HYDROLOGICAL MODELLING OF XEBANGFAI RIVER BASIN IN LAO PDR USING SWAT MODEL

Abstract

In this study ArcSWAT model has been used to simulate runoff and to evaluate the Best Management Practices (BMPs) from XeBangfai river basin. ArcSWAT is a long term, physically based; watershed model developed to quantify the impact of land management practices on water, sediment and agricultural chemical yields in complex watersheds and is developed by Agricultural Research Service of United States, Department of Agriculture. The present study is undertaken with an aim to test the performance of SWAT model on XeBangfai River located in Center part Lao PDR draining an area of 10064 km2. For model application, the catchment area is divided into 9 sub-watersheds. The whole catchment area is classified into seven land use categories. Available hydrological data 1997-2008, split into two groups is used for calibrating and validating parameters of the model. The calibrated model parameters are also used to test the performance of the model at upstream intermediate gauging site at Mahaxay station downstream at XeBangfai Br13 KM station. Model performance is adjudged base on visual comparison of observed and model simulated as well as on statistical measures of mass balance error, coefficient of determination (R2), Nash-Sutcliffe efficiency (ENS), Index of agreement ( d) and Relative Error (RE) Comparative analysis of results is done on daily and monthly time scale. Analysis reveals that during calibration period, Value of the coefficient of determination (R2) before calibration and after calibration of the SWAT model was found to be 0.376 and 0.754 for the Mahaxay gauging site. Similarly, the value of coefficient of determination (R2) before calibration and after calibration of the SWAT model was found to be 0.459 and 0.747 for the XeBangfai Br13 KM gauging site, both gauging site on daily. Annual average discharge at the Mahaxay gauging site was found to be 220.3 m3/s. The runoff depth is about 1,650 mm, higher than the average annual rainfall at mouth. The monthly discharge is highest in the month of August followed by September and July. Value of the coefficient of determination (R2) of 0.754 (daily simulation) and 0.948 (monthly simulation), Nash-Sutcliff efficiency (NSE) of 0.754 (daily simulation) and 0.942 (monthly simulation), index of agreement (d) of 0.924 (daily simulation) and 0.998 (monthly iii simulation) and Relative Error (RE) of - 0.023 (daily simulation) and 0.023 (monthly simulation) indicates satisfactory calibration of the ArcSWAT model. The average annual flow at the XeBangfai Brl3 KM gauging site is estimated at 441.2m3/s. Value of coefficient of determination (R2) of 0.747 (daily simulation) and 0.876 (monthly simulation), Nash-Sutcliff efficiency (NSE) of 0.737 (daily simulation) and 0.839 (monthly simulation), index of agreement (d) of 0.912 (daily simulation) and 0.946 (monthly simulation) and Relative Error (RE) of -0.113 (daily simulation) and 0.112 (monthly simulation) indicates satisfactory calibration of the ArcSWAT model. For the Mahaxay gauging site for model validation period (2003-2008), Value of coefficient of determination (R2) of 0.750 (daily simulation) and 0.945 (monthly simulation), Nash-Sutcliff efficiency (NSE) of 0.732 (daily simulation) and 0.908 (monthly simulation), index of agreement (d) of 0.907 (daily simulation) and 0.971 (monthly simulation) and Relative Error (RE) of -0.133 (daily simulation) and 0.132 (monthly simulation) indicates satisfactory validation of the ArcSWAT model. For the XeBangfai Br13 KM site for model validation period (2003-2008), Value of coefficient of determination (R2) of 0.740 (daily simulation) and 0.875 (monthly simulation), Nash-Sutcliff efficiency (NSE) of 0.702 (daily simulation) and 0.851 (monthly simulation), index of agreement (d) of 0.902 (daily simulation) and 0.952 (monthly simulation) and Relative Error (RE) of -0.082 (daily simulation) and 0.083 (monthly simulation) indicates satisfactory validation of the SWAT model. From the ArcSWAT modeling it can be concluded that the discharge of the XeBangfai Watershed is 442.1m3/sec. From December 2009 onward the discharge of the XeBangfai watershed will be augmented with the flow of Nam Theun2 River via the regulating dam and downstream channel of the hydro-power plant and this will increase the average discharge of the XeBangfai by 220 m3/s. The Best Management Practices of XeBangfai watershed was done by SWAT model, the BMPs indicated decreased runoff, percent runoff decreased iv due to influence of each crops such as irrigation rice 1.954%, soybean 0.720% and mung bean 0.090%. Sediment was increased from 4.83 to 9.37 t/ha after irrigation in the basin, whereas sediment was decreased again to 5.36 tlha after applies Best Management Practices. Thus, based on visual and statistical evaluation, it can be concluded that the SWAT model can be used with confidence to simulate runoff response on daily, monthly and annual basis and to evaluate the Best Management Practices in XeBangfai river basin in LAO PDR