## 4 Discussion  
In case of models M1-M2 and M3-M8, direct comparison is not possible because different results (SD vs. total sediment load) are presented. To obtain SD values for comparison between the two sets of models, the total sediment load values predicted based on the models M3, M6, M7, and M8 and daily discharge data from 2010 to 2020 in the gauging points of Jeokpo Bridge and Jindong (Figure 6b,c) located downstream of the Hwang River confluence and Nam River confluence, respectively, were used to present flow duration curves (Figures 10 and 11). Additionally, the sediment rating curves presented using the predicted total sediment load results are shown in Figures 9 and 10. M4 and M5 were excluded from comparisons between the models because their predictability was low. Considering Jindong (SD = 102 ton/km2-year) used as calibration data for the model development and Waegwan (SD = 56 ton/km2-year) located in the main stream of the Nakdong River near the validation section, the predictabilities of M6 and M8 yielded the best performances. This contradicts the expectation that models with more submodels will produce superior prediction results, thus showing that in fact, simplified models (with fewer submodels) yielded superior prediction results. The reason M6 and M8 yielded the best predictive results is attributable to the fact that the submodels directly used for validation (M6-E1 and M8-E1) were developed using 227 calibration data (Table 12). In the cases of M1 and M2, the models were developed using a small number of samples (34); however, the response variable was developed using the SD values calculated considering total sediment load and flow conditions, and the SD values (ranges for calibration data: 10-1000 tons/km2-year) did not exhibit a large variation unlike the total sediment load results (range for calibration data: 1-1,000,000 tons/day). Thus, these models yielded consistent results. In addition, in this study, in the case of models M3-8 developed based on data from 2006 to 2012, considering that the data directly measured for validation were obtained from 2016 to 2019, a temporal lumping problem, which is one of the drawbacks of an empirical model, was expected. However, in the case of models M6 and M8, contrary to the expectation, a superior prediction performance was obtained compared to models M1 and M2, which were developed based on the data obtained from 2005 to 2015. This is considered to be attributed to a combination of reasons. First, as calibration data, M1 and M2 used most of the data from other rivers, unlike the models M3-M8, which mostly used the sediment measurement outcomes for Nakdong River. Second, the data measured for validation are shown to be affected by the spatial lumping problem with high-sediment yield value compared with other data used for model development located in the main Nakdong River. In addition, in this study, there were no measurements for Jindong and Waegwan from 2016 onward. Thus, the analysis was not possible, but it is expected that there will be an effect of the temporal lumping problem owing to changes in the river environment. Considering this aspect, data-based empirical models for sediment yield predictions must be updated periodically in consideration of the spatial and lumping problem. Although the presence of spatial and temporal lumping problems was confirmed in the case of the data mining technique based on the results of this study, the existing models