201-1 A Century Written in the Valley Floor: 100 years of Subsidence in California's Central Valley

Abstract:

Land subsidence in California's Central Valley is both an old and a new problem. Historical leveling surveys documented widespread subsidence exceeding 30 ft by the late twentieth century and established the Central Valley as one of the world's most extensively studied examples of aquifer-system compaction. However, most discussions of subsidence focus on historical groundwater overdraft and provide a limited perspective on how subsidence patterns have evolved over the past century.

This study integrates historical leveling data, extensometer records, Global Navigation Satellite System (GNSS) measurements, Interferometric Synthetic Aperture Radar (InSAR) observations, and groundwater-level records to reconstruct and evaluate 100 years of subsidence across California's Central Valley. A previously published map of cumulative subsidence from 1926–1970 is combined with newly developed reconstructions for 1970–2026 and 1926–2026, providing the first consistent century-scale assessment of regional subsidence patterns. The results show that while the locations of maximum subsidence have shifted substantially through time, significant subsidence persisted well beyond the period commonly associated with historical overdraft. Some areas that experienced severe historical subsidence have stabilized, while new centers of subsidence have emerged in response to changing groundwater-use patterns, prolonged droughts, and continued reliance on groundwater pumping.

To investigate the causes of these changes, long-term groundwater-level and subsidence records from 50 representative locations throughout the Central Valley were analyzed using one-dimensional aquifer-system compaction models. Estimated critical heads and site-specific subsidographs demonstrate that modern subsidence behavior is strongly influenced by historical groundwater stresses, preconsolidation conditions, and aquifer-system properties. Sites with similar contemporary groundwater levels can exhibit markedly different subsidence responses because of differences in stress history and aquifer-system memory accumulated over decades of groundwater development.

Together, the mapping and modeling results reveal how the magnitude, distribution, and controls on land subsidence have changed over the past century. The findings provide a long-term perspective on subsidence evolution in California and demonstrate how historical groundwater stresses continue to influence present-day subsidence risk and groundwater-management decisions.

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