<p>The accurate streamflow projection, which is crucial for flood risk mitigation and water resources management, highlights the significance of development of advanced hydrological models, which can be capable of producing precise flow predictions and projections considering the changes in climate and land use patterns. In this study, a novel approach is proposed to evaluate how climatic variability and changes in land use affect flow patterns, improving the accuracy and reliability of streamflow projections using the Hydrologic Engineering Center-Hydrologic Modeling System (HEC-HMS) model using data derived for the Mahanadi River basin in Chhattisgarh, India. The deficit and constant loss (DC) method which is least explored method for the loss rate and the SCS unit hydrograph as the transformation method are used in this study. Further, the climate variables from the best identified Coupled Model Intercomparison Project Phase 6 (CMIP6) model for the SSP2-4.5 and SSP5-8.5 scenarios are used to project streamflow for the near future (2019–2051) and the far future (2052–2084). The novelty of this study is to simulate streamflow for extended future periods, enhancing the understanding of long-term hydrological impacts under changing climate scenarios by integrating CMIP6 future climate projections into the HEC-HMS model considering the impact of current and future land use/land cover changes. Novelty also lies in the uses of the DC method in HEC-HMS, which is not frequently used in comparable research that generally employs the SCS Curve Number method. The results revealed that streamflow is increased in both the near and the far futures under the both scenarios for the study region and also affected by the land used in the study area. The findings of this study may be helpful for hydraulic structure design and construction, flood forecasting, and making well-informed decisions to prevent flooding.</p>