Abstract:
Abstract: To investigate water movement through the unsaturated zone under different kinds of irrigation modes can provide scientific basis for decreasing dryland salinity in arid area. Most researches focused on controlling water and salt conditions of the topsoil or creating a suitable environment within the root zone for plants growth. However, there are few studies on the mechanism of soil water movement in the unsaturated zone under drip irrigation and its impacts on groundwater recharge, or the relationship between soil water and groundwater. In this study, we analyzed water movement through the unsaturated zone in severe (Plot 1) and mild (Plot 2) saline-alkali cotton fields of the Karamay Agricultural Development Area (KADA) accompanied by interbasin water transfer, based on in-situ observation, stable isotopes tracing, and numerical simulation. The results show that under matric potential controlled drip irrigation, infiltration depths are about 0-150 cm for both plots, and the dynamics of soil water content (SWC) and soil water potential (SWP) show the responses to irrigation and evapotranspiration. Soil water in this layer is the mixture of antecedent soil water and irrigation water during the observation. The depths of 150-260 cm for Plot 1 and 250-350 cm for Plot 2 are affected by capillary upflow from groundwater, and the dynamics of the SWC and SWP vary with the water table depth. Soil water in these depths is the mixture of the antecedent water and groundwater. There is a middle layer (150-250 cm) in Plot 2, neither affected by infiltration nor groundwater capillary rise, along with dynamic balance of SWC and SWP. And soil water in this layer is dominated by the antecedent water. According to HYDRUS-1D modelling results, for Plot 1, irrigation (precipitation) and groundwater respectively account for 92.1% and 7.9% of the sources of the unsaturated zone, while evapotranspiration and soil water storage increase account for 64.3% and 35.7% of the sinks of the unsaturated zone, respectively. For Plot 2, irrigation (precipitation) and groundwater account for 85.0% and 15.0% of the sources of the unsaturated zone, respectively, while evapotranspiration and soil water storage increase account for 92.4% and 7.6% of the sinks of the unsaturated zone, respectively. The ratio difference at these 2 plots is caused by higher irrigation amount along with higher matric potential control level at Plot 1. There is a two-way exchange between deep soil water and groundwater, and groundwater recharges soil water in general. The current drip irrigation mode in the cotton fields can be conductive to salt leaching out of the main root zone, i.e., within the 60 cm depth below ground surface in this area. Plot 2 was less prone to secondary salinization in comparison with Plot 1 due to the middle layer extending the distance between the root zone and groundwater. However, the water table rise of about 50-60 cm during the observation period implies the potential risk of secondary soil salinization and groundwater salinization. To prevent above phenomena, enhancing water-saving irrigation and improving current drainage systems are in urgent need.