Effect of climate and land use changes on sediment load for Bbrahmaputra river basin using HEC-HMS

Abstract

Brahmaputra River Basin (BRB), the largest contributor of sediment load in Ganges-Brahmaputra-Meghna (GBM) delta, is highly vulnerable to future climate change. The present to future assessment of sediment load from BRB is challenging due to unavailability of measured data and uncertainty in climatic parameters. The Brahmaputra river and its’ nature together with eco-hydrology and livelihood of people surrounding it, are highly dependent on incoming sediment load. Many researchers give assessment of climate changes of BRB on flow, but assessment on sediment load due to future climatic scenarios is unavailable. The present study focuses on developing a hydrological and sediment routing model of BRB using HEC-HMS to estimate present to future sediment load considering climate and land use change. The catchment and river network of BRB have been delineated using SRTM 90m DEM. Soil Moisture Accounting for computing runoff volume, Snyder unit hydrograph for direct runoff, linear reservoir for baseflow and Muskingum Cunge for routing have been considered for representing hydrological processes of BRB. Land cover map from 0.5 km MODIS-based Global Land Cover analysis and soil map from FAO have been used to estimate impervious area, maximum infiltration rate, surface storage and MUSLE factors. Using climatic data from NASA POWER and sensitivity analysis of SMA algorithm parameters, this hydrological model has been calibrated from 1983 to 1996 and validated for 1997-2010 at Bahadurabad transit (SW 46.9L). The NSE, PBIAS, RSR, R2 values are 0.65, -20.92, 0.59, 0.76 and 0.54, -23.40, 0.68, 0.71 for calibration and validation period respectively. Using MUSLE and Engelund Hansen equation and obtaining the boundary conditions from calibrated hydrological model and secondary literature, surface erosion and sediment routing model has been calibrated and validated for observed sediment load of 1983-2010 collected from BWDB. The simulated annual average sediment load at Bahadurabad transit from calibrated and validated HEC-HMS model is about 370.20 Mt/year. For future climate changes, synthetic approach for changing precipitation about -25% to 40% have been performed. The result shows that BRB is almost equally sensitive to a reduction and increase in precipitation for discharge and sediment load. For -25% to 40% changes of baseline precipitation in future, average annual sediment load varies from -31.34% to 41.92% of iii climatic base period values. For 40% increase of precipitation in future, annual sediment load from BRB will increase up to 525.38 Mt/year. The calibrated surface erosion and sediment routing model of BRB then has been used to assess the impact of climate change on sediment load together with water availability of BRB by applying RCP 8.5 scenario of selected RCMs outputs obtained from CORDEX database. HEC-HMS simulated mean annual and seasonal (pre-monsoon, monsoon, post-monsoon and dry) streamflow of BRB for the 2010-2039 (2020s), 2040-2069 (2050s), and 2070-2099 (2080s) of the 21st century was compared with the corresponding climate normal streamflow. A gradual increase in annual average flow is found from 2020s to 2080s which indicates that uncertainty associated with the projected streamflow of BRB increases as we go distant future. For impact analysis on sediment load at Bahadurabad transit of BRB, mean annual and mean seasonal (pre-monsoon, monsoon, post-monsoon and dry) simulated sediment load have been assessed. It reveals that though the uncertainty in percentage change in seasonal sediment load of pre-monsoon is higher than the monsoon season, the contribution of sediment load of pre-monsoon is very much lower than the monsoon season. So, increase in sediment load for monsoon season will have much effect on changes in total sediment load at Bahadurabad transit of BRB. The percentage changes in average annual sediment load from baseline period for 2020s, 2050s and 2080s are 34%, 67% and 115% respectively and it increases gradually. Also, uncertainty associated with the projected sediment load of BRB increases as we go distant future can be stated due to growing nature of range between 75th and 25th percentile from 2020s to 2080s. An increase in urbanization area, about 5% and 15% for all over BRB, have been done to analyze the changes of sediment load. The average monthly, seasonal and annual sediment load coming from basin area decreases almost linearly with respective increase in urbanization. But, at Bahadurabad transit (SW 46.9L), these values remain almost same as the present condition. It reveals that the decrease amount of sediment load from the basin area has been adjusted with the increased volume of sediment load carried by the increased amount of overland flow due to urbanization.