Thesis Defense – Amanda Gateley

Evaluating Future Groudwater Sustainability in Kern County, California under SGMA Regulations using MODFLOW

Abstract

California’s Central Valley is a large sediment-filled valley that trends northwest to southeast, surrounded by the Coast Ranges to the west and the Sierra Nevada to the east (Faunt et al., 2009). These two mountain ranges contribute alluvial deposits to the valley’s aquifer system. Droughts and groundwater overdrafts in California prompted government intervention at the state level, resulting in the passage of the Sustainable Groundwater Management Act (SGMA). In compliance with SGMA, local Groundwater Sustainability Agencies (GSAs) were prompted to outline their Groundwater Sustainability Plans (GSPs) with the goal of maintaining or achieving sustainable groundwater levels by 2040.

Kern County is one of 21 critically overdrafted basins, where groundwater pumping has become unsustainable. Agriculture in Kern County is significantly dependent on groundwater extraction. In order to discern whether a GSA’s sustainability plan can be realistically achieved within 20 years, a simplified hypothetical groundwater model was developed using the ModelMuse and MODFLOW hydrologic modeling software. This model’s domain encompassed two GSAs: the Buena Vista Water Storage District and the Semitropic Water Storage District. Pumping wells were placed in the Buena Vista GSA to pump groundwater at different rates over different time frames to determine if long-term pumping rates are sustainable.

Three hypothetical pumping scenarios were run over the 50-year period from 2020 to 2070 in order to determine the long-term effects of changing pumping rates. Pumping Scenario 1 considered pumping at a constant rate equal to the historical average pumping rate from the past 27 years. Pumping Scenario 2 involved decreasing from historical pumping rates by 40% (done gradually over the first 20 years), and in Pumping Scenario 3 pumping was decreased from historical pumping rates by 70% (done immediately within the next decade). The results from these three scenarios may be useful in determining whether GSAs can realistically expect to exceed minimum thresholds and reach their measurable objectives in groundwater levels.