Integrated Surface and Groundwater Flow Modeling in Neishaboor Watershed with SWAT-MODFLOW

During the last decades, arid and semi-arid regions has faced a severe problem of depletion of groundwater resources due to the over-exploitation of the aquifer. Moreover, groundwater and surface water are not isolated components of the hydrologic system, but instead interact in a variety of aspects in which development of one commonly affects the other. Additionally, the interaction is often complicated by agricultural activities including surface water diversion, groundwater pumping and irrigation. This study presents an integrated SWAT-MODFLOW model that couples land surface hydrology and groundwater hydrology to determine spatial groundwater percolation patterns considering allowable groundwater pumping rates for the Neishaboor watershed, Iran. Within the integrated model, the pumped groundwater is applied as irrigation to the cultivated fields within the SWAT model, with deep percolation from the soil profile bottom applied to the MODFLOW model as recharge. The model is tested against observed stream flow and water table elevation, with model output then used to assess and quantify spatial-temporal patterns of groundwater recharge to the aquifer.

The recently developed SWAT-MODFLOW modeling code simulates spatially-distributed hydrologic processes in the coupled land surface / aquifer system, with SWAT simulating land surface, soil zone, and stream flow routing processes and MODFLOW simulating groundwater flow and groundwater/surface water interaction processes. Modifications which is done to the modeling code includes: 1) Linking pumping from MODFLOW cells to SWAT HRUs for groundwater irrigation and 2) Imposing shallow water table percolation and lateral flow conditions for SWAT HRUs when the MODFLOW-simulated water table is within the soil profile of the HRU. The integrated SWAT-MODFLOW framework is tested in the Neishaboor watershed (9157 km2) for the 1998 to 2011 time period. Climate of the region is classified as semi-arid, with an average annual precipitation of 265 mm that varies considerably from one year to another. The mean annual temperatures changes from 13°C in the mountainous area to 13.8°C in the plain area and the annual potential evapotranspiration is about 2,335 mm. The main crops that are grown in the watershed is irrigated and rain fed wheat during fall and winter and corn silage during summer. Regarding previous studies, about 93.5% of the groundwater withdrawals in the Neishaboor watershed are consumed in agriculture, mostly for irrigation. Therefore, irrigation practices play a crucial role in the water resources balance in the study area. Within the integrated model, the pumped groundwater is applied as irrigation to the cultivated fields within the SWAT model, with deep percolation from the soil profile bottom applied to the MODFLOW model as recharge. The SWAT model was calibrated and tested in SWAT-CUP for the 2001-2009 and 2010-2011 periods, against stream flow and developed model was calibrated manually against groundwater level data.

Annual average recharge, calculated from the daily recharge values pass from SWAT to MODFLOW, demonstrating higher recharge rates in the alluvial fans and upland plain. Observed and simulated stream discharge in four hydrometric stations demonstrate good similarity results with the observed hydrograph. The NS values for monthly discharge rates are considered acceptable, however, the field-estimated stream flow estimates contain a high degree of uncertainty. Simulated cell-wise groundwater hydraulic head at the end of the simulation is compared with observation values with the highest water table elevation occurring in the north east and low water table elevation occurring in the outlet. Comparing observed and simulated average groundwater levels at the 48 monitoring wells, the deviation from the 45-degree line is less than 2.5 m for over 73% of the circles. The manual calibrated model can capture the main temporal trend. Overall, the model well captures the long-term characteristics of the regional groundwater level.

In this study, a new coupled model, referred to as SWAT-MODFLOW was used to model a dry and semi-arid region with a complicated irrigation system with groundwater pumping. A comprehensive model, will enable accurate simulations of stream flow and water table fluctuations in watersheds and aquifers respectably. In short, surface water infiltration is passed from SWAT to MODFLOW based on the contributing areas of the HRUs to the groundwater grid. Pumping agriculture water is then calculated and passed back to SWAT. The need for such a model is highlighted by the Neishaboor basin, where the agriculture is completely based on groundwater pumping. The case study in the Neishaboor basin demonstrated the applicability of the model for large, dry basins. The model will be used to determine best management practices for groundwater pumping in the region.