<p>Water availability is widely used to assess water resources and climate impacts, but its interpretation often depends on model assumptions. In regulated basins, it reflects political decisions (management objectives, demand representation, reliability criteria, and environmental constraints) more than intrinsic hydrological properties. This study offers a policy-oriented interpretation of water availability in Mediterranean basins using the WAAPA (Water Availability and Adaptation Policy Analysis) model. A coherent framework tests management hypotheses that include reservoir coordination, demand modulation, guarantee parameters, and the implementation of ecological flows. Results show that variations in management assumptions produce changes in water availability comparable in magnitude to those induced by climate change. By explicitly quantifying these relative contributions, the study demonstrates that water availability is a policy-dependent indicator, not an intrinsic hydrological property. These findings highlight the need for transparent definitions and consistent assumptions when using water availability metrics for planning climate change adaptation in highly regulated water-scarce regions.</p>