## 5 Discussion  
Because the watershed in the distributed watershed models was divided into multiple grid networks, the topography and Rainfall-runoff characteristics of the watershed varied depending on the DEM grid cell size. If the resolution of the DEM grid cell size is very high, it can accurately represent the characteristics of actual Rainfall-runoff events in a watershed. However, the simulation time for the runoff estimation and topography analysis in the model increase. If the resolution of the DEM grid cell size is too low, the geographical characteristics of the actual watershed are very simple, and a risk of computational errors exists [35]. Therefore, the present study proposed optimum DEM resolution on the basis of the work efficiency and runoff output accuracy in the Dongkok and leemokjung watersheds.  
Figure 14 shows the NSE box plots in the Dongkok and Ieemokjung watersheds when the parameters were optimized in each grid for all Rainfall-runoff events. TOPMODEL hydrographs obtained using the optimized parameters in each DEM resolution were similar to the observed runoff data, which indicated that the resolution did not affect the model because the input parameters were well optimized depending on the DEM resolution.  
Figure 15a shows the efficiency under a constant parameter set in the Dongkok watershed when the resolution was varied. The efficiencies of the 5 and 40 m DEM resolution slightly changed, but the efficiencies of the 80 m or higher DEM resolution significantly changed with the constant parameter set. Similarly, Figure 15b shows the efficiency under a constant parameter set in the leemokjung watershed depending on various DEM resolution values. Similar to Figure 15a, the variation in the efficiency was very small when the DEM resolution values were 5 and 40 m. However, the change in the efficiency  
Figure 13: Rainfall–runoff events by applying optimized parameters in the leemokjung watershed depending on each DEM resolution. (**a**) 24 September 1992. (**b**) 1 August 1994. (**c**) 4 July 1996. (**d**) 25 August 1996.  
was very large with the constant parameter set depending on the 80, 160, and 300 m DEM resolution values. Figure 15 shows that the combination of the input parameters and DEM resolution smaller than 40 m was not sensitive, which implied that the estimated topographic index, which was the area of the slope per unit contour length divided by the slope of the surface, was similarly smaller in the 40 m DEM grid cell size. This analysis agreed with that shown in Figures 4 and 5. The frequencies of the topographic indexes of the 40 m DEM grid cell size shown in Figures 4 and 5 were slightly different from those of the 5 and 20 m DEM grid cell size. Therefore, the result suggested that at least the 40 m resolution showed reliable results with the variation in the input parameters in the TOPMODEL application to the Dongkok and Ieemokjung watersheds. The proposed resolutions in this study were similar to the good-performance resolutions DEM that used 30 m resolution [40], greater than 10 m resolution [35], or smaller than 100 m resolution [37; 41]. However, it is necessary to investigate resolutions that provide similar results as this study to the different characteristics of watersheds or to other hydrological models.