Date of Award:

5-2025

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Civil and Environmental Engineering

Committee Chair(s)

David G. Tarboton

Committee

David G. Tarboton

Committee

Jeffery S. Horsburgh

Committee

Sarah Null

Abstract

The Great Salt Lake (GSL) is a lake of environmental and economic importance for Utah, and in recent years lake levels have declined to critically low levels. The lake level is dependent on the balance between the inflow and outflow. Inflow into the lake is governed primarily by the streamflow originating from the Bear, Weber and Jordan River basins (GSL subbasins), direct precipitation over the lake and groundwater entering into the lake. The GSL is a terminal lake, with outflow via evaporation from the lake.

In this study, we examined hydrologic processes within the river basins that relate to streamflow entering into the GSL. First, we examined trends in streamflow, precipitation, temperature, and evapotranspiration to assess natural variability and their potential role in the decline lake level. The result for the study period of 2003 to 2021 showed that there was no statistically significant trend in most processes across the GSL subbasins, except for a decreasing evapotranspiration trend in the Jordan River basin.

Land cover is an important factor in water management and water availability, that is linked to evapotranspiration. Using publicly available remotely sensed data from satellites, we estimated the changes between land cover classes (land cover transformation) in the GSL subbasins. The major land cover transformations in the GSL subbasins were from Grass to Tree and Tree to Grass. Other significant land cover transformations were from Cropland to Developed and Grass to Developed.

Precipitation over the basin is partitioned into streamflow or infiltrates into the soil and subsurface groundwater, where it supplies evapotranspiration, or ultimately drains to streams. Thus an increase in evapotranspiration leads to decrease in streamflow and vice versa. We quantified evapotranspiration from primary land cover classes using the land cover map and a gridded evapotranspiration dataset. Estimating evapotranspiration from these land cover classes help us assess the amount of water returning to the atmosphere, without reaching the Great Salt Lake. In addition to determining which land cover classes have higher or lower evapotranspiration depths, this information also helps to comprehend how changes in area of these land cover classes affect the basin-wide evapotranspiration. Besides water bodies, evapotranspiration depth from the Tree land cover class was the highest, while evapotranspiration from Grass land cover class was one of the lowest. One reason for evapotranspiration being highest in the Tree land cover class is that tree’ d areas receive most precipitation, because that is where the trees grow, typically at higher elevations. Additionally, changes in basin-wide evapotranspiration due to land cover changes were estimated. The results showed that for the Jordan River basin where evapotranspiration decreased overall the dominant land cover change was from Tree to Grass suggesting that this land cover change played a role in evapotranspiration changes and the overall water balance relating to streamflow.

Overall, this study has provided knowledge and insights on the hydroclimatic components, land cover changes, and evapotranspiration from different land cover classes across the GSL subbasins. It has also quantified basin-wide changes in evapotranspiration due to land cover change in the basin. These contributions provide important scientific information about the processes associated with streamflow entering into the GSL, information that may aid water managers and decision makers in developing strategies for managing water resources to help restore the GSL.

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