Decoding groundwater level patterns and abrupt changes in Central and Southern California's alluvial regions

The variability in groundwater levels (GWL) in California's Central Valley and Southern California Coastal Basin, driven by climatic and hydrological shifts, poses significant challenges for ecosystems and agricultural sustainability. This study employs a dual-method approach, integrating the Seasonal Kendall (SK) test and the Bayesian Estimator of Abrupt Change, Seasonality, and Trend (BEAST) algorithm, to analyze long-term trends and abrupt shifts in GWL. The SK test reveals statistically significant declines in GWL across most wells, with particularly severe reductions observed in the Central Valley and the counties of San Bernardino and San Diego. For instance, well A5 in the Central Valley recorded a Z-value of 23.83 and a β of 2.36, marking acute groundwater depletion. Similarly, in San Bernardino County, wells S11 (Z = 24.09, β = 14.50) and S17 (Z = 24.20, β = 9.53) demonstrated alarming declines. These findings suggest that reduced recharge rates and intensified extraction are driving the depletion, which in turn threatens local ecosystems through diminished streamflows and wetland contraction. However, some wells exhibited rising GWL, attributed to localized recharge, underscoring the spatial heterogeneity of groundwater dynamics. BEAST analysis further identified both positive and negative abrupt changes in GWL, reflecting complex responses to environmental variability. While several wells recorded sharp drops in GWL, such as up to −7.48 m in the Central Valley and −44.00 m in Southern California, others demonstrated notable recoveries, including up to 4.20 m in the Central Valley and 9.31 m in Southern California. These results emphasize the urgent need for tailored groundwater management strategies that address both declining and rising trends, while accounting for seasonal variability. Adaptive water management practices, which are flexible and responsive to changing conditions, will be crucial to safeguarding ecosystem integrity and sustaining agricultural productivity.