Quantifying Streamflow Alteration in a River Basin of South-Western India

Climate change and anthropogenic activities have significantly altered the natural flow regimes, leading to hydrological instability and exacerbating extreme events in different parts of the world. In this study, long-term changes in the hydrological regime of the Chaliyar River Basin were evaluated using daily streamflow data from 1986 to 2022. For this purpose, the Indicators of Hydrologic Alteration (IHA), quantile-based environmental flow metrics, and Flow Duration Curves (FDCs) were employed to quantify flow patterns in the river basin. The range of variability approach indicated that 42.4% of the hydrologic parameters fall into the high-alteration category. The mean river discharge increased by 105% to 438% from January to May during the study period, indicating a pronounced shift in flow during the dry and pre-monsoon seasons. The analysis of low-flow characteristics revealed the strongest response with a 440% increase in the 1-day minimum discharge and a 448% rise in the Baseflow Index, signifying a substantial baseflow contribution. On the other hand, the flow regime became more unstable, as evidenced by a 155% increase in the frequency of high-flow pulses and a 193% increase in the frequency of low-flow pulses. The quantile analysis underpins this transition, indicating a significant increase in the normal low-flow discharge (Q90–Q95), whereas the intensity of the most substantial floods (Q1–Q10) remained relatively stable. These findings provide a quantitative roadmap for evidence-based river management, which can enable policymakers to address the challenges of intensified flow variability in the basin under changing environmental conditions.