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A network of 9-m-tall surface flux measurement stations were deployed at eight sparsely vegetated sites during the Monsoon '90 experiment to measure net radiation, Q, soil heat flux, G, sensible heat flux, H (using eddy correlation), and latent heat flux, ire (using the energy balance equation). At four of these sites, 2-m-tall eddy correlation systems were used to measure all four fluxes directly. Also a 2-m-tall Bowen ratio system was deployed at one site. Magnitudes of the energy balance closure (Q + G + H + /rE) increased as the complexity of terrain increased. The daytime Bowen ratio decreased from about 10 before the monsoon season to about 0.3 during the monsoons. Source areas of the measurements are developed and compared to scales of heterogeneity arising from the sparse vegetation and the topography. There was very good agreement among simultaneous measurements of Q with the same model sensor at different heights (representing different source areas), but poor agreement among different brands of sensors. Comparisons of simultaneous measurements of G suggest that because of the extremely small source area, extreme care in sensor deployment is necessary for accurate measurement in sparse canopies.A recently published model to estimate fetch is used to interpret measurements of H at the 2 rn and 9 rn heights. Three sites were characterized by undulating topography, with ridgetops separated by about 200-600 m. At these sites, sensors were located on ridgetops, and the 9-m fetch included the adjacent valley, whereas the 2-m fetch was limited to the immediate ridgetop and hillside. Before the monsoons began, vegetation was mostly dormant, the watershed was uniformly hot and dry, and the two measurements of H were in close agreement. After the monsoons began and vegetation fully matured, the 2-m measurements of H were significantly greater than the 9-m measurements, presumably because the vegetation in the valleys was denser and cooler than on the ridgetops and hillsides. At one lowland site with little topographic relief, the vegetation was more uniform, and the two measurements of H were in close agreement during peak vegetation. Values of XE could only be compared at two sites, but the 9-m values were greater than the 2-m values, suggesting ire from the dense vegetation in the valleys was greater than elsewhere.

An equation describing the mean daily discharge of groundwater by transpiration from phreatophyte shrubs as a function of plant density, leaf area index, and depth to groundwater was developed using an energy combination model calibrated with energy fluxes calculated from micrometeorological data. The energy combination model partitions the energy budget between the soil and canopy permitting plant transpiration to be separated from evaporation from the soil. The shrubs include greasewood, rabbitbrush, shadscale, and sagebrush. Converting a daily groundwater discharge rate calculated by the equation to an annual rate requires an estimate of the number of days the plants used only groundwater. Rates used during previous studies in the Great Basin range from 0.030 to 0.152 m yr-•; rates calculated with the equation developed during this study range from 0.024 to 0.308 m yr -• for the reported field conditions. Annual rates estimated in this study differ from the estimated annual rates used in previous studies by factors ranging from 0.8 to 5.0. Introduction Evapotranspiration is the principal, and in some areas the sole, mechanism of groundwater discharge in the Great Basin and can be a significant method of groundwater consumption in other areas of the arid and semiarid western United States. Estimates of groundwater discharge by phreatophyte transpiration have been based on results of a study by White [1932] and have been used to estimate groundwater budgets for much of the Great Basin. It is now recognized that the results of this study are based on a flawed analysis [Nichols, 1993]. Additionally, previous studies applied the results of White [1932] in a qualitative and inconsistent fashion. Given the increasing demand for water resources in the region and the need for better estimates of regional water budgets, it is essential to develop quantitative methods that can be applied systematically for estimating groundwater discharge by phreatophytes in the Great Basin and elsewhere in the arid and semiarid west. The present study extends earlier work [Nichols, 1993] and presents a physically based method for estimating groundwater discharge by transpiration from phreatophyte shrubs in the northern Great Basin as a function of depth to groundwater, plant density, and leaf area index. Most closed basins and valleys of the Great Basin have a central bare playa, commonly underlain by a shallow water table (less than 2.5 m). Surrounding the playa are plants of the salt desert community, including iodine bush (Allenrolfea occidentalis, also called pickleweed), saltsage (Atriplex tridentata), and saltgrass (DistichIls spicata vat. stricta). Iodine bush is reported to grow in areas with a depth to water of as much as 6 m [Robinson, 1958, p. 32]. Saltgrass, the most important phreatophyte in this zone, grows most commonly in areas where the depth to water is less than about 2.5 m but has been reported to grow in areas This paper is not subject to U.S. Paper number 94WR02274. where the water table is as much as 3.6 m deep [B...

Inch-pound units of measure used in this report may be converted to International System of units (SI) by using the following factors Multiply By To obtain Area acre 4,047 square meter square foot (ft 2) 0.09290 square meter Length foot (ft) 0.3048 meter inch (in.) 2.540 centimeter mile (mi) 1.609 kilometer square mile (mi 2) 2.590 square kilometer Volume acre-foot (acre-ft) 1,233 cubic meter acre-foot per year (acre-ft/yr) 1,233 cubic meter per year Flow rate foot per day (ft/d) 0.3048 meter per day foot per year (ft/yr) 0.3048 meter per year foot squared per day (ft 2 /d) 0.09290 meter squared per day Temperature: Degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) by using the formula °C = [°F-32]/1.8. Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929, formerly called "Sea-Level Datum of 1929"), which is derived from a general adjustment of the first-order leveling networks of the United States and Canada.

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