The impacts of climate and land use changes on river discharge in the Lam Chi subwatershed in northeast Thailand were assessed using the H08 model. We calibrated and validated the model based on land use type using observational data from the Royal Irrigation Department and evaluated it using the Nash-Sutcliffe Efficiency Coefficient. After calibration, we used climate data from CMIP5 for 2022–2031 and land use data projected by the CLUE model to estimate future river discharge. We found that discharge will increase due to increases in precipitation between the past (1986–1995) and future (2022–2031). Discharge in the forested area is lower than that in the agricultural area due to differences in soil depth, the subsurface flow rate, and evapotranspiration. The main factor affecting discharge is plant evapotranspiration. Furthermore, the impact of land use change is smaller than that of climate change in determining the progression from current to future discharge, whereas the opposite was observed for the transition from the historical to more recent past.