Stretching Water Availability: Hydrosolidarity in Drought-Prone Regions

In Northeast Brazil, rainwater-harvesting cisterns implemented through the One Million Cisterns Program (P1MC) aim to secure dry-season water supply for low-income households. In the Forquilha Valley, where 88% of households own cisterns, unequal access persists for those without them. In practice, however, cisterns are used year-round and frequently support neighboring households through hydrosolidarity, diverging from program assumptions. This study combines monitored cistern data, household surveys, and rainfall records to assess water management practices. Scenario-based reconstructions were developed to evaluate cistern performance and hydrosolidarity potential during drought across different rainfall capture areas and consumption modalities. Results show predominantly conservative consumption, with a daily rate below 75% of P1MC recommendations. Although sharing occurred on fewer days, it accounted for the majority of extracted volumes, reaching up to 69% annually. Scenario results indicate that cistern performance is primarily controlled by rainfall capture area: mean storage increased from 3% (12 m2) to 46% (40 m2) and 81% (77 m2). Under adaptive conditions, dryness reduction was limited for small capture areas (by 3%) and higher for both intermediate (8%) and large areas (10%). These findings highlight that while capture area governs reliability, sharing and adaptive practices shape cistern water availability under drought, providing grounded socio-hydrological insights.