Effect in the water balance of the teesta river basin due to different climate changes and upstream development

Abstract

The Teesta River is one of the most important trans-boundary river of Bangladesh with only the Ganges and Brahmaputra have a higher annual runoff than the Teesta. Bangladesh has long argued that India’s construction of the Gazaldoba Barrage upstream of Dalia has significantly reduced the availability of water in the dry season. Furthermore, the release of water during the monsoon season causes flooding and bank erosion in downstream. The availability of water for irrigation, particularly in the lean or dry season, has been at the crux of the longstanding dispute between the two countries. The analysis of secondary flow data shows that annual maximum and annual mean flow at the Dalia point is on the decline. The annual minimum flow and minimum water level at Dalia shows very low water availability during lean period. The maximum water level shows an increasing trend meaning increasing intensity of floods. But increasing maximum water level with decreasing flow means that the Teesta Basin is experiencing siltation or aggradation. And with the impending threat of climate change, the water balance of the Teesta Basin is going to become a more important factor in an already water stressed region. With this issues in mind, a semi-distributed hydrological model of the Teesta basin has been developed using Soil Water Assessment Tool (SWAT). The model was calibrated and validated for the stream flow measured at Dalia, upstream of the Teesta barrage for 1999 up to 2013. Potential change in the water balance due to climate change and upstream development in the Teesta River Basin has been assessed with this developed SWAT hydrological model. The climate change scenarios have been developed based on the projection of couple of GCM models (HAD GEM2 ES and CSIRO MK 3.6.0) for all the RCP scenarios of IPCC AR-5, during the 2050s and 2080s. While the upstream control scenarios were prepared by withdrawing the stream flow at three different points (Gozoldoba, Teetsa III and Rammam III) and analyzing the impact of it further downstream. The downscaled GCM model data of temperature and precipitation for all the RCPs were analyzed at different points in and around the Teesta to find the driest, wettest, warmest and coolest scenario of the Teesta basin in 2080s. For the wettest scenario the precipitation had increased by 11.71% while it decreased by 1.76% for the driest scenario. The increase x in temperature for the coolest and the warmest scenario is found to be 2.24⁰C and 5.34⁰C. These scenarios were used as the input data for SWAT model to assess the changes in water balance due to climate change. The developed hydrological model of 1998-2013 timeframe also served as the base model output to be compared against climate change model results. Comparing the water balance of the climate change model with the base model, it was clearly evident that the monsoon season will become more wetter and the dry season become more drier due to climate change for all the climate change scenarios. The monsoon may see as much as 80% increase in surface runoff (wettest scenario) while the dry season might see a 70% decrease in surface runoff (driest scenario). The base flow is expected to decrease in the month of April, May and June while increase in temperature means the evapotranspiration is expected to increase all throughout the year. The outputs of water yield shows it might increase by more than 50% in the monsoon season and reduce by 30% for the dry seasons. The flow comparison at the Dalia point, upstream of Teesta Barrage for different climate change scenarios shows similar kind of trend to that of the water balance comparison. The general trend emerging from the flow analysis is that the Dalia point will experience a more severe shortage of water during the lean season even without further upstream controls. The impact of upstream development (control structures) by analyzing different flow diversion scenarios shows that Gozoldoba has a severe impact on water availability at Dalia. Being hydroelectric structures, the Teesta III and Rammam III stores the water during monsoon reducing the peak flow but releases the storage during lean season hence increases the lean season flow. A regression analysis was carried out to develop an equation to find the percent flow reduction at Dalia point due to different flow control scenarios. The equation can be used to assess the impact on Bangladesh side of the Teesta River flow due of upstream development (control structures) of the basin.