Learning and utilizing the downscaling approaches in the climate change impact studies are burdensome and protracted. Indeed, the downscaling approaches are uninterruptedly amended, and they introduce the climatic variables with inappropriate formats. Moreover, observation data are not obtainable for some synoptic stations. However, NASA Earth Exchange Global Daily Downscaled Projections (NEX-GDDP) data set has been developed for 21 General Circulation Models (GCMs) under two Representative Concentration Pathways (RCPs), i.e. RCP4.5 and RCP8.5, with the spatial resolution of 25°, i.e. 25km × 25km (625 Km2) and diurnal temporal resolution. In this study, firstly, the climatic variables, i.e. precipitation, minimum temperature and maximum temperature, were extracted from the NEX-GDDP data set for Lali region, Khouzestan Province, southwest Iran for the present (1961-1990) and future (2021-2050) time periods. Secondly, the climatic data were verified by the observation data so that the determination coefficient (R2) between the NEX-GDDP data set's outputs and the observation data is 0.89 for precipitation and approximately one for the average temperature in the verification time period (2007-2016). Thirdly, the discharge rates of Taraz-Harkesh river were simulated by IHACRES rainfall-runoff model. Results demonstrated the average precipitation would be 28.66, 27.41 and 27 mm/month and the average temperature would be 14.18, 15.98 and 16.31°C both for the present, future under RCP4.5 and future under RCP8.5 time periods, respectively. Furthermore, results depicted the ability of the rainfall-runoff model in simulating the river's discharge rates with R2=0.6 and monthly R2=0.9 for the calibration time period, i.e. from 23/09/2009 to 10/01/2011 for which the observation data had been obtained in daily time step, and R2=0.51 for the verification time period, i.e. from 06/11/2012 to 06/10/2015 for which the observation data had been obtained for one day over the month. Finally, the river's discharge rates were simulated as 340, 304 and 295 l/s for the present, future under RCP4.5 and future under RCP8.5 time periods, respectively.