Climate change impact assessment on the khowai river flow using HBV model

Abstract

Water is one of the most indispensable resources on earth which is potentially useful for all living things. Consequential impacts of climate change are mediated through the changes in the water or hydrologic cycle due to the close connection between the climate and the atmosphere, hydrosphere, land surface and biosphere. Hydrologic models are being used in understanding the behavior of hydrologic systems of a watershed to make better predictions and to address the major challenges induced by climate change.

Khowai is one of the trans-boundary rivers that plays an important role in the fields of irrigation, transportation and flood events of the north-eastern region of Bangladesh. It also contributes to the flash flood hazard in this region damaging agricultural products of large areas. Anticipated climate change may exacerbate the current situation as climate change will have a profound impact on the availability and variability of fresh water throughout the world due to a change in rainfall pattern in response to the global warming. This response may vary from region to region. Hence assessing the impact of climate change on the streamflow of the Khowai river basin is very important for sustainable water resources management in this region.

In this study, HBV (Hydrologiska Byrans Vattenavdelning) hydrologic model has been set, calibrated and validated to assess the future flow of Khowai river and the suitability of two gridded datasets has been assessed in this regard. To test the datasets, three models have been set using three different datasets. Each model has been calibrated for 1980 to 2010 and validated for 2011 to 2019. In terms of Nash-Sutcliffe efficiency (NSE), ERA5 data driven model (NSE = 0.50 and 0.21) shows better result than NOAA data driven model (NSE = 0.49 and 0.19) in both calibration and validation, respectively. But, in the validation period (2011-2019), both ERA5 and NOAA show unsatisfactory results than the calibration results. For this reason, another model has been set using the average rainfall from the global gridded products (NOAA, ERA5) and measured precipitation of Habiganj station. This model shows better results for both calibration and validation periods and has been used for simulating future flow. For the calibration and validation period R2 have been found to be 0.78 and 0.75 while NSE have been found to be 0.75 and 0.67, respectively. Future precipitation, temperature and evaporation from the CANESM2 (The second-generation Canadian Earth System Model) climate model under RCP8.5 scenario and bias corrected by Quantile mapping method have been used to simulate future flow. Projections show an increase in precipitation by 7.69, 15.86 and 21.05%, in 2020-2040 (2020s), 2041-2070 (2050s) and 2071-2099 (2080s), respectively, from the baseline period (1980-2019) whereas temperature is expected to increase by 0.82, 1.46 and 2.41oC, in those periods, respectively. Evaporation is projected to increase in all future periods while actual evapotranspiration is projected to decrease as warmer environment may decrease vegetation coverage and transpiration due to water stress in the soil system which will reduce the overall actual ET in future.

The future (2020 to 2099) flow of the Khowai river has been simulated by the calibrated and validated model driven by bias corrected CANESM2 climate datasets. Analysis of monthly flow data indicates that wet months will be wetter while some dry months such as January, February and March will be drier with respect to the base period. Flow may decrease up to 61% (during February of 2020) while wet season flow may increase up to 50% (during July-August of 2070). Flow duration curve also indicates that at 1%-time, discharge may equal or exceed 117, 163, 184 and 192 m3/sec during the base period (1980-2019), 2020s, 2050s and 2080s, respectively. Frequency analysis shows that discharge corresponding to 100-year return period may increase by 13, 25 and 48% during the 2020s, 2050s and 2080s, respectively.

This study may play a vital role in assessing the future flow of the Khowai river basin for the proper planning and water resources management in this basin. Furthermore, as Khowai river is a transboundary river, the results of this study can also be used for the development of an effective water sharing policy which will ensure that both countries (India and Bangladesh) can get enough water for the purpose of agriculture, livelihood, economic growth, and maintenance of ecology and biodiversity in this region in the coming decades.