Groundwater discharge by phreatophyte shrubs in the Great Basin as related to depth to groundwater

Abstract

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...

Estimating Riparian and Agricultural Actual Evapotranspiration by Reference Evapotranspiration and MODIS Enhanced Vegetation Index

“…Riparian plant communities are subject to environmental factors that can reduce ET a below ET o . ET a typically decreases with increasing depth to groundwater below about 2 m [27]. Depth to groundwater frequently increases over the summer as water is withdrawn to support ET a , hence ET a /ET o might not be constant over a growing season [28].…”

Groundwater discharge by evapotranspiration, flow of water in unsaturated soil, and stable isotope water sourcing in areas of sparse vegetation, Amargosa Desert, Nye County, Nevada

“…Nevertheless, it has been speculated that shadscale will use groundwater opportunistically (Nichols, 1994). Prior to this study, however, this process had not been directly investigated, and the results presented in sections, "Flow of Water in Unsaturated Soil" and "Stable Isotope Water Sourcing" show that groundwater was being lost through evapotranspirational processes and strongly support the upward flux of groundwater at this site (and AFS).…”

“…The AFD site was established within the Sonora-Mohave Creosotebush-White Bursage Desert Scrub ecological system (Prior-Magee and others, 2007) (Branson and others, 1976) that is widely distributed in the Mojave Desert. Shadscale is not commonly considered a phreatophyte, but is assumed to transpire groundwater when occurring with phreatophytes such as greasewood (Sarcobatus vermiculatis) in areas of shallow groundwater (Nichols, 1994). It is a desert halophyte (xerohalophyte) classified as a salt excluder because its leaves sequester excess salts in bladder cells, which release the salt back into the environment when the cells rupture (Schirmer and Breckle, 1982).…”

Conceptual evaluation of ground-water flow and simulated effects of changing irrigation practices on the shallow aquifer in the Fallon and Stillwater areas, Churchill County, Nevada

“…The rate decreases linearly from a maximum at a specified elevation (land surface) to zero when the water table is below a specified depth ( 10 ft below land surface). This depth is based on work by Nichols (1994) m which ground-water discharge from ET by phreatophytes is small below a depth of 10 ft. 3 Assumes 90 percent of annual rate of 42 inches per year is applied during stress periods 2, 3, and 4, and I 0 percent is applied during stress period 5. Percentages based on applications reported by Guitjens and Mahannah (1974).…”