Impacts of climate change on river hydrology and energy economics in Budhigandaki river basin

Abstract

Water management has become a challenging task due to the increasing population, rapid urbanization and industrialization. Availability of observed hydro-meteorological data plays a crucial role in water budgeting for the country like Nepal that relies heavily on hydroelectricity for its energy needs. Quantification of available water at the local scale and examining how it is impacted by climate change (CC) is extremely important from the water management perspective at the river basin level. Budhigandaki River Basin (BRB) of Nepal, was chosen for this study in assessing climate change impact on river hydrology utilizing well calibrated and validated Soil Water Assessment Tool (SWAT), consequent impact on hydroelectric energy generation. Extending this aspect further, the micro-economic assessment of the Budhigandaki hydroelectric project was also made in this study. This study assessed the interannual variability of hydroclimatic condition of the BRB using daily hydrological and meteorological data for the period from 1983 to 2012. To evaluate the impact of climate change on hydrological phenomenon in the study basin, future climate data under two Representative Concentration Pathways (RCP 4.5 and RCP 8.5) with four climatic conditions (cold-wet, warm-wet, warm-dry and cold-dry) for each RCP were considered. Digital elevation model (DEM) data, land use and land cover, and soil data of the basin are the spatial data required in the hydrological simulation that were utilized. The climate change impact on flow, hydroelectric energy generation and energy economics were evaluated by comparing these variables with the baseline. Historical data shows that there is a very high variability in daily, monthly, seasonal and interannual flow in the study basin. Future annual precipitation in BRB varies significantly and is projected from -9% to 23% for RCP 4.5 and -11% to 21% for RCP 8.5 scenarios compared to the baseline value (1530 mm). The mean annual temperature increases 1.7 o C for RCP 4.5 and 3.9 o C for RCP 8.5 by the end of this century. SWAT model was calibrated and validated at Arughat gauging station considering 30 years daily flow data. Model evaluation using four statistical parameters (NSE, PBIAS, RSR and KGE) showed that the developed model performed very well to simulate river flow. Additional validation of the model done at three supplementary points; two in the upstream and one in the downstream of calibration point (Arughat) also showed that vi the developed SWAT model for BRB is well calibrated. To compare the performance of flow simulation methods in the basin level, a new evaluation statistical index, the Global Performance Index (GPI), was introduced in this study. SWAT hydrological model preformed the best among the different methods considered for flow estimation as evaluated by GPI in BRB. Annual mean flows are projected to increase in the future scenarios; 10 to 31% in RCP 4.5 and 5 to 57% in RCP 8.5 scenarios with respect to the baseline flow of 240 m /s. The analysis of future extreme flow shows an increasing trend in case of annual maximum one-day flow and a decreasing trend in low flow case. These results indicate a need to alter the design of hydraulic structures and selection of storage project over runoff-river project for climate resilience. Future annual energy of the Budhigandaki Hydroelectric Project is expected to increase by 9 to 13% compared to the baseline value (3385 GWh) that is equivalent to annual revenue of 20 to 28 million USD. Results of this study show that storage hydroelectric projects with the provision of flexible operating rules are desirable. Financial policies related to hydroelectricity need to be revised with the changes in the future climatic conditions. The findings of this study are expected to be useful for hydrologists, economists and decision-makers to plan the use of available water judiciously in the future. Keywords: Climate change, river hydrology, hydroelectricity, energy economics, GPI, SWAT, Budhigandaki