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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5135

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Introduction

Evapotranspiration (ET) measurements are essential when quantifying soil-plant-atmosphere controls on hydrologic transport processes in arid environments. Current research at the U.S. Geological Survey’s Amargosa Desert Research Site (ADRS), adjacent to a low-level radioactive and hazardous chemical-waste facility, includes analysis of soil-plant atmosphere interactions as they relate to transport and release of waterborne contaminants. In support of this ongoing research, the ET components of transpiration and evaporation from distinct plant species and bare soil were differentiated with periodic portable chamber measurements of ET. The components were combined in a one-layer, multi-component canopy model to estimate total landscape-scale ET (Stannard, 1988; Stannard and Weltz, 2006).

Portable chambers have been used to measure ET from cultivated alfalfa fields (Reicosky and others, 1983), bare soil, and sparsely vegetated plant communities (Stannard, 1988; Stannard and Weltz, 2006). Chambers also have been used to determine ET from distinct vegetation types within mixed species communities (Stannard, 1988). Chambers measure the water vapor exchange between the Earth’s surface and the atmosphere from small areas (Dugas and others, 1997) by enclosing a known volume of a plant canopy, soil surface, or both, and then measuring the increase in vapor density within the chamber. The maximum rate of change in vapor density with time is proportional to the ET flux from the surface area enclosed by the chamber (Stannard, 1988).

Purpose and Scope

This report documents the measuring of ET from different desert-shrub species and bare soil using a portable chamber at an arid mixed-vegetation site, partitioning of ET measurements into transpiration and evaporation, and compiling chamber measurements into landscape-scale ET. From 2003 to 2006, chamber ET measurements over creosote bush, shadescale, and bare soil were collected approximately every 3 months; measurements over burrobush and wolfberry were collected annually. Partitioning of chamber ET measurements into evaporation and transpiration components and assembly of these components into landscape-scale ET using a one-layer, multi-component canopy model are described. The magnitude and variability in component- and landscape-scale ET are compared between shrub species and bare soil.

Description of Study Area

The ADRS borders a waste-disposal facility in the Mojave Desert, 17 km southeast of Beatty, Nev., near Death Valley (fig. 1A). The ADRS is in one of the most arid regions of the United States. Annual precipitation averaged 130 mm during 2001–05 and was highly variable from month to month and year to year (Johnson and others, 2007). Annual potential evaporation is about 1,900 mm (Nichols, 1987). The Amargosa Desert in the Basin and Range Province is bounded by lower Paleozoic rock and Tertiary volcanic-rock mountains (Fischer, 1992). Surface soils are mapped as the Yermo-Arizo association. The Yermo soils are loamy-skeletal and the Arizo soils are sandy-skeletal. Subsurface sediments are predominantly fluvial deposits, consisting of several sand and gravel sequences (Andraski, 1996). Depth to the water table ranges from 85 to 115 m below land surface (Fischer, 1992).

The sparsely vegetated (6–8 percent of the landscape; Andraski and others, 2005) study site is dominated by creosote bush [Larrea tridentata (Sessé & Moc. Ex DC.) Coville], an evergreen shrub (Smith and others, 1997). The root system of a creosote bush can exceed 4 m radially (Gile and others, 1998) and the root system depth generally corresponds with the penetration depth of annual precipitation, about 0.75 to 1 m at the ADRS (Andraski, 1997). Root-zone (0–1 m depth) volumetric soil-water content ranges from 0.02 to 0.12 m³/m³ and near-surface soil-water content ranges from 0.02 to 0.31 m³/m³ (2001–05; Johnson and others, 2007). Drought deciduous plant species include shadscale [Atriplex confertifolia (Torr. & Frém) S. Wats.], burrobush [Ambrosia dumosa (Gray) Payne], and wolfberry [Lycium pallidum Miers].

Portable chamber measurements of ET were collected periodically at locations 1 and 2 shown in figure 1B. Site selections were based on four 400-m transects measured in 2001 using the line-transect method (Smith, 1974); and chosen for their proximity to an eddy-covariance ET station (fig. 1B), for representative plant size and shape, and for having replicate plants within close proximity.

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