6. Conclusions and Recommendations
6.1. Conclusions
In this study, a conceptual, distributed parameter, continuous time, river basin model, SWAT2012 was used to simulate runoff and sediment from Kulekhani watershed of Bagmati river basin in Nepal. The model operates on a daily time step and allows a basin to be subdivided into grid cells or natural sub-watersheds. The objective of the study was to determine whether the SWAT could be used to simulate stream flow and sediment yield giving priority to the later from Kulekhani watershed where soil erosion is not solely driven by rill and sheet erosion. A GIS interface was used to prepare and process a geospatial data required to run the model. Automatic calibration of SWAT model using Sequential Uncertainty Fitting version two was used together with enormous support of manual calibration.
The available stream flow and sediment data for calibration and validation were limited. A model was calibrated by using two years (2007 to 2008) of daily stream flow data collected from Nepalese Department of Hydrology and Meteorology (DHM) at the outlet of the study watershed. The model calibration for sediment was carried out for daily sediment data from 2004 available for only four months of the monsoon season (June, July, August and September). The sediment data was obtained from Sangroula (2005). The flow was validated for the year 2009 and 2004 at three outlets. The flow from 2009 was collected at the outlet of the Kulekhani watershed and the 2004 was measured at the outlet of two sub-watersheds: Palung Khola and Chitlang Khola. The model was not validated for erosion or sediment since there was no available length of data enough for validation. The average simulated daily runoff and daily sediment yield by SWAT were compared with the corresponding average values of the observation using graphical and statistical methods.
Coefficient of determination
R2 and the Nash-Sutcliffe efficiency
NS have been used tomeasure the performance of the model. The coefficient of determination
R2 and the Nash- Sutcliffe efficiency
NS for the daily runoff was obtained as 0.60 and 0.44 for the calibrationperiod respectively. The coefficient of determination
R2 and the Nash-Sutcliffe efficiency
NS for the daily runoff at the outlet of the Kulekhani watershed for validation period (2009)was obtained as 0.6 and -0.6 respectively. The validation of the daily runoff for the year 2004 at Palung Khola sub-watershed gives
R2 and
NS value of 0.66 and 0.29 respectively.88
The validation of the daily runoff for the year 2004 at Chitlang Khola sub-watershed gives
2R and
NS value of 0.81 and 0.74 respectively. The calibration of the model for the dailysediment observed at Palung Khola gives
R2 and
NS value 0.54 and 0.53 respectively. Thecoefficient of determination
R2 and the Nash-Sutcliffe efficiency
NS in estimating dailysediment yield during calibration period was 0.4 and 0.07 respectively.
In general, the daily stream flow and the daily sediment yield predicted by SWAT corresponded well with observed values. However, the model seems to over predicted surface runoff and under predicted the base flow in some years. The reason for this was that the daily stream flow data used for calibration and validation was not reliable. Three years of flow data (2007 to 2009) was a calculated from power production and reservoir water level at Kulekhani hydropower plant located at the outlet of the Kulekhani watershed. It was believed that there might be some error during calculation or particularly the data used for calculating the flow might be erroneous. This was clearly indicated during flow validation in 2009 (Figure 5.3). The model response for each rainfall event was quite better than what the observation showed.
In addition to predicting the daily stream flow satisfactorily, SWAT also simulated soil erosion and sediment transport within Kulekhani watershed in a promising way. But, the simulation of runoff was better than that of sediment yield. The relatively poor performance of the SWAT model in simulating the sediment yield from Kulekhani watershed is due the incapability of the SWAT model to realistically model gully erosion and landslide which are believed to be common in the watershed.
Calibration and validation of the SWAT model show that the simulated daily stream flow and sediment yields were in reasonable agreement with measured values. The study demonstrated that the river basin scale model, SWAT has the capability of simulating runoff and sediment from Kulekhani watershed.
The global sensitivity analysis of the SWAT parameters showed that runoff is most sensitive to curve number (CN), Manning’s “n” value for overland flow (OV_N) and effective hydraulic conductivity in the main channel alluvium (CH_K2). The sensitivity analysis of the SWAT parameters showed that sediment yield is most sensitive to upland factors such as USLE soil erodibility factor (USLE_K), USLE cover and management factor (USLE_C) and USLE support and practice factor (USLE_P).
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In general, it can be concluded that the ability of the SWAT model in predicting the sediment from a watershed depends on which erosion or sediment transport mechanism is dominant in the watershed. If most of the sediment added to the channel is caused by gully erosion and landslide, then the SWAT prediction could not match with the observation as it only considers rill and sheet erosion by MUSLE equation.
On the basis of the results obtained in this study, SWAT may be believed to be a reasonable selection for the simulation of runoff and sediment from Kulekhani watershed. The result of this study could have been better if spatially distributed precipitation data, long period of runoff and sediment yield data, high resolution of land use and soil data, good knowledge of the watershed area and enough time had applied.