Estimating Annual Groundwater Discharge by Evapotranspiration Along the Wild and Scenic Portion of the Amargosa River Completed

PREVIOUS STUDIES

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Of the three main components of a groundwater budget–natural discharge, recharge, and subsurface flow–estimating natural groundwater discharge often is the most pragmatic. Unlike recharge and subsurface flow components of the water budget, groundwater discharge from shallow groundwater is observable at land surface and can be estimated from energy-budget measurements made at or near land surface. Reliable estimates of groundwater discharge can be used to constrain these other, more-difficult-to-quantify components of the water budget. Groundwater discharge to the Amargosa River was previously estimated at 8,400 acre-feet per year. For this estimate, evapotranspiration rates measured mostly in Ash Meadows were applied to a vegetation-index-based model to estimate groundwater discharge in the AWSR. Uncertainty in the discharge estimate is likely greater than plus or minus 32 percent because the following issues were not considered in the uncertainty analysis:

1. About 25 percent of estimated groundwater discharge from the AWSR is from the “dense meadow and forested vegetation” ET unit. The ET unit in Ash Meadows is comprised primarily of dense meadow; forested vegetation accounted for only a small portion of the ET unit. By contrast, in AWSR forested vegetation predominates and in most cases dense meadow is absent. The differences in vegetation between Ash Meadows, where the ET was measured, and the AWSR, where the discharge was estimated, increases the uncertainty of the AWSR estimate.
2. The “moist bare soil” ET unit identified in Ash Meadows also accounts for about 25 percent of estimated groundwater discharge; in the AWSR area, the moist-bare soil designation is dominated by salt-encrusted soils, leading to additional uncertainty for the AWSR area.

A surface salt encrustation was observed during preliminary field work raising concerns (1) of a seemingly high groundwater discharge rate and (2) that the source of the evaporated water is other than groundwater since most of this area is in a floodplain. Thus, although the Laczniak and others groundwater discharge estimate is sufficient to better understand regional-scale hydrologic processes, these first-order estimates are insufficient for understanding local-scale hydrologic processes of the AWSR, or for supporting a scientifically defined federal reserved water right. Moreover, estimates of natural groundwater discharge provide baseline data from which to quantify changes in riparian ecosystems caused by anticipated increased pumping in the study area.

CURRENT STUDY

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Previous estimates of annual groundwater discharge by ET will be refined by 

1. measuring groundwater discharge rates within the study area, and
2. applying these rates to groundwater discharge areas delineated using high-resolution remotely-sensed imagery.

Local precipitation and surface water flooding are subtracted from the measured ET resulting in ET only from the groundwater system. To complete this micro-scale water budget, the change in soil-moisture storage is calculated as the difference between the soil moisture at the beginning and end of the period of record. The combination of high-resolution imagery (1 to 10-meter resolution) and in-situ groundwater discharge measurements will refine existing estimates of ET.

The initial locations for ET site placement will be determined based on preliminary remote sensing analysis and field verification. The two ET units most in need of refinement and representing proportionally larger sections of the groundwater discharge area will be targeted first. These areas most likely will be

1. the woody riparian corridor of predominately mesquite, willow, and tamarisk south of Tecopa, CA and classified by Laczniak and others as “dense meadow and forested vegetation”, and
2. a large area within the floodplain near Tecopa, CA classified by Laczniak and others as “moist bare soil”.

Groundwater Discharge Rates

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Annual groundwater discharge rates will be quantified at a minimum of two sites by concurrently measuring ET, precipitation, groundwater levels, and soil moisture over a period of up to two years. Sites will contain

• a micrometeorological station collecting eddy-covariance ET and energy-budget data,
• a bulk precipitation gage to accurately measure precipitation,
• a tipping bucket rain gage to record event timing and intensity,
• soil-water-content sensors to measure soil moisture in the upper 25 cm of soil, and
• an observation well to monitor fluctuations in groundwater levels.

An attempt will be made to identify and remove any flood-water contribution to measured ET. Measured ET rates may be higher due to surface water run on in the event of flooding. The Nevada Water Science Center operates two streamflow gaging stations (10251300, 10251330) on the Amargosa River. ET site visits will be made, if possible, during or shortly after flood events to determine the extent of surface water run on within groundwater discharge areas. Other parameters recorded at each ET site such as net radiation, sensible heat, and soil heat flux also will provide evidence of flood occurrence and duration. Continuous ET data measured during flood periods will be evaluated, compared against previous and subsequent periods, and adjusted if necessary. 

Remote Sensing Analysis

High-resolution remotely-sensed imagery will be used to (1) define the potential area of groundwater discharge (PAGD), and (2) classify ET units. The combination of high-resolution imagery and additional field validation will result in an improvement in PAGD boundaries and ET unit classification. The PAGD boundaries will be mapped using 1-meter resolution National Agriculture Imagery Program (NAIP) aerial imagery acquired June 4, 5, and 7, 2010. The accuracy of mapped boundaries will be validated with field-based observations. 

ET unit classification is the remote-sensing process of up-scaling measured groundwater discharge rates to the basin scale, by using a vegetation index created from imagery. Land-cover analysis uses all available bands of the electromagnetic spectrum to group similar vegetation types and density. Vegetation-index analysis relates the near-infrared and infrared bands resulting in a reliable and proven method to estimate vegetation density. Past groundwater discharge studies by the USGS Nevada Water Science Center have related ET and/or groundwater discharge rates with land-cover and/or vegetation index analyses to classify ET units. A combination of both methods will be used for this study. Both land-cover and vegetation-index analyses will be completed prior to site selection using 10-meter resolution, multispectral Systeme Pour l’Observation de la Terre (SPOT 5) imagery. The accuracy of classified ET units will be validated with field-based observations.

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