This study aimed at quantifying the impacts of climate and land use changes on flood damage on different flood occurrences. A Hydrologic Engineering Center’s Hydrologic Modeling System (HEC-HMS) model was calibrated for the period 2005–2011 and validated in the period 2012–2017, and was used to generate hydrographs using rainfall during the period 2020–2039 from CNRM-CM5, IPSL-CM5A-MR, and MPI-ESM-LR climate models under Representative Concentration Pathways (RCPs) 4.5 and 8.5. A Hydrologic Engineering Center’s River Analysis System (HEC-RAS) model for use in generating inundation maps from hydrographs produced by HEC-HMS was calibrated and validated for 2010 and 2011 period, respectively. The climate and land use changes showed insignificant impacts on the extent of floods during 25-, 50-, and 100-year flood events, i.e., inundation in 2039 under RCP 4.5 is smaller than baseline (2000–2017) by 4.97–8.59 km2, whereas a larger difference of inundation is found for RCP 8.5 (0.39–5.30 km2). In contrast, the flood damage under RCP 4.5 (14.84–18.02 million US$) is higher than the baseline by 4.32–5.33 million US$, while the highest was found for RCP 8.5 (16.24–18.67 million US$). The agriculture was the most vulnerable, with a damage of 4.50–5.44 million US$ in RCP 4.5 and 4.94–5.72 million US$ in RCP 8.5, whereas baseline damages were 4.49–6.09 million US$. Finally, the findings are useful in the delivery of flood mitigation strategies to minimize flood risks in the lower Nam Phong River Basin.

When the severity of exposure to flood is being addressed, several related concerns have always been raised to draw attention on a growing flood threat. In relation to this, the extraordinary insight into the seriousness of land use and rainfall changes that could greatly exacerbate flood impacts would need to be highlighted. The importance of the aforementioned issue lies in the main objective of quantifying consequences of how changes in land use and rainfall affect the hydrological processes in the lower Nam Phong River Basin. The use of Hydrologic Modeling System (HEC-HMS) simulation model would add robustness and predictability to the overall results. It was apparent from the calibration and validation processes that there are reasonably close agreement between observed and simulated discharges at Ban Nong Hin gauging station (E.22A), with good correlation coefficients (ENS= 0.78, r2= 0.81 and ENS= 0.77, r2= 0.82, respectively). Thereafter, different what-if scenarios were conducted to determine impacts of land use changes in 2001, 2011 and 2057 and extreme rainfall with different return periods of 10-, 50-and 100-years on hydrological responses. A slight increase in peak flows were equal to 4% and 1%, as a consequence of the change from 2001 land use conditions to 2011 and 2057, respectively. Conversely, a large increase in peak discharges was expected to be 13%, 20% and 27% when the 2001 rainfall event was adjusted to the projected changes in rainfall corresponding to 10-, 50-and 100-year return periods, respectively. In brief, insignificant relation between hydrological response and land use changes was obviously found, but it was of particular significance due to changes in rainfall extremes. Taken together, obtained findings can then be used as a baseline for water resources planning, development and management, as well as flood management perspective.

Rapid socio-economic development along with exceptional rainfall can potentially exacerbate risk of flood damage to life and property in the lower Nam Phong River Basin. In relation to this, the non-structural measures including risk-based zoning could be considered as an effective solution in mitigating the flood threat in the future. Thus, a coupling of the hydrological model HEC-HMS and hydrodynamic model HEC-RAS, which increases the robustness and predictability to the overall findings, was applied to assess flood hazard in this study. The outcomes highlighted that the applications of the HEC-HMS and HEC-RAS models are suitable for the study area with the Nash-Sutcliffe Efficiency (E NS ) varied between 0.75 to 0.87 and the coefficient of determination (R 2 ) ranged between 0.81 to 0.92. Moreover, the flood zone mapping was also carried out based on the Flood Hazard Rating (FHR) analysis. As a result, the flood hazard areas were determined which covers about 16.5% of the total river basin areas, and it was classified into four zones, i.e. extreme (18.79% of inundated area), high (46.33% of inundated area), moderate (18.24% of inundated area), and low (16.64% of inundated area), respectively. The obtained findings can be useful as the adaptation guideline for water resources planning and flood management in the lower Nam Phong River Basin and other parts of Thailand.