Quantifying Groundwater Evapotranspiration with Remote Sensing for the Humboldt River Basin, NV

The Desert Research Institute in cooperation with the USGS and the Nevada Division of Water Resources is currently evaluating groundwater discharge via evapotranspiration from phreatophyte shrubs in the Humboldt River Basin, NV. The overarching goal of this work is to improve the modeling of groundwater for the purposes of evaluating the interplay between groundwater and surface water.

Accurate estimates of groundwater ET (ETg) are essential to developing groundwater budgets, modeling boundary conditions, and ultimately constraining model calibration of aquifer properties (i.e. non-uniqueness between recharge, transmissivity, and discharge). Methods used to estimate ETg are often based on intensive field based micrometeorological measurements, assumed or fixed rates, or a combination of remote sensing and empirical relationships based on past measurement studies.

The approach used in this study uses a regression based on enhanced vegetation index (EVI) calculated from remotely sensed optical data acquired by the Landsat series of satellites, along with precipitation and evaporative demand in the area of interest. This regression was developed through the evaluation of 40 site years of micrometeorological data collected at 26 unique sites, primarily located within phreatophyte areas of Nevada. Google Earth Engine (GEE), a massively parallel, cloud-computing platform, was used to apply this method to 30 years of Landsat images and climate archives, in order to produce annual estimates of rates and volumes of ETg over the entire Humboldt River Basin. Because volume calculations of ETg are sensitive to estimates of phreatophyte area, an evaluation was conducted to determine the accuracy of previously defined phreatophyte boundaries. This review was performed through a combination of field evaluations and an examination of aerial imagery and optical and thermal Landsat data.

This presentation will describe methods used to quantify ETg in the Humboldt River Basin, preliminary results, and the relevance of the data to ongoing groundwater modeling efforts.