JWSS - Journal of Water and Soil Science

One of the most important and complex processes in the watersheds is the identification and prediction of surface water changes. The main processes associated with surface water include precipitation, percolation, evapotranspiration and runoff. In this research, the semi-distributed model, SWAT, was used to simulate ground water and surface water in Semnan catchment in a monthly scale. A sensitivity analysis was perfomed to evaluate and demonstrate the influence of the model parameters on the four major components of water budget including surface runoff, lateral flow, groundwater and evapotranspiration. River discharge data from 2004 to 2014 were used for the calibration and those of 2014 to 2016 were applied for the validation. The results of sensitivity analysis showed that the most sensitive parameters were: SoL_K(Saturated hydraulic conductivity), CH_K2 (Effective hydraulic conductivity in main channel), RCHRG_DP(Deep aquifer percolation fraction and CN2 (Moisture condition II curve number). The simulation accuracy using Nash-Sutcliffe and coefficient of determination for Shahrmirzad, Darjazin, and Haji Abad hydrometric stations was about 0.60 to 0.80 and 0.80 to 0.90 for the calibration and validation period, respectively, showing a good performance in the simulation of river flow. According to the water balance results, about 87.6% of the total inflow into the watershed was actual evapotranspiration, 3% was surface run off, 3% was percolation, and the rest was related to the soil moisture storage.

Reducing surface water resources and successive droughts and consequently excessive use of groundwater resources, especially for agricultural purposes, have caused irreparable damage to the natural resources of the country. In the meantime, knowing the status of the water balance of the plain can help to effective management of water resources in the region. Samalqan plain is located in a semi-arid climate in North Khorasan Province. Since the surface water resources for water supply are not very reliable, so, the main source of water supply in the region is well. Due to the existence of rivers in the plain, the low thickness of the alluvium, groundwater level fluctuations, and the high uncertainty in the calculation of hydrodynamic coefficients, the need for careful hydrogeological studies and determining the role of each parameter affecting groundwater is necessary. This study was conducted to simulate the Samalqan aquifer and analysis of water balance for the years 2003 to 2013 using the MODFLOW model. To identify the groundwater recharge rate, this component was estimated by the SWAT model. Calibration and validation of the model with an error of 1.1% and 1.2%, respectively, indicated that an appropriate estimation between the simulated and observed heads. Assessment of the groundwater hydrograph in the observation wells showed that the groundwater level in most places has many monthly and seasonal fluctuations. After drawing the potential lines of the plain, the inputs and outputs were identified, and using the reserve volume changes, the water balance was determined. The results showed that the water balance of the plain was negative and the reservoir deficit was estimated at 9.14 million cubic meters. Therefore, this model can be used to predict the future situation of aquifer and the management of water resources in the region.