Assessing the impacts of irrigation and soil types on the water balance and groundwater depletion in an irrigation district of the North China Plain based on a coupled SWAT-MODFLOW model

Well irrigation can alter water balance and threaten the sustainability of groundwater resources globally. It is a challenge to quantitatively evaluate the effect of complex irrigation water use in different soils on the water balance in the vadose zone and in the phreatic aquifer at a regional scale. An irrigation district in the North China Plain was chosen as the study site, where the soil types were mainly sandy loam and silt loam in well-irrigated areas and silt in surface water irrigation areas. The coupled SWAT-MODFLOW model was robustly verified by the two-year monthly soil water content and groundwater level data. Evaluation of water balance components across the entire study area showed that under the precipitation of 426 mm yr−1 and irrigation of 300 mm yr−1 (5.7 % surface water and 94.3 % groundwater), the actual evapotranspiration (actual ET) was 514 mm yr−1, soil water recharge to groundwater was 252 mm yr−1, and the groundwater amount declined 90 mm yr−1. Irrigation increased the actual ET by 191 mm yr−1 and soil water percolation to groundwater by 134 mm yr−1 compared with the non-irrigation scenario. Groundwater abstraction for irrigation resulted in less soil water percolation (220 mm yr−1) and higher groundwater depletion (181 mm yr−1) in the sandy loam region than in the silt loam region where soil water percolation was 281 mm yr−1 and groundwater depletion was 161 mm yr−1. The higher antecedent soil water content resulted in higher soil water percolation in fine-textured soil. Irrigation optimization measures are necessary to mitigate groundwater depletion in coarse-textured soil in the irrigated region of the North China Plain.