Hydrochemical Characteristics and controlling factors in different irrigation periods of Saline-alkali landin YanghuangIrrigation District: A case study of a typical small watershed in Jingtai County, Gansu Province

Soil secondary salinization caused by irrigation poses a serious threat to soil quality in arid areas. In irrigation districts, farmland drainage ditches are used to lower groundwater levels and discharge salts, thereby mitigating soil salinization. However, the patterns of salt discharge through irrigation and drainage vary significantly across different periods, and the mechanisms governing the hydrogeochemical processes affecting salinity remain unclear. This study was conducted in a small watershed within a typical Yellow River irrigation district in Jingtai County, Gansu Province. Samples of irrigation water, drainage ditch water, and groundwater were collected. Combined with field investigations and hydrochemical analysis, this research investigated the patterns of salt discharge and associated hydrogeochemical effects during two typical irrigation periods (summer and winter irrigation). The results indicate that: 1) The hydrochemical characteristics of groundwater and drainage water differed significantly between different regions and irrigation periods. The total dissolved solids (TDS) and major ion concentrations (e.g., Na⁺, SO₄^2-, Cl^-) in drainage water from the secondary ditch in moderately to severely saline-alkali areas were higher than those in drainage water from the primary ditch in mildly saline-alkali areas but lower than those in the surrounding groundwater of the moderately to severely saline-alkali areas. The difference in salinity between drainage water and groundwater was most pronounced during the summer irrigation period. 2) The salinity in the drainage ditches is influenced by the mixing of irrigation water and groundwater, as well as cation exchange during irrigation. Specifically, large-volume summer irrigation raises the groundwater level, allowing irrigation water to enter the drainage ditches without fully leaching salts. In contrast, winter irrigation water slowly infiltrates and replenishes groundwater before entering the drainage ditches, resulting in more effective salt discharge. 3) Calculations of mineral saturation indices indicate that both groundwater and drainage water are saturated or supersaturated with respect to calcite (CaCO₃) and dolomite (CaMg(CO₃)₂), while they are close to saturation for gypsum (CaSO₄: 2H₂O). This suggests that long-term irrigation intensifies the precipitation of carbonate minerals, potentially affecting the permeability of the soil for salt leaching. Therefore, the application of calcareous soil amendments should be reduced in soil reclamation practices. This study provides a scientific basis for optimizing irrigation and drainage management and improving soil salinization in Yellow River irrigation areas with saline-alkali soils.