Soil water utilization patterns of Salix based on particle tracking and isotope hydrodynamics

This study aims to in situ monitor the soil water use patterns of the vadose zone in the Mu Us sandy land during the growing season (March-September). The water-heat-δ¹⁸O simulation was coupled with particle tracking. Three fractionation models were selected as the No Fractionation (NF), Gonfiantini (GF), and Craig-Gordon (CG). The results showed that the kinetic characteristics of δ¹⁸O in the study area were best in the GF model, compared with the NF and CG models. The evaporation of the soil water was characterized by a temperature-controlled isotope fractionation migration. The particle and δ18O migration paths were established to clarify the spatial and temporal occurrence of the soil water recharge-evaporation, according to the precipitation with the vegetation water uptake. When the precipitation was scarce in spring and summer, the evaporation intensity and drought duration were used to determine the evaporation rate and evaporation depth of soil water. Among them, the evaporation depth of the soil reached 0-120 cm and 0-135 cm in April-May and June-July, respectively, due mainly to the strong evaporation. In addition, the priority of the infiltration-evaporation process depended on the initial water content of the soil caused by the precipitation distribution. That is, the infiltration rate of the dry soil was slow after light rainfall recharge. The evaporation capacity was enhanced significantly, where the depth of evaporation (0-13 cm) was equal to the depth of infiltration. At the same time, all particles entering the soil with the precipitation were dominated by the evaporation to leave from the ground surface; The depth of infiltration (20 cm) was greater than that of evaporation (16 cm) after the effective recharge followed by light rainfall. As such, the soil was effectively recharged after the process. Therefore, the soil water in the root zone was sourced from the frequently small-medium precipitation and a single large precipitation event, and then passed through the evaporation susceptibility zone (0-40 cm), finally remaining at a depth of 40-90 cm for a long period of time. Water use strategies of Salix soil depended on the root distribution and shallow soil water availability. The ablated soil stagnant water from the shallow (0 to 40 cm) zone was the main source of water consumption during the rejuvenation season. If the precipitation was abundant during the growing season, the shallow soil water (0-40 cm) was still the main source of the water uptake. The monthly precipitation also fully met the growth demand; Once the precipitation was scarce, the soil water was reduced at the surface soil to transfer the uptake layer. The main source of water uptake was attributed to the middle soil water (60-100 cm) that was retained by the precipitation in the previous period. As such, the process further clarified the hydrological coupling of the precipitation-soil-vegetation. The finding can also provide a scientific basis for the effective management of water resources and vegetation restoration in the dry zone.