<![CDATA[Reliable streamflow is a critical indicator of water availability and hydrological resilience in tropical watersheds under increasing land-use pressure. This study quantified reliable low-flow conditions (Q95) and characterized hydrological regime dominance in the Upper Wampu Watershed, Karo Regency, Indonesia, using daily and monthly streamflow data from 2017–2024 analyzed through Flow Duration Curves (FDC), low-flow indices, and baseflow separation supported by SWAT model simulations. Mean annual discharge ranged from 45.1 to 66.4 m³/s, increasing by about 8.7% between 2017–2020 and 2021–2024. Overall dependable flow (Q95) was estimated at 19.27 m³/s, while monthly Q95 values ranged from 10.62 to 38.50 m³/s, reflecting strong seasonal dependence on rainfall. Baseflow Q95 values were comparatively stable (21.47–41.59 m³/s), yet the quickflow-to-baseflow ratio (Qq/Qb) exceeded 0.8 during wet months, indicating a runoff-dominated regime. Water balance analysis showed that surface runoff contributed more strongly to water yield than percolation, suggesting limited groundwater buffering capacity. SWAT model performance improved from calibration (PBIAS of -31.3% to -15.4%) to validation (+1.7%), supporting its reliability for hydrological regime assessment. These findings indicate that dependable water availability in the Upper Wampu Watershed is highly seasonal and rainfall-driven, with limited subsurface storage to buffer dry-season conditions. Strengthening infiltration through land conservation and sustainable watershed management is therefore essential to reduce hydrological vulnerability, particularly during dry seasons and intense rainfall events, in this and other environmentally stressed tropical mountainous watersheds.]]>
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