Hydraulic response of a river reach supplying urban water under extreme drought conditions

Study region Pirapó River Basin in Paraná State, southern Brazil, is a surface-water-dependent system that supplies the city of Maringá through a 90 km river reach from headwaters to main intake. Study focus This study investigates how hydrological drought propagates into hydraulic conditions along a river channel and affects water availability for urban supply. A one-dimensional hydraulic model (HEC-RAS) was developed using channel geometry derived from DEM correction and field bathymetric surveys. Model calibration was performed using Manning’s roughness coefficients and evaluated using deterministic metrics, event-based low-flow statistics, and residual-based uncertainty analysis. Historical drought scenarios (2000–2020) were simulated to assess low-flow hydraulic behavior along the river. New hydrological insights for the region Results show that drought conditions do not translate uniformly along the river but instead generate strong spatial heterogeneity in hydraulic response. During severe drought events, more than 45% of the 90 km reach exhibits discharges below 2 m³ s⁻¹, with upstream and midstream sections experiencing critical flow reductions, while downstream reaches partially recover due to cumulative inflows. These findings demonstrate that basin-scale discharge indicators alone are insufficient to assess water supply reliability, as hydraulic conditions controlling abstraction are spatially variable. Despite uncertainties in groundwater contributions and withdrawals, the main hydraulic patterns remain robust. Results highlight the importance of integrating reach-scale hydraulic modeling into water resource management to support drought resilience planning.