Modeling the behavior of shallow groundwater system in sustaining arid agroecosystems with fragmented land use

Shallow groundwater often presents multiple behaviors in supporting the sustainability of arid agroecosystems. The behaviors are especially important and complicated for the arid irrigated areas with fragmented land-use types. In the view of understanding the role of shallow groundwater in such areas, the MODFLOW and MT3DMS (with the assistance of HYDRUS-1D) were used to simulate groundwater flow and salt transport in a typical observation site of the arid upper Yellow River basin (YRB). The original MODFLOW was modified with introducing a variable expression of specific yield, aiming to improve the modeling accuracy in shallow groundwater environments. The model was calibrated and validated using the observation data of groundwater depth and drainage in 2013 and 2012, respectively. Groundwater evapotranspiration and lateral water-salt exchange among different land-use types were simulated and analyzed; meanwhile, the dry drainage effect of non-cultivated natural land was quantitatively interpreted. Results indicated that the fragmented land-use/planting pattern (with different irrigation scheduling) resulted in significant lateral groundwater flow and water/salt redistribution among different land-use fields during irrigation. The groundwater evapotranspiration ranged at about 160–220 mm for different land-use types, reaching to 0.30–0.45 of plant evapotranspiration (ETpt) during the growing seasons. This effectively supplemented the plant water use in arid climate, especially for the unirrigated cultivated land (during some stages) and the non-irrigated natural land. Meanwhile, the lateral groundwater flow drove salt migrating from the cultivated land to the natural land. The natural land accumulated 27–40% of salts introduced by irrigation, playing a significant role in accommodating salts (i.e. the dry drainage effect). Moreover, our modeling work further implied that the reliable groundwater modeling in arid and shallow groundwater areas required a more detailed model conceptualization and more accurate information of land-use distribution, irrigation scheduling, and local microrelief than the general understanding in the past.