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Water-Resources Investigations Report 01-4234

Estimates of Evapotranspiration from the Ruby Lake National Wildlife Refuge Area, Ruby Valley, Northeastern Nevada, May 1999-October 2000


The Ruby Lake NWR in the southern part of Ruby Valley is the largest perennial wetland area in northeastern Nevada. The long-term preservation of the refuge depends on the availability of sufficient water to maintain optimal habitat conditions. ET from the refuge is thought to be the largest natural outflow component of the water budget for Ruby Valley. To refine the estimate of the annual water budget for Ruby Valley and to facilitate water management on the refuge, estimates of ET were made at nine sites that represented the major habitats found in the Ruby Lake NWR.

Ruby Valley is about 65 mi southeast of Elko, Nev., and encompasses about 1,000 mi2 in Elko and White Pine Counties. The Ruby Lake NWR includes about 38,000 acres of the southern part of Ruby Valley and consists of wetland and adjacent areas of meadow, grassland, and shrub upland. The existence of Ruby Lake stems in large part from the permeability and stratigraphic positions of the carbonate rocks that make up the Ruby Mountains, which form the western border of Ruby Valley. The predominant water source for Ruby Lake and associated wetland is spring discharge that originates in the southern Ruby Mountains.

Micrometeorological data were collected during an 18-month period at nine sites. These sites represented five of the most aerially extensive habitats in the refuge. The Bowen-ratio method was used to estimate daily ET for more than 500 consecutive days, from mid-May 1999 to mid-November 2000, over an open-water site, in a moderate-to-dense cover of bulrush marsh, in a moderate cover of mixed phreatophytic shrubs, and in a desert-shrub upland. The eddy-correlation method was used to estimate daily ET for periods of 2 to 12 weeks during May-September 2000 at a meadow site and at four sites in sparse-to-moderate cover of phreatophytic shrubs.

Daily ET rates ranged from less than 0.010 in/d at all of the sites to a daily maximum of 0.464 in/d at the open-water site. Average daily ET rates estimated at the open-water and bulrush-marsh sites were about four to five times greater than at the phreatophyte-1 and desert-shrub upland sites. Winter ET at the open-water site was almost twice that at the bulrush-marsh site due to the effect of shading from dead plant material. ET, computed for 84 days during the summer at the meadow site, was more than two-thirds of the annual estimate of ET at the phreatophyte-1 site. Differences in average daily ET for corresponding days between the phreatophyte-1 and the other four phreatophyte sites, where the eddy-correlation method was used, ranged from 0.001 to 0.021 in/d.

Seasonal estimates of ET derived from daily ET rates, along with corresponding habitat areas, were used to compute annual ET for the 2000 water year (October 1999-September 2000). Annual ET rates for habitats with limited ET data were derived from basis of satellite data, adjusted to reflect total ET by adding precipitation, and seasonally proportioned to correspond with changes in habitat area. An estimated 89,000 acre-ft of water was consumed by ET on the refuge during the 2000 water year based on seasonal distribution of habitats and computed winter and summer ET rates. Of this total, 49,800 acre-ft is accounted for by ET in the wetland area.

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