Evaluating SUDS Efficiency in Urban Environments: A Dual-Scale Methodology Applied to the City of Madrid

Although Sustainable Urban Drainage Systems (SUDS) are widely recognised as essential components of resilient urban water management, the large-scale planning and evaluation of such systems remain challenging. This study assesses the hydrological and economic performance of SUDS in Madrid (Spain) under the SSP1-2.6 and SSP5-8.5 climate scenarios, applying a dual-scale methodology based on the Curve Number (CN) model. At the catchment scale, SUDS show substantial potential for irrigation reuse, with runoff-rich catchments reaching hydrological saturation earlier (plateau at r ≈ 0.4) and runoff-limited catchments stabilising at higher implementation levels (plateau at r ≈ 0.6). At the parcel scale, partial-coverage configurations (50% irrigation coverage) outperform full-coverage solutions (100% irrigation coverage), achieving maximum retention levels of 70% in SSP1-2.6 and 50% in SSP5-8.5 while requiring less surface area (10–15%). From an economic perspective, positive net present values (NPVs), acceptable internal rates of return (IRRs), and feasible payback periods occur only at very low retention levels (r < 0.05), with financial performance declining rapidly as storage capacity increases.