Land Cover Maps from Landsat Data

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4.2.2 Model Calibration and Verification

There are three years paired observed rainfalls and discharge data were used to calibrate and validate the model. The calculation methods that have been employed to the model are: NRCS curve number method to calculate loss, Snyder for transform method, and recession for base-flow method. Table 4.8 shows the adjusted parameters during calibration period. Table 4.8 Set of calibrated parameters on Palu Catchment Parameter Range Value Initial abstraction scale factor 0 – 0.5 0.05 Baseflow threshold for sub-basin 1 0 – 1 0.22 Baseflow threshold for sub-basin 2 0 – 1 0.06 Baseflow threshold for sub-basin 3 0 – 1 0.06 Baseflow threshold for sub-basin 4 0 – 1 0.1 Baseflow threshold for sub-basin 5 0 – 1 0.04 Baseflow threshold for sub-basin 6 0 – 1 0.22 Baseflow threshold for sub-basin 7 0 – 1 0.05 Comparison between simulated and observed hydrograph during calibration process shows the good fit, even though there are some deviations or errors. During calibration process by using one-year daily data, the accuracy achieved is quite good with 0.758 of R 2 . It might be due to the areal rainfall pattern spatially doesn’t represent the real pattern in each sub-basin area and also due to the simplification of some parameters under the model. 5 10 15 20 25 30 50 100 150 200 250 300 350 400 450 Di sch a rge m 3 s Date Precipita\on mm Simulated Observed 47 Figure 4.8 Simulation versus observation hydrograph, and correlation between simulation and observed discharges during calibration process. Performance of the model was objectively evaluated by using Nash- Sutcliffe efficiency and relative volume error, in which it gave good efficiency value of 0.81 and 42.9 of relative volume errors. By using three tests it can be stated that the model is satisfactory accepted. For advanced evaluation, the calibrated parameters was verified using two years independent paired observed rainfall-discharge data on 2006 and 2007. R² = 0.75882 100 200 300 400 500 50 100 150 200 250 300 350 400 450 Q Sim ulate d m 3 s Q Observed m 3 s 5 10 15 20 25 30 35 40 50 100 150 200 250 300 350 400 450 Preci p it a Uo n mm Di sch a rge m 3 s Date Precipita\on Simulated Observed 48 Figure 4.9 Simulation versus observation hydrograph, and correlation between simulation and observed discharges during verification process. Figures 4.9 demonstrate verification of calibrated parameters that have been done previously. Here the performance of calibrated parameters was evaluated using Nash-Sutcliffe efficiency and relative volume errors. Overall it gave values 0.9 of Nash-Sutcliffe efficiency and 12.1 of relative volume errors. The correlation coefficient of R 2 during verification process was quite low, achieving only 0.58 of RMS value. From the correlation graph between observed and simulation discharges shows that both values have strong correlation.

4.2.3 Changes in Seasonal Stream Flow

In order to analyze the annual seasonal stream flow, twenty years of daily hydrograph was simulated. The simulation used twenty years rainfall data that recorded by eight rain gauges spread out on the catchment. Based on the simulated hydrograph, it can be indicated that annual peak flow occurred in 1994, 2000, and 2007. The dash-circle indicates the peak flow of the years. R² = 0.57642 100 200 300 400 100 200 300 400 Q Sim ulate d m 3 s Q Observed m 3 s 49 Figure 4.10 Twenty years simulated hydrograph in Palu River. Wet season high flow; in order to analyze the seasonal stream flow, period of April - June and October – December was selected as the wettest months, while the peak on May and November. January-March and August-September were considered as driest month. During the wet season, rainfall increased and reached the peak on May and November. Due to the increasing of rainfall intensity, large amount of stream flow was generated from surface runoff. During the 1990 to 2009 period, the stream flows fluctuated about 73 to 288 m 3 s, and reach the peak flow on May 2007 as 2007 big flood was occurring in Palu. Figure 4.11. Variations between high wet seasons flow, low dry season flow and peak to low ratio. 5 10 15 20 25 30 35 40 50 100 150 200 250 300 350 P mm Discharge m 3 s Years 50 100 150 200 250 300 350 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 Di sch a rge M 3 s Years Wet Dry Ra\o