Impacts of climate change and upstream intervention on the hydrology of the Meghna river basin using SWAT

Abstract

The Meghna river basin is expected to suffer from the adverse impacts of climate change according to the IPCC 5thassessment report and other hydrological studies. Previous studies and models indicated that the Meghna river basin area may experience frequent floods, high variability in rainfall patterns, and an increase in surface temperature which will disrupt the haor ecosystem. Moreover, any human-made intervention (dam/barrage) in the upstream part of the basin, may pose a great threat to the downstream country like Bangladesh.The present study developed a semi-distributed hydrological model for the Meghna river basin using SWAT to simulate the impact of changing climate and upstream intervention on the hydrologic cycle of the basin area. The model was setup using the topographic data, land cover data, soil property, and meteorological data. The simulated and observed hydrographs of the dailydischarge showed a good agreement during calibration (2000-2008) and validation (2009-2018) and the results are NSE: 0.60, PBIAS: 23.55, RSR: 0.39, R2:0.71,and NSE: 0.64, PBIAS: 19.19, RSR: 0.36, R2: 0.68 respectively.

Initially,projected future precipitation and temperature from four RCM model outputs were analyzed under RCP 4.5 and RCP 8.5. Maximum changes in rainfall (-14.49% to +26.16%)and in average temperature (+2.200C to +4.250C) were observed under the dry-cold and wet-warm scenario. Predicted maximumchanges (-250% to >400%)in mean monthly dischargeis observedunder wet-warm scenario (the 2080s, RCP 8.5). Whereas, the least changes in flow volumewas observed (-100% to +100%) under the dry-cold scenario (in 2050s, RCP 4.5). Analyses of seasonal flow variation show that river flow will decrease (-7% to >50%) in every season under both RCP scenariosif the dry-cold scenario prevails. Contributions from the lateral flow and percolation losses will be much higher in future.

The probable impact of an upstream intervention (reservoir)on the Surma-Kushiyara river system at Amalshid was simulated for the present state (1998-2018) and near future (2049-2069). Results reveal that peak flow at Amalshid could be greatly affected in the monsoon and post-monsoon (> -60%) and may increase in the dry season (> +200%).The study will assist researchers to understand the hydrological changes and to study further how the chnages will impact the nature and the wetlands of the basin area.