Hydro Visi Damage Done: Causes and Impacts of Land Subsidence in the San Joaquin Valley by Pete Dennehy, Bill Mok, and Trey Driscoll Land subsidence is a costly and problematic result of groundwater overdraft in San Joaquin Valley, where prime agricultural farmland is ideal for producing high yields of valuable crops. However, overplanting, hardening of demand with perennial crops, and limited surface water availability have allowed groundwater demands to outpace groundwater supply. Growing cities and degraded water quality in shallower aquifers further stress the subsidence prone deeper aquifers. These factors lead to groundwater overdraft that drives subsidence, despite extensive efforts to slow the processes. This article focuses on the history and impacts of subsidence in the San Joaquin Valley, particularly the Tulare Lake subregion, where recent subsidence rates are the highest in the state. This article is the fourth installment of a five-part HydroVisions series on land subsidence, including: • California’s Sinking Feeling: An Introduction to Subsidence (Fall 2023) • Subsidence Data: Techniques, Availability, and Interpretation (Winter 2024) • Models: Tools for Estimating and Predicting Subsidence (Spring 2024) • Damage Done: Causes and Impacts of Land Subsidence in the San Joaquin Valley (this issue) • Arrested Displacement: Subsidence Mitigation (planned for Winter 2025) The final article in the series, planned for spring 2025, will focus on actions being taken in the San Joaquin Valley and other parts of the state to avoid and minimize subsidence impacts going forward.
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2024 Fall Issue
Groundwater Overdraft Driving Historical and Recent Subsidence Subsidence in the San Joaquin Valley has been called the greatest human alteration of the Earth’s surface (Galloway et al., 1999). The primary cause of this subsidence is reduced groundwater levels due to overdraft. Improvements to the vertical turbine pump and rural electrification in the 1930s allowed wells to be drilled deeper and pump more water from greater depths (Galloway et al., 1999). The extensive clay interbeds in the deep sedimentary basin near Tulare Lake are particularly susceptible to subsidence from decreased pore pressure, which is caused by groundwater overdraft. Other localized drivers of land subsidence include oil and gas extraction, natural processes such as faulting and tectonic down warping, and expansive soil types susceptible to hydrocompaction (LSCE, 2014). Historical water level declines in the San Joaquin Valley have been significant, up to 400 feet during the 1950s and 1960s, coinciding with subsidence rates of up to 1 foot per year. The state and federal water projects completed in the late 1960s lessened groundwater pumping, allowing groundwater levels to rebound by up to 200 feet in some areas (LSCE et al., 2014). Despite rising groundwater levels in the 1970s and 1980s, subsidence continued at a reduced rate due to residual subsidence – the continued land sinking after groundwater levels stabilize or rise. Modeling by Stanford University researchers suggests that residual subsidence may continue for decades after overdraft is reversed (Lees et al., 2022). For example, subsidence numerical model using data compiled near Hanford, CA simulated 3 feet of subsidence between 1965 and 1985, despite stable to increasing groundwater levels during this time.
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