Impact of climate change on future flow of Brahmaputra river basin using swat model

Abstract

Brahmaputra River Basin(BRB) is one of the largest basins among Ganges-Brahmaputra-Meghna (GBM) river system carrying enormous volume of water through Bangladesh. The response of this basin due to climate changes is one of the key issues to be investigated due to its socio-economic and environmental vulnerability. A semi-distributed hydrological model of the BRB has been developed using the Soil Water Assessment Tool (SWAT). It has been calibrated and validated for the streamflow measured at the Bahadurabad station for the climate normal period (1981 to 2010).

Synthetic approach of climate change modeling has been applied to assess the changes in water availability due to future potential changes in temperature and precipitation in BRB. Twenty hypothetical climate change scenarios (perturbed temperatures and precipitation: precipitationfrom -20% of climate normal period to +40% at 10% interval and temperature change of 0◦C to 6◦C at 2◦C interval) were applied to the calibrated and validated model in order to investigate the sensitivity of BRB mean annual and mean seasonal streamflow under the impact of climate change in the 21st century. The results revealed that the changes in annual streamflow due to changes in precipitation and temperature are linear. It appears that with respect to the climate normal (1981-2010), the changes in average annual streamflow (keeping temperature unchanged) are ± 12.83% per ±10% change in precipitation. In contrast, streamflow response to the increase in temperature (keeping precipitation unchanged) is -2.49% per ◦C.

The calibrated hydrological model of BRB then has been used to assess the impact of climate change on water availability of BRB by applying different climate change scenarios of selected General Circulation Models (GCM). The selection of GCM was based on the Representative Concentration Pathways (RCPs) scenarios of eight Intergovernmental Panel on Climate Change (IPCC) GCMs for the 21st century. Six climate change scenarios, viz. as warmest, coolest, driest, wettest, moderate warm and moderate wet were selected based on the projected precipitation and temperature of the 21st century obtained from four RCPs (RCP 2.6, RCP 4.5, RCP 6.0 and RCP 8.5) of eight GCMs (BCC-CSM1.1, BCC-CSM1.1(m), GISS-E2-H, GISS-E2-R, Had-GEM2-ES, MIROC-ESM, MIROC-ESM-CHEM, MRI-CGCM3). From the analysis of temperature and precipitation data BCC-CSM1.1 RCP 8.5, HadGEM2-ES RCP 8.5, MIROC-ESM-CHEM RCP 8.5, GISS-E2-R RCP 2.6, MRI-CGCM3 RCP 6.0 and GISS-E2-H RCP 4.5 were selected as the wettest, driest, warmest, coolest, moderate wet and moderate warm scenario, respectively. The high resolution spatial distribution of temperature and precipitation of these GCMs were obtained using the pattern scaling technique and were further applied to the SWAT hydrological model.

SWAT simulated mean annual, mean dry period (December to May), and mean wet period (June to November) seasonal streamflow of BRB for the 2010-2039 (2020s), 2040-2069 (2050s), and 2070-2099 (2080s) of the 21st centurywere compared with the corresponding climate normal (1981-2010) streamflow. In general, BRB projected an increase in the mean annual streamflow for 21st century under the climate projections for almost all the six scenarios considered in this study. However, GISS-E2-R RCP2.6 (coolest) and HadGem2-ES RCP8.5 (driest) projected decrease in annual average flow at Bahadurabad station during 2080s and 2020s respectively. The maximum projected increase in mean annual streamflow found are 15.019%, 32.457% and 47.436% for MIROC-ESM-CHEM RCP8.5 (warmest), GISS-E2-H RCP4.5 (moderate warm) and BCC-CSM1.1 RCP8.5 (wettest) in 2020s, 2050s and 2080s respectively. The minimum projected change in streamflow found are -3.290%, 1.800% and -0.908% for HadGEM2-ES RCP8.5 (driest), GISS-E2-R RCP2.6 (coolest) and GISS-E2-R RCP2.6 (coolest) in 2020s, 2050s and 2080s respectively. On average, at the end of 21st century (2080s), the mean dry and wet period streamflow of BRB is projected to increase by about 177.93% and 11% of their mean dry and wet period discharge in climate normal period, respectively. In 2080s, maximum increases in dry and wet period flow were found for GISS-E2-H RCP 4.5 (moderate warming) and BCC-CSM1.1 RCP8.5 (wettest), respectively. Lowest dry and wet period flows were found for MRI-CGCM3 RCP6.0 (moderate wet) and GISS-E2-R RCP2.6 (coolest) scenarios, respectively.