In order to use the successfully hydrological model must model parameters be determined carefully. The lack of physical data specific and as well as the measurement field that are very costly. Measuring all values of model parameters is not possible. The estimation parameters are usually output by model and observational data in a process of trial and error done. On the other hand the successful application of hydrological models depends on the accuracy of the calibration model. So before you apply the results to make different decisions, should be carefully calibrated to increase the reliability model. In this study IHACRES model is used to study rainfall - runoff process in Gharasoo Basin located in Kermanshah. IHACRES model has 3 input variables: Daily time series data of rainfall, stream flow and temperature. Model with daily data flow (2000-1981) was calibrated and then during the period (2010-2001) were about validation Although the model could not simulate the maximum discharges suitably, but in total with regard to low deviations and suitable simulation of the minimum discharges and based on two indices (R2=0.66) in the calibration and (R2=0.61) in evaluation and (CE=0.008) in the calibration and (CE=0.029) in evaluation, it can be said that the model performance in the studied basin was reasonable.The average amount of predicted daily effective precipitation was estimated at 0.36 mm in the calibration stage of the model. In general, the amount of effective daily precipitation in this stage fluctuated between 0 and 12.31 mm, which is in close agreement with the natural conditions of the region. The average amount of effective daily precipitation is 0.23 mm in the validation phase. The results of the statistical evaluation related to the stages of calibration and validation of the IHACRES model are specified .The parameter tq (the time constant of rapid flow response reduction) indicates the time it takes for the rapid flow to decrease, the higher the value of this parameter, the later the basin responds to the flow and the later the rapid flow decreases, this parameter is for the basin in question. The study was estimated to be 12.43 days, since the parameter (the time constant of slow current response reduction) was estimated to be 25.83 days, so it can be concluded that slow current responds to fast current in a shorter time and more time is needed than slow current. The watershed should be reduced It is obvious that the parameters obtained by this model should be determined separately based on the conditions of different regions and based on the recalibration of the model. In general, according to the obtained results, it can be said that the model simulates low watershed flows well, but it has little ability in simulating maximum flows and simulates smaller values. But in general, considering the low deviations of the model and the good simulation of the values, at least it can be said that the performance of the model in the studied basin is satisfactory, and it can be said that this model is easy to use, has more limited inputs, and reduces time spent according to the level. Its accuracy shown in this study was used in various fields such as evaluation and estimation of hydrological effects, runoff forecast for future periods.The actual values of observed flow volume in the time period of 1981-2020 is studied. There are a limited number of evapotranspiration and rain gauge stations in the Qara-Su watershed. In this research, the basic data used includes the observational data of temperature, precipitation and runoff in the period of 1981 to 2020 from selected stations in the region. For the temperature variable, the daily data of Kermanshah synoptic station was chosen as the basis. According to the difference between the figure of this station and the average figure of the Qarasu basin, using the temperature and altitude gradient, the average temperature data of the basin was calculated. For the rainfall variable, after completing the daily data for 7 existing stations, the average daily rainfall of the basin was obtained by considering the amount of each station compared to the average amount of the basinAlso, the amount of effective daily precipitation for all days has been determined based on the parameters estimated by the model. The values of drought severity parameters, temperature adjustment factor, storage moisture, time constant of rapid flow response reduction, soil moisture index threshold and slow flow volume ratio, related to the calibration stage.These parameters were calculated for the studied area as 4.9 days, 2.6 degrees Celsius, 0.13 mm, 12.43 days, 0.11 and 0.9 respectively.