Quantifying unregistered groundwater via census-registry integration for Chile

Sustainable groundwater governance requires accurate extraction accounting, yet unregistered domestic wells create persistent blind spots worldwide. Chile exemplifies this challenge: its 1981 Water Code established market-oriented water rights while exempting domestic wells from registration. This study develops an integrated framework quantifying discrepancies between registered entitlements and unregistered household extraction by combining spatially audited national water rights records with geocoded census microdata (2017, 2024), alongside piezometric analysis from 474 monitoring wells.Results revealed approximately 200 000 unregistered extraction points, indicating that approximately 76% of domestic groundwater withdrawal occurs outside formal visibility. Well dependence increased 12.3% between census periods despite network expansion. After climate correction, 86.7% of monitored wells exhibited declining trends significantly correlated with discrepancy metrics. Ensemble projections indicate high-discrepancy alluvial systems face potential exhaustion by 2030. This census-registry integration offers replicable methodology for bridging administrative records with physical aquifer conditions, supporting SDG 6 monitoring across regions with substantial unregistered groundwater use.