Groundwater Discharge by Evapotranspiration from Areas of Spring-Fed Riparian Vegetation, Stump Spring and Hiko Springs, Nev.

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Stump Spring has been designated as an Area of Critical Environmental Concern (ACEC) by the BLM, and Hiko Springs currently is in the designation process. Both springs flow intermittently in drainages where the depth to groundwater is shallow. The shallow groundwater flowing to and from the springs sustain scarce desert riparian habitats. The BLM has recognized a need for accurate estimates of GWET from these riparian spring-discharge areas to scientifically support

1.  water right applications, and
2.  a baseline from which to quantify future changes in these riparian ecosystems caused either by groundwater pumping or climate change.

Annual GWET will be estimated by applying GWET rates measured in situ in spring discharge areas delineated by mapping riparian vegetation extents. This methodology has been well-established by the USGS-NVWSC. Earlier studies in the modern era combined remote-sensing techniques with evapotranspiration (ET) rates measured using the Bowen-ratio method. More recent studies have measured ET using the eddy-covariance method rather than the Bowen-ratio method. The approach for this study will be most similar to Laczniak and others (2006) with the primary exception being that eddy-covariance instead of the Bowen-ratio method will be used.

Annual GWET rates will be quantified with concurrent measurements of ET, precipitation, and soil moisture in each spring discharge area. Instrumentation for continuous monitoring will include

• eddy-covariance and energy-balance sensors to compute ET,
• tipping-bucket and volumetric precipitation gages, and
• water content reflectometers to measure soil moisture and determine changes in soil-moisture storage.

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Local precipitation will be subtracted from measured ET resulting in ET derived solely from groundwater. The change in soil-moisture storage will be calculated as the difference between the soil moisture at the beginning and end of each period-of-record. Site-scale energy and water balances will be computed. Preliminary fieldwork and mapping efforts suggest that a single GWET rate measured in situ in each spring-discharge area will be representative and applicable to each area. The instrumented ET station will be deployed first to Stump Spring, and then will be relocated to Hiko Springs. The station will remain in each area for 12-14 months. At least one location in each spring-discharge area was identified during field visits where the fetch is mostly adequate in the prevailing wind direction. The prevailing wind direction in southern Nevada is from the southwest (Houghton and others, 1975). The ideal location for an ET station is one where the fetch for the surface-of-interest is longer than the ET source area. Fetch is defined in this study as the upwind distance from the ET station to the riparian vegetation edge. The fetch should be long enough for airflow measured by the eddy-covariance sensors to equilibrate to riparian vegetation ET. Evapotranspiration measurements recorded when the wind is from a direction that does not represent riparian vegetation will be filtered and replaced with estimated values as necessary.

Riparian vegetation extents will be mapped using 1-meter resolution National Agriculture Imagery Program (NAIP) aerial imagery. The riparian vegetation in both spring-discharge areas comprise primarily honey mesquite (Prosopis glandulosa) with lesser amounts of catclaw (Acacia greggii). Preliminary mapping indicates that the riparian areas for Stump Spring and Hiko Springs are limited to about 150 and 120 acres, respectively. The accuracy of mapped boundaries will be validated with field-based observations. A simple vegetation-index analysis may be done if high-resolution multispectral imagery are available to evaluate differences in vegetation density within each area.