STUDY OF IMPACT OF CLIMATE CHANGE ON WATER AVAILABILITY IN BAGMATI RIVER BASIN NEPAL

Abstract

Bagmati river basin is a mid-class river basin (of Nepal) originating from northern part of the capital city, Kathmandu of Nepal. It drains south to the plain area of Nepal called Terai and ultimately joins to the Koshi and then Ganges river at Indian territory. It is a transboundary river between Nepal and India also. The river has extreme diverse flow of water as flood water during rainy season and lean flow in non-monsoon season. The power generation potentiality of river is much more, being its head, in hilly regions as DEM shows vast level difference in the short reach and potentiality to command the large agricultural area being there in the Terai (plain) region for irrigation purpose in both banks. The river has water availability problem in month & season in a year as well as in a decade. The river has less water in non-monsoon season flow and excess flood water as shown by observed data. Due to impact of climate change in the study area, the observed trends are also changing in respective month & season. In this study, hydrological modeling of Bagmati river basin at upper catchment using physically based, spatially distributed, continuous model Soil and Water Assessment tool (SWAT) for assessment of Impact of Climate Change on Water Availability in the basin (area 2737.4 km2) has been envisaged. The key objective of this study is to develop a suitable model to represent hydrology of Bagmati river basin for calibration and validation of hydrological model (SWAT) to study the rainfall-runoff relationship, simulation of the daily flow using the hydrological model under various scenarios of climate change and water availability study in Bagmati river basin considering various climate change scenarios. SWAT model simulation was carried out for discharge on daily basis for 8 years (1998 to 2005). The SWAT model was calibrated and validated for the years 1998-2002 and 2003-2005 respectively, considering observed stream flow using SWAT-CUP model and sequential uncertainty fitting (SUFI2) technique. Calibration of the SWAT model was carried out using: (i) p-factor which is the percentage of data lying within the 95% prediction uncertainty, and (ii) r-factor, which is the ratio of the average thickness of the 95PPU band and the standard deviation of the observed value of discharge or sediment yield. Furthermore, Nash-Sutcliffe Efficiency (NSE), Coefficient of determination (R2), Percentage bias (PBIAS), and ratio of root mean square error to the standard deviation of measured data (RSR) were also used to evaluate the model performance. For daily flow, the results obtained from the model calibration and validation showed reliable estimate of daily stream flow (R2 = 0.75, NSE =0.74, PBIAS=17.28 and RSR=0.0) for calibration period. However, for the validation period, v model performance in PBIAS was low as compared to the calibration for period with parameter for flow (R2 =0.76, NSE =0.70, PBIAS=-35.37 and RSR=0.0). Based on the statistical results obtained from the SWAT simulation, it is seen that the performance of the SWAT model in the Bagmati river basin is very good. The climate data was downloaded from CORDEX for historic and two emission scenario, extracted NetCDF data into ASCII format and bias correction applied using CMhyd software for precipitation data and developed macro enabled excel sheets for correction of maximum and minimum temperature data. The linear bias correction method is applied to model downloaded climate data. The water balance study of the basin showed that 32.7 % of the average annual rainfall of the basin contribute to evapotranspiration. The annual volume of water availability at the basin outlet is 3.942 BCM. Further, the calibrated SWAT model was used for assessment of water availability through the flow simulation of different climate data of different climate change scenarios. The water availability in the basin has been assessed in 75%, 90% and 99% dependable flow available in different time steps like month, season, year, decade and full time series using daily generated flow considering daily operation. Future monsoon dependable flow of 75%, 90% and 99% in monthly, seasonal, decadal time series are decreasing and non-monsoon flows are increasing as compared to the base line flow in all scenario. It seems that temporal shifting of flow is going on in the future as compared to the observed historic flow. 3.942 BCM volume of water in baseline flow is available annually at the Pandheradovan outlet of the basin. The baseline average annual discharge available is 125 m3/s while for RCP4.5 and RCP8.5, available annual water volume are 3.377 & 3.40 BCM and annual discharge available are 107.11 m3/s & 107.81 m3/s respectively. The total water availability is decreasing in future as compared to the baseline and thus RCP4.5 impacts more than RCP8.5 with lower water availability. This study would be useful for assessment of possibility of storage type project in the basin in terms of water availability. This study will also help in integrated water resources management and sustainable development of the Bagmati river basin, Nepal.