Modeling the hydrologic response of the Meghna basin Under cmip6 climate projections

Abstract

Global climate change is brought on by an increase in atmospheric greenhouse gases and human actions such as altering land usage, producing industrial effluents, and other things associated with societal development. As a direct result of climate change, a number of different alterations are taking place in different hydrologic regions all around the world. The Upper Meghna River Basin (UMRB), being a component of the Ganges-Brahmaputra-Meghna basin, is likely to experience an alteration of its hydrological cycle due to the rise in the regional average temperature. It is anticipated that the effects of climate change would be more severe for the Meghna Basin compared to the other two adjacentbasins, the Brahmaputra and the Ganges.Therefore, this study is designed to conduct future climate change analysis and assess the impact of this change on the hydrologic response of the Upper Meghna River Basin (UMRB). In order to address the above issues, this study first sets up, calibrate and validate the Soil and Water Assessment Tool (SWAT) for the three major sub-basins (Barak, Meghalaya, Tripura) of UMRB. In this study, observed climate data (precipitation, temperature, wind speed, humidity, radiation) were collected from Indian Meteorological Department (IMD) and WFDE5. WFDE5 is a new meteorological forcing dataset for land surface and hydrological models based on the ERA5 reanalysis. Moreover, 13 global climate model (GCM)datasets of CMIP6 under two emission scenarios (SSP2-4.5 and SSP5-8.5) were used as projected climate data. Initially, future climate change in the Meghna River basin was assessed by estimating 12 climate extreme indices among the 27 indices recommended by Expert Team on Climate Change Detection and Indices (ETCCDI). From the analysis, it was found that most of the indices (both temperature and precipitation indices) showed positive trend as the timeline progressed but only few of them were significant (p-value<0.05). The analysis of future precipitation revealed that the amount of annual rainfall would increase by 16-20% and 41-54% by the year 2100compared to the baseline period (1985-2014) under the SSP2-4.5 and SSP5-8.5 scenarios, respectively, over the UMRB. The probability distribution of number of very wet days (R95p) and extremely wet days(R99p) showed significant rightward movement indicating an increase in heavy rainfall daysin the future. In addition, RX1dayand RX5day (1-day and 5-day maximum rainfall, respectively) are also projected to be increased and the most affected period will be pre-monsoon period and monsoon period. Analysis ofmonthly maxima of daily maximum temperature (TXx) and minimum of daily minimum temperature (TNn) revealed that the daily maximum and minimum temperature would get raised in all months. In general, the trends and shifting of these indices strongly indicate that the future climate in the Meghna River basin would be much warmer and wetter. For the three sub-basins with unique characteristics, separate SWAT hydrologic models weredeveloped. From the analysis different sets of parameters were found to be sensitive for different models. Moreover, the SWAT models developed for the Meghalaya and the Tripura sub-basins were calibrated at both single and multiple locations to identify whether multi-site modeling can provide better model performance.But no definite conclusion could be drawn from the results. For the Tripura basin single-site model performed well while for the Meghalaya basin multi-site (Meghalaya-02) model performed well. The calibrated SWAT models that gave the best results for each basin were used later to estimate future flows using future climate data (precipitation, temperature) of 13 GCMs from CMIP6 under two emission scenarios, SSP2-4.5 and SSP5-8.5. Here, the time periodfrom 1985 to 2014 was selected asbaseline period and the future timeline was split into three slices: near future (2026-2050), mid-future (2051-2075) and far-future (2076-2100). The results revealed that the annual flow volume will keep increasing gradually with time under both emission scenarios. From the basin-wise results, it was found that the outlets of the Meghalaya sub-basins were the ones with the most increased flows in the upcoming decades compared to the other two sub-basins. Results revealed that dry season flows (increased by 37%) would be less affected compared to the wet periods (increased by 59%). The probability distribution of future flows also tended to shift rightwards indicating that floods will be more frequent in the coming decades. Monthly assessment of future flow disclosed that April, May and June will be the most affected months due to climate change. By the end of this century, the 100-year design flow is projected to increase by 16% (40%), 13-16%(41-44%) and 20-28%(46-58%) in the Barak, Tripura Meghalaya sub-basin, respectively under SSP2-4.5 (SSP5-8.5) emission scenario.