Migration process of soil water in the unsaturated zone of the Loess Plateau

The mechanism of soil water migration in the reconstruction and restoration of the Loess Plateau is a key scientific problem that must be solved. Isotopic tracers can provide valuable information associated with complex hydrological problems and are difficult to obtain by other methods. This study investigated soil water migration in the unsaturated zone in the hilly and gully region of the Loess Plateau. Samples were collected from the 0-150 cm unsaturated zone, including precipitation, soil water content, plant xylems, and plant roots, and were analysed. The results showed that the isotopic composition of precipitation in the Yangjuangou River basin was affected by evaporation, as indicated by isotopic fractionation phenomena. The δD and δ¹⁸O isotopic profiles of the soil water exhibited monthly variations. The soil water was recharged by precipitation. The soil water and precipitation were taken up by shrubland. Water migration exhibited a transformation pathway from precipitation to soil water to plant water. Shrubland root systems were distributed in the shallow layer. Root and water uptake increased at a depth of 20-40 cm. The isotopic composition of xylem water was close to that of soil water at a depth of 20-40 cm. These values reflect soil water signatures associated with shrubland uptake at a depth of 20-40 cm. This study investigate the migration process of soil water, identify the source of plant water use, and provide a scientific basis for the migration process of soil water, including model structure and parameter identification.