Integration of Hydrological and Flood Inundation Models for Assessing Flood Events in the Lower Prek Thnot River Basin under Climate Change

The exacerbation of flood inundation has been highlighted by revolving around climate change, given that this global phenomenon aggravates the flooding status quo and poses a multitude of collateral damages to humans and societies. Climate change leads to changes in precipitation patterns and thereby changes in patterns of discharge and flood inundation. Consequently, estimating these changes using any numerical models is deemed essential to develop countermeasures. There is growing anticipation of climate change over Cambodia and the lower Prek Thnot River Basin, yet it is unknown how serious climate change is on the basin’s hydrological and especially flood characteristics. Thus, this study aims to assess the change impacts by integrating the Rainfall-Runoff-Inundation (RRI) model and the Soil and Water Assessment Tool (SWAT) model to apply in the lower Prek Thnot River Basin, Cambodia, whose susceptibility to climate change is relatively high. The RRI model was integrated with the discharge outputs from the SWAT model to simulate four flood events: 2000, the biggest flood event, 2001, 2010, and 2020 flood events. Hydrographs, water levels, and flood extent maps for these events were generated. The biggest flood event of 2000 was set as the baseline period for a possible flood event in the future projected climate in the 2030s and 2060s using three general circulation models (GCMs) [Geophysical Fluid Dynamics Laboratory Climate Model, version 3 (GFDL-CM3), Goddard Institute for Space Studies Model E2, coupled with Russell and interactive terrestrial Carbon Cycle (GISS-E2-R-CC), and Institut Pierre-Simon Laplace Coupled Model, version 5A, Medium Resolution (IPSL-CM5A-MR)] under two representative concentration pathways (RCPs) including RCP2.6 and RCP8.5 emission scenarios. The results of climate change suggested that flood magnitude in the lower Prek Thnot River Basin will vary slightly in the 2030s and greatly in the 2060s. In the 2030s, relative changes in precipitation between $-8.2\%$ and 21.6% could lead to relative changes in extreme flow ($Q_5$, flow exceeding 5% of the time) of $-21.4\%-8.4\%$, changes in peak water level of $-6.3\%-1.3\%$, and changes in inundated area of $-12\%-7.4\%$. In the 2060s, relative changes in precipitation between $-17.7\%$ and 46.4% could lead to relative changes in $Q_5$ of $-39.3\%-13.5\%$, in peak water level of $-19.6\%-5.2\%$, and in inundated area from $-31.4\%$ to 20.7%. These relative changes imply the increases and decreases due to climate change compared with the present climate. Overall, the resultant outputs of this study revealed how impactful climate change is on the basin’s hydrological and flood characteristics, which have long been in question. The results can thus be deployed to underpin decision making and offset the impending negative impacts of climate change on flood inundation within the lower Prek Thnot River Basin.