Document Type

Honors Thesis

Publication Date

Spring 5-8-2026

Abstract

The Greenfields Bench is an irrigation-sustained, shallow, unconfined gravel aquifer in Teton County, Montana, where irrigation modernization and canal operations affect groundwater recharge, storage, and discharge to drains and streams. This study integrates a 30 m remote sensing soil water balance in Google Earth Engine using OpenET evapotranspiration and gridded climate inputs (2020 to 2024) with a two-epoch time-lapse microgravity survey (July and November 2025), groundwater-level measurements, Greenfields Irrigation District operational records, and U.S. Geological Survey streamflow observations to quantify irrigation water partitioning and develop a groundwater budget. Microgravity paired with water level change yields provisional, high-uncertainty, effective specific yield estimates of 0.07 to 0.12; a value of 0.07 was adopted for storage calculations. In the 2024 aquifer budget, total inflow was 42,789 acre-feet and was dominated by canal leakage (33,957 acre-feet; 1.08 cfs per mile of canal), with smaller modeled irrigation recharge (8,773 acre-feet) and precipitation derived areal recharge (59 acre-feet) was negligible. Groundwater discharge to drains and springs (44,449 acre-feet) dominated outflow. A net annual storage loss of 1,995 acre-feet was estimated. A corresponding Muddy Creek surface water budget attributed only 4.7 percent of its measured discharge in 2024 to natural precipitation runoff with the remainder being irrigation return flow. Results indicate that contemporary recharge to the aquifer is strongly focused through canal leakage, implying that canal improvements or operational changes may materially affect aquifer storage and return flows to Muddy Creek. The water budget framework provides calibrated inputs for scenario evaluation in future groundwater flow modeling.

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