Journal of Watershed Management Research

Introduction: The application of hydrological models in watersheds has always been considered by water resources researchers. This subject is of special importance in arid and semi-arid regions due to the greater complexity of hydrological processes in these region. Optimal utilization and consumption of water resources and its optimal management requires a better understanding of the hydrological model. Precipitation and subsequently surface runoff are important phases of the hydrological cycle Given the present situation in most of the country's watersheds in terms of missing statistics and high complexity and impossibility of fully understanding hydrological ecosystem, to use some approaches and method to estimate runoff in watersheds with missing data or incomplete statistics is essential. Rainfall-runoff models can simulate processes within watershed and serves as a tool to estimate runoff and study hydrological processes. In the present research, WetSpa distributed hydrological model was applied to simulate river flow in Sarbaz watershed of Sistand and baluchistan Province.
Materials and Methods:Sarbaz watershed is located in the southeast of Iran in Sistan and Baluchistan province. This basin has an area of 6324.29 km2. The discharge of the Sarbaz river is more affected than the northern areas of the basin, the base discharge of the Sarbaz river is low, but its flood discharge is high even for the periods of low return. Wetspa distributed hydrological model has been used to simulate the runoff of this area. WetSpa is a distributed, continuous and physical model with daily or hourly time step that explains processes of precipitation, runoff and evapotranspiration for both simple and complex contexts. WetSpa model first calculates water balance in root zone because this is the most important area in water retention and at the same time it controls surface and subsurface runoff, evapotranspiration and underground water flow. The model uses modified rational method to calculate runoff is and to estimate snow melt runoff, growth it applies day- degrees coefficient method. In this study, first the initial values ​​of the parameters were determined and the model was calibrated manually. Then, PEST software was used to obtain more accurate results in the calibration. PEST software adjusts the parameters step by step according to its algorithm and the program execution is repeated using the local search method until the best parameter values ​​are obtained. To run model, Daily data on flow, precipitation, temperature and evaporation for years 2018 to 2022 were considered for calibration and validation. From this statistical period, the years 2018 to 2020 were considered for model recalibration and the years 2020 to 2022 were considered for model evaluation. In order to evaluate the efficiency of the model in the calibration and validation stage, Nash-Sutcliffe evaluation indices and the mean squared error and bias model were used.
Results: After preparing the WetSpa model based on the daily data of discharge, precipitation, temperature, evaporation and maps of land use, soil texture and digital elevation map, the model was implemented for two periods of calibration and validation.Based on Nash-Sutcliffe general criterion, the model showed daily hydrographs with an accuracy of more than 69.02 Percent for the calibration period and based on the adapted Nash-Sutcliffe criterion for maximum discharges with an accuracy of 88.18 Percent. In the validation period, the model based on the general Nash-Sutcliffe criterion simulates daily hydrographs with an accuracy of more than 57.4 Percent and based on the adapted Nash-Sutcliffe criterion for maximum flow rates with an accuracy of 69.39 Percent. has made The results showed that the model is more effective in estimating high flows than low flows, but overall, the model has simulated the total flow with acceptable accuracy. The results showed that the model did not simulate the discharge well at some points during the validation stage, which could be due to the structure of the model or the field conditions or the data. The results showed that the response of the model to precipitation in the area is reasonable, so the lack of simulation in the specified areas can be caused by the conditions of the area or errors in the recorded data. With the investigations carried out at the field level, it was found that before the outlet hydrometric station, a diversion dam was built to extract water, which caused the simulated flow rate to be higher than the observed amount, and this lack of accurate simulation is due to the construction of this dam. be And this shows that in the case that the simulation results are not suitable, it is not related to the inefficiency of the model.
General conclusion: According to the results of the evaluation indicators in the calibration and validation stage, it was determined that the model has the necessary efficiency to simulate the flow in Sarbaz watershed. According to the investigations, most of the studies related to the WetSpa model in humid and semi-humid conditions of most European countries have confirmed the appropriate use and efficiency of the model in these conditions. But considering that a large part of our country has a dry and semi-arid climate, it is necessary to evaluate the effectiveness of this model in dry and semi-arid regions of the country. Due to the fact that the climatic conditions of Sarzab watershed are dry to semi-arid, the research results indicate that the WetSpa model has the necessary efficiency for hydrograph simulation in these climatic conditions.