Flow regime alteration due to anthropogenic and climatic changes in the Kangsabati River, India

According to the 'natural flow paradigm', any departure from the natural flow condition will alter the river ecosystem. Flow regimes have been modified by anthropogenic interventions and climate change is expected to cause additional impacts by altering precipitation extremes. This study aims to evaluate the observed hydrologic alteration caused by dam construction and simulate alteration due to expected climatic changes in a monsoon dominated mesoscale river basin in India. To analyze the natural flow regime, 15 years of observed streamflow (1950-1965) prior to dam construction is used. Future flow regime is simulated by a validated hydrological model Soil and Water Assessment Tool (SWAT), using four high resolution (∼25 km) Regional Climate Model (RCM) outputs for the near future (2021-2050) based on the SRES A1B scenario. Finally, to quantify the hydrological alterations of different flow characteristics, the Indicators of Hydrologic Alteration (IHA) method which is based on the Range of Variability approach is used. This approach enables the assessment of ecologically sensitive streamflow parameters for the pre- and post-impact periods. Results of our analysis indicate that flow variability in the river has been significantly reduced due to dam construction with high flows being reduced and low flows during non-monsoon months considerably enhanced. Streamflow simulated based on projected climatic changes reveals reduced monsoonal flows with marginal changes in non-monsoon streamflow. The combined effect will reduce flow variability, potentially affecting the ecosystem. We conclude that in such modified basins, adaptive river basin management will be necessary to maintain such an extreme river flow regime for the long term viability of riverine ecosystems.