Groundwater Discharge from Phreatophyte Vegetation, Humboldt River Basin, Nevada
Groundwater evapotranspiration (ETg) from phreatophyte vegetation is the primary component of natural groundwater discharge within the Humboldt River Basin. This report summarizes previous study estimates of ETg, and details methods and results of updated groundwater discharge areas, ETg rates, and ETg volume estimates developed in this study. Estimates derived in this study are summarized for the period of 1985-2015 and were based on a consistent place-based approach that relies on Geographic Information System and groundwater level data and a least-squares regression model that relates Landsat vegetation indices with evaporative demand, precipitation, and in-situ estimates of phreatophyte ET. Median annual ETg rates and volumes reported in this study are representative of pre-development conditions. Where irrigated areas were identified, ETg rates were adjusted to reflect the phreatophyte vegetation that likely occupied irrigated areas prior to cultivation. Results from this study were used to inform groundwater modeling studies by the U.S. Geological Survey and the Desert Research Institute, in cooperation with Nevada Division of Water Resources, to support conjunctive water management.
Results and datasets are summarized and documented in the form of maps, graphs, tables, geodatabases, and metadata following Federal Geographic Data Committee standards and are available at www.dri.edu/humboldt-etg. Estimated pre-development total annual ETg volumes for the upper, middle, and lower Humboldt River basin are 158,500, 361,600, 55,900 ac-ft/yr, and 85,700, 248,400, and 46,100 ac-ft/yr when riparian lands are excluded, respectively. Discharge areas and median annual ETg rates and volumes were compared to previous estimates for respective ET Units and Hydrographic Areas. Results reported for the upper Humboldt River Basin indicate that potential areas of groundwater discharge are generally lower, and ETg rates and volumes are generally less than one half of the ETg rates and volumes reported by Plume and Smith (2013). Results reported for the middle Humboldt River Basin indicate that ETg volumes are higher in six, and lower in seven HAs when compared to previous estimates reported in Water Resource Bulletin and Reconnaissance Series reports. ETg rates and volumes in the middle Humboldt River Basin are also generally less than one half when compared to those reported by Berger (2000). Differences in ETg volumes are primarily due to differences in ETg rates and differences in groundwater discharge areas.
This study used place-based satellite remote sensing, climate and GIS datasets, groundwater levels, and in-situ based phreatophyte ET empirical regression models to estimate potential areas of groundwater discharge, and ETg rates and volumes within the Humboldt River Basin. Future study estimates of ETg within the Humboldt River Basin could be improved by refining delineation of groundwater discharge areas, variability in ETg with respect to climate and land use change, and collection of in-situ ET estimates in areas where large uncertainty exists.
Appendix C – Percent Change in Median ETg for Select Basins
GIS Data – Potential areas of groundwater discharge