Estimation and evaluation of water balance components by calibrated SWAT Model,, case study: Mahabad Plain

The SWAT Model has been accepted as a comprehensive surface water simulation model in estimating water balance components with the ability to study different management scenarios on water resources. In most studies, the SWAT Model is calibrated based on the accuracy of surface runoff estimation, and its comparison with field measurements in hydrometric stations. In a previous study, the performance of the SWAT Model in estimating runoff, evapotranspiration, and yield of wheat, barley, corn, sugar beet, alfalfa, apple, and grape crops in the Mahabad Plain was satisfactory and acceptable. The purpose of the present study was to use the SWAT calibrated model in estimating other components of water balance including deep percolation, lateral flow, return flow from the aquifer to the river in the same area. Assessing the components of water balance in the unsaturated zone, and its effectiveness in aquifer balance is important in managing the utilization of integrated water resources. The results showed that the calibrated model was able to estimate the difference between the deep percolation in the wetted year compared to normal years, and the difference in the infiltration in the wet year compared to the dry year under the influence of increasing rainfall. The results showed that the amount of deep percolation in the wetted year compared to normal and dry years has increased by about 23% and 43%, respectively. The average depth percolation is estimated 157.86 mm (17.1%). At the same time, the model has been able to estimate the difference in the amount of infiltration in different areas of the plain, and in different seasons of the year according to the type of land use and land management patterns. The amount of lateral flow has also increased in the wetted year compared to the dry year, so that the amount of this component has doubled. In this study, the average return flow from the aquifer to the river was estimated to be 20.9% (129.6 mm). Surveys have shown that the presence of shallow groundwater level has a significant role in creating surface return flow from the aquifer to the river. In general, the results showed that recharge due to rainfall and irrigation water from unsaturated zone is one of the most important input components of groundwater models such as MODFLOW in arid and semi-arid regions. The amount of recharge is important in more accurately estimating water level fluctuations. Given that the MODFLOW Model is not well able to estimate the recharge processes and lateral flow in the unsaturated zone. Therefore, the simultaneous use of SWAT and MODFLOW models in estimating the water balance components of the unsaturated zone and combining it with the groundwater model is important in studies and management of operation of integrated water resources.