Award Date

5-2009

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Biological Sciences

Department

Life Sciences

First Committee Member

Michael Young, Chair

Second Committee Member

Stanley Smith

Third Committee Member

Paul Schulte

Graduate Faculty Representative

Zhongbo Yu

Number of Pages

112

Abstract

Global climate change in addition to a growing population can have a strong impact on our natural resources. In particular, cities in the semi-arid and arid southwestern US are currently faced with the challenge of providing sustainable fresh water to a growing population. In Las Vegas, dwindling water levels in Lake Mead have forced water managers to seek water resources from Spring Valley and Snake Valley, NV, which are located 402 km north of the city. The goals of this research were to investigate the water use of shrubs located in these valleys by using a large aperture scintillometer (LAS) to scale up energy balance components (needed to close a water balance) from the canopy level and to determine the potential impacts of altering the water source available to these native vegetation communities. Due to proposed groundwater pumping to Las Vegas, we focused mainly on the facultative phreatophytic shrub Sarcobatus vermiculatus (greasewood). Phreatophytic shrubs have a large tap root that accesses ground water within a phreatophytic zone directly above the water table, and they are characterized by being either obligate (living almost entirely off of ground water) or facultative (the ability to survive off of either groundwater or vadose zone soil moisture).

This study demonstrates the validity of using a LAS to measure energy balance components over heterogeneous landscapes in a semi-arid environment, as well as the potential impacts that a rapidly falling water table can have on native vegetation. This information could be combined to help mitigate negative impacts from long term water acquisition from these valleys, and help to combat against drastic changes to the natural ecology existing in these valleys.

In Chapter 2 we demonstrate the ability of the LAS to be used in a semi-arid environment within a heterogeneous landscape to estimate energy balance components by comparing LAS sensible heat values (HLAS ) to those obtained from an eddy covariance station (HEC ). Correlation coefficients between the two techniques for four measurement periods over the growing season of 2005 were as follows: 0.98 on 4-14-05; 0.85 on 6-1-05; 0.85 on 7-19-05, and 0.89 on 9-5-05, with all p<0.001. These results suggest an alternative technique for measuring sensible heat flux over much larger footprints than possible by the EC station alone, which are needed to close a water balance. Future research is needed to determine the effectiveness of the LAS for determining basin wide evapotranspiration values when combined with satellite data.

The plant water use of S. vermiculatus during the growing season of 2007 is investigated in Chapter 3. Here we use a sap flow system to estimate transpiration across three treatments: severed tap (ST), infiltration prevention (IP) and True Control (TC). Our data suggest that severing the tap root of this phreatophytic shrub has a greater effect on sap flow than preventing rainwater infiltration. Furthermore, we demonstrate the ability of this species to respond to episodic rainfall events by increasing their sap flow in response to increased soil moisture. Results from this study may provide valuable information on how this species could respond to a rapidly declining water table in Spring Valley and Snake Valley, NV.

Keywords

Arid regions--Water-supply; Groundwater; Plants--Effect of global warming on; Plants--Effect of water levels on; Water table

Disciplines

Desert Ecology | Ecology and Evolutionary Biology | Life Sciences | Plant Biology | Plant Sciences

Language

English

Comments

Signatures have been redacted for privacy and security measures.


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