Environmental flow assessment in the Chirchik River Basin using multi-method hydrological indicators

Environmental flow assessment is increasingly important for maintaining riverine ecosystems amid climate variability, reservoir regulation, and rising water demand. However, information on environmental flow requirements remains limited for regulated river basins in Central Asia, including the Chirchik River Basin in Uzbekistan. This study aims to estimate and compare environmental flow requirements at three hydrological stations—Chinaz, Ugam, and G’azalkent—using long-term monthly discharge data from 1971 to 2020. Seven hydrological methods were applied: Tennant, Tessman, average annual flow, flow duration curve, dynamic mean flow, median flow, and flushing-flow assessment. Trend analysis was conducted using the Mann–Kendall test to examine station-wise environmental flow gradients across the basin. The results showed substantial differences among the stations: mean annual flows were 80.4, 23.1, and 227.7 m3/s at Chinaz, Ugam, and G’azalkent, respectively. The Tessman method yielded seasonal environmental flow ranges of 30.4–63.3 m3/s, 8.6–22.8 m3/s, and 91.1–147.1 m3/s, respectively. The findings indicate that fixed-percentage methods underestimate seasonal ecological requirements, especially in reaches affected by snowmelt and flow regulation. Seasonal and station-specific environmental flow rules are recommended to support sustainable basin management.

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