil data were collected at three ET stations in Topock Marsh, from May 2002 to June 2004 (table 3). The stations are in areas that are representative of common phreatophyte types and densities found along the LCR; homogeneous saltcedar (SC), medium-density mixed vegetation (MV), and homogeneous low-to-medium density arrowweed (AW). Monthly Bowen-ratio ET was computed (sum of daily ET) for each SC, MV, and AW station (figs. 8A, 8B, 8C, respectively).
Monthly ET was compiled for six LCRAS phreatophyte groups for 2002 and 2003, including barren soil. Based on the LCRAS group descriptions (table 4), five LCRAS phreatophyte groups were selected that have vegetation characteristics similar to vegetation at the Bowen-ratio ET stations (table 3). Monthly ET was compiled for groups sc_high, ms/aw, sc/ms/aw, aw, low_veg, and barren (figs. 9A–9F, respectively).
Monthly Bowen-ratio ET rates for each ET station (SC, MV, and AW) were compared to monthly 
rates for LCRAS vegetation groups associated with each ET station, based on descriptions of phreatophyte groups (table 4), for part of 2002 and all of 2003 (table 5). Generally, the LCRAS method yielded higher estimated phreatophytic ET than estimates using the Bowen-ratio method (table 5, fig. 10).
Monthly Bowen-ratio 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.
Monthly Bowen-ratio 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 84 percent (ms/aw) and 105 percent (sc/ms/aw).
Monthly Bowen-ratio 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).
Barren ET was not measured directly as part of the study because (1) a limited number of energy-budget stations could be installed and operated and (2) because the estimated quantity of LCR water lost in barren areas is low compared to other phreatophyte groups. Monthly LCRAS barren for 2002 was 1.14 ft (annual precipitation at the Mohave AZMET station was 0.07 in.) and for 2003 was 1.03 ft (fig. 9F). These rates are as high as those reported for sparse grassland and sparse shrubland for other ET studies (table 1). Based on relative comparisons to other estimates of ET with similar environmental characteristics, the LCRAS estimated monthly ET for bare-soil areas appear reasonable from May through November, but seem high for January, February, and March.
Higher ET estimates using the LCRAS method likely is due to uncertainties associated with the derivation of 
. LCRAS ET estimates are the product of two variables; the daily 
and ET (eq. 10). The standardized reference ET equation used by LCRAS (eq. 11) was developed rigorously specifically for well-watered crops and was derived from a widely accepted physics-based ET model. However, which are normally empirically derived, were not specifically derived for the LCR due to the lack of long-term ET measurements for crops and phreatophytes. LCRAS agricultural ET coefficients were derived from coefficients used in other Western United States agricultural areas and modified to reflect characteristics of crops along the LCR (the average day of year for planting and harvesting; Jensen, 1998). LCRAS phreatophyte ET coefficients were derived from a correlation of compiled phreatophyte ET data and daily data (Jensen, 1998).
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