Environmental flow assessment in a tropical karst catchment using hydrological methods

Rivers in tropical karst regions often face flooding and dry-season droughts, challenging their ability to sustain discharge and ecological integrity. This research evaluates the environmental flow (EF) in tropical karst catchment using a hydrological approach, with the Upper Bengawan Solo as the case study. We analyzed 15 years (2009–2023) of AWLR Ngadipiro data and applied the Sacramento model. Sensitivity analysis identified LZPK and LZFPM as the key parameters influencing low-flow conditions. This study evaluated Q99, Q95, Q90, 7Q10, 7Q2, and 3Q2 as key hydrological indices for EF determination. The Sacramento model demonstrated good performance, achieving R² = 0.654, NSE = 0.673, RSR = 0.573, PBIAS = 6.040, and KGE = 0.760 with a MoE of ± 0.167 m³/s at a 95 % confidence level. The findings show that the assessed hydrological indices varied within a limited range. Significantly, Q95 results closely mirrored 3Q2 values obtained by the Gumbel minimum distribution (Gumbelmin). At AWLR Ngadipiro, Q95 was 0.73 m³/s, while 3Q2 Gumbelmin was 0.75 m³/s. In the Sacramento model, Q95 was 0.67 m³/s, whereas 3Q2 Gumbelmin was 0.69 m³/s. These results confirm the consistency of representing low-flow conditions, and both can serve as reliable alternatives for EF in tropical karst catchment. The methodology is cost-effective, data-efficient, and yields reliable EF assessment. Furthermore, it offers replicable guidelines for tropical karst river basins calibrated using local data to assist policymakers in aligning ecological integrity with water resource needs. © 2025