A replicable six-method framework for quantifying legislative efficacy in surface water quality governance: evidence from four decades of monitoring in ten São Paulo River Basins, Brazil (1978–2026)

Abstract Surface water quality governance in developing economies frequently fails to reverse long-term deterioration despite successive legislative interventions, yet rigorous quantitative evaluation of policy effectiveness at the basin scale remains rare. This study applies a transferable six-method analytical framework to 48 years (1978–2026) of monitoring data from ten São Paulo river basins, Brazil, comprising 646,356 physicochemical records from 141 stations. Five legislative milestones are evaluated, from the 1997 National Water Resources Policy to the 2020 Legal Sanitation Framework. A nine-parameter water quality index was computed over the full record, with the 2012 microbiological-indicator transition handled by continuity to avoid a spurious discontinuity. Seasonal–trend decomposition revealed heterogeneous rather than uniform trajectories: five basins show negative trend slopes and five positive (− 0.67 to + 0.42 index units per year), with the metropolitan Tietê corridor degrading while reservoir and peri-urban basins improve. Changepoint analysis identified structural breaks temporally proximate to legislative milestones, but only the creation of the National Water Agency (2000) shows a robust degradation-leaning signal; two milestones are statistically indistinguishable from null, and one window is confounded by the index transition and reported as non-interpretable. Spatial autocorrelation is positive and significant throughout but non-monotonic, peaking in the 2012–2019 period. Copula analysis showed that joint pollutant exceedance probabilities exceed univariate expectations, and machine-learning forecasts project divergent basin trajectories to 2030. Because the framework establishes temporal coincidence rather than causation, it identifies where change accompanied legislation without attributing that change to legislation alone.