Evaluating long-term water quality variations to climate variability and land use change in California's Sacramento watershed.

This study aimed to evaluate how shifts in land use and climate extremes have jointly shaped long-term water quality trends in the Sacramento River watershed in California between 1985 and 2023. We integrated satellite-derived land use maps, daily climate indices, and monthly water quality records for six parameters (hardness, pH, specific conductance, total dissolved solids (TDS), total suspended solids (TSS), and turbidity). Analyses included seasonal trend assessments, spectrogram analysis of periodicity, and Random Forest models to identify key drivers of water quality trends. Three modeling cases were compared: climate-only, land use-only, and combined (climate + land use). A 10-fold cross-validation framework was implemented, and differences among models were assessed using the Friedman test with Wilcoxon signed-rank post-hoc comparisons. The combined model integrating climate and land use variables best explained long-term water quality variability, followed by land use alone and climate alone. Seasonal analysis further highlighted a dual stress regime: winter storms drove sediment and turbidity peaks, while summer droughts intensified salinity and pH. Water quality deterioration peaked during 2002-2012, with elevated salinity (conductance, TDS) and sediment-related parameters (TSS, turbidity). Land use predictors explain more site-to-site variation in water quality than climate predictors (median absolute ΔR2 LU - Clim ≈ 0.18; LU outperforms Clim for 5/6 variables). Persistence of elevated salinity underscores the need for long-term management, while the partial recovery of sediment indicators highlights opportunities for targeted erosion control. These findings show the interacting influence of land use transitions and climate extremes, offering a transferable framework for adaptive watershed management.