Date of Award

Spring 5-4-2024

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Geosciences

Committee Chair

John LaFave

First Advisor

Zach Hoylman

Second Advisor

Glenn Shaw

Abstract

Drought is a worldwide phenomenon, and many metrics have been developed to help monitor and understand drought. In this study, the drought sensitivity of two different western Montana intermontane-basin groundwater systems were analyzed: the Bitterroot Valley and the Gallatin Valley. Different standardized meteorological drought indices, namely, the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI), were compared against the Standardized Groundwater level Index (SGI) to better understand groundwater sensitivity to meteorological forcing. The SPI and SPEI were computed using precipitation and reference evapotranspiration data from the gridMET dataset. Groundwater level data from the Montana Bureau of Mines and Geology’s Ground Water Information Center (GWIC) were used to compute the SGI. In general, groundwater in the Bitterroot Valley was shown to be less sensitive to drought with increasing total well depth. Results for the Gallatin Valley did not show any apparent trend in groundwater sensitivity with total well depth. Groundwater in both basins was shown to be more sensitive for wells completed in unconfined material compared to confined material. Wells completed in unconfined material were shown to correlate more strongly to SPI/SPEI accumulation periods of 2-27 months whereas wells completed in confined material correlated more strongly to accumulation periods of 19-48 months. Groundwater level data from both basins showed influence from irrigation recharge, and thus land use plays a significant role in this analysis. The results for SPI vs. SGI and SPEI vs. SGI cross-correlation also showed slight differences depending on the normalization procedure used for the meteorological indices (i.e., parametric vs. non-parametric). These results in sum, and of the Gallatin Valley in particular, highlight the need for localized studies in order to determine groundwater sensitivity to drought.

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