Estimation of annual Groundwater Evapotranspiration from Phreatophyte Vegetation in the Great Basin using Remotely Sensed Vegetation Indices and Ground Based Flux Tower Measurements
AuthorBeamer, Jordan P.
AdvisorPohll, Greg M.
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Escalating concerns about the future of water resource management in arid regions of the American Southwest have sparked numerous hydrologic studies looking into available water resources for in-basin and inter-basin transfers. Groundwater is the primary water supply source for much of the state of Nevada and the Great Basin, thus accurate estimates of the regional scale groundwater recharge and discharge components are critical for regional groundwater budgets. Groundwater discharge from phreatophyte vegetation by evapotranspiration (ET) is the dominant component of groundwater discharge in many hydrologically closed valleys of the Great Basin, and can be measured directly from eddy-covariance (EC) and Bowen-ratio (BR) flux tower systems. The purpose of this project was to develop a predictive equation based on relationship between annual ET and meteorological data from EC and BR sites in phreatophyte vegetation with remote sensing data. Annual total ET (ETa) measured from forty site/year combinations of flux tower data from Carson Valley, Walker River Basin, Oasis Valley, Snake Valley, Spring Valley, White River Valley, and the lower Colorado River Flow system were correlated with the Enhanced Vegetation Index (EVI) from Landsat Thematic Mapper (TM) satellite. EVI was extracted from source areas at corresponding locations from 15 mid-summer Landsat TM scenes. ETa was transformed into ET* by subtracting annual precipitation and normalizing by annual reference ET (ETo) (ET*=(ETa-precipitation)/(ETo-precipitation)). ET* correlated well with EVI (r2=0.97), and because it takes basin specific climate measurements into account, it is transferable to many shallow groundwater discharge areas in the Great Basin. This relationship was used to provide a first order estimate of the mean annual groundwater ET (ETg) from four phreatophyte groundwater discharge areas in Nevada using only a mid-summer Landsat EVI image, annual ETo and precipitation data. This simple approach provided estimates of mean annual ETg volumes which compare well with estimates from previous studies from these phreatophyte areas.