This study simulated the inundation process in the Lower Mekong River Basin (LMB). The LMB has suffered from severe floods, especially in 2000 and 2011. To quantify the inundation of water in a basin where large-scale inundation by river water occurs, understanding the conveyance of a river channel during a flood is particularly important. Therefore, we conducted a field survey using an acoustic Doppler current profiler (aDcp) to understand the longitudinal distribution of the width and depth of the river channel and the variation in hydraulic resistance with respect to shear stress on the riverbed. It was found that the width and depth vary longitudinally, and the relationship between them can be estimated by an equation derived from governing equations of water and sediment and the bed load formula. Furthermore, it was revealed that hydraulic resistance decreases with increasing non-dimensional shear stress. Then, the characteristics of the river channel were incorporated into the runoff-inundation simulation. Furthermore, inundation water should be validated not only in terms of inundation extent but also with respect to water depth and velocity. These were estimated using 8-day composite surface reflectance data from the Moderate Resolution Imaging Spectrometer (MODIS) and the SRTM. Simulation results indicated that water level and discharge within the river channel were able to reproduce observed values. Additionally, simulated inundation extent, water velocity, and water depth over the floodplain showed reasonable agreement with the results using the data from the MODIS and the SRTM. Although there are some elements that should be improved, the inundation process in the LMB was simulated appropriately despite its complexity. The method described in this study to set a calculation condition and to validate variables over a floodplain should be useful for runoff-inundation simulation in various large-scale basins.
River Levels Derived with CryoSat-2 SAR Data Classification—A Case Study in the Mekong River Basin