Long-term responses of the water cycle to climate variability and human activities in a large arid irrigation district with shallow groundwater: Insights from agro-hydrological modeling

Climate variability and human activities are the two major driving forces of changes in the hydrological cycle. However, how these two factors separately influence the water cycle in arid irrigation districts with shallow groundwater tables is still unknown. This study aimed to investigate the long-term changes in the water cycle and then quantify the individual contributions of climate variability and human activities to the changes in the water cycle in a large irrigation district. The Hetao Irrigation District (HID), located in the upper Yellow River Basin in Northwest China, was selected as the case study area, and an agro-hydrological model (SWAT-AG), which has powerful capabilities to model complex agro-hydrological processes, was employed as the simulation tool. The results showed that from 2000 to 2018, evapotranspiration (ET) of growing season in the HID remained relatively stable, but there were significant decreases in evaporation and dramatic increases in transpiration. For the subsurface hydrological processes, deep percolation and capillary rise of growing season both showed a decreasing trend, while groundwater drainage showed an increasing trend. The water exchange between the atmosphere layer and the root zone (i.e., ET, including evaporation and transpiration) was more significantly dominated by climatic conditions. In contrast, the subsurface hydrological processes were mainly affected by human activities, e.g., irrigation management, canal seepage, farmland area and cropping pattern. Climate variability contributed to approximately 60% of the changes in ET, whereas human activities introduced over 70% of the changes in deep percolation, capillary rise, and groundwater drainage, respectively. This study provides valuable insights into the impacts of climate variability and human activities on the changes in the water cycle and enhancements of water management practices in arid irrigation districts with shallow groundwater tables.