Abstract
Abstract. Soil water is an important driving force of the ecosystems, especially in the semiarid hill and gully region of the northwestern Loess Plateau in China. The mechanism of soil water migration in the reconstruction and restoration of Loess Plateau is a key scientific problem that must be solved. Isotopic tracers can provide valuable information associated with complex hydrological problems, difficult to obtain using other methods. In this study, the oxygen and hydrogen isotopes are used as tracers to investigate the migration processes of soil water in the unsaturated zone in an arid region of China's Loess Plateau. Samples of precipitation, soil water, plant xylems and plant roots are collected and analysed. The conservative elements deuterium (D) and oxygen (18O) are used as tracers to identify variable source and mixing processes. The mixing model is used to quantify the contribution of each end member and calculate mixing amounts. The results show that the isotopic composition of precipitation in the Anjiagou River basin is affected by isotopic fractionation due to evaporation. The isotopic compositions of soil waters are plotted between or near the local meteoric water lines, indicating that soil waters are recharged by precipitation. The soil water migration is dominated by piston-type flow in the study area and rarely preferential flow. Water migration exhibited a transformation pathway from precipitation to soil water to plant water. δ18O and δD are enriched in the shallow (< 20 cm depth) soil water in most soil profiles due to evaporation. The isotopic composition of xylem water is close to that of soil water at the depth of 40–60 cm. These values reflect soil water signatures associated with Caragana korshinskii Kom. uptake at the depth of 40–60 cm. Soil water from the surface soil layer (20–40 cm) comprised 6–12 % of plant xylem water, while soil water at the depth of 40–60 cm is the largest component of plant xylem water (ranging from 60 to 66 %), soil water below 60 cm depth comprised 8–14 % of plant xylem water and only 5–8 % is derived directly from precipitation. This study investigates the migration process of soil water, identifies the source of plant water and finally provides a scientific basis for identification of model structures and parameters. It can provide a scientific basis for ecological water demand, ecological restoration, and management of water resources.
Highlights
Water in the soil environment plays a crucial role as a carrier of dissolved and solid species and as a reservoir in the hydrological cycle
Compared with global meteoric water line (GMWL), the slope and intercept of local meteoric water line (LMWL) are lower than that of GMWL (Fig. 2), indicating that they are affected by the local climate and environment with less precipitation and lower humidity
The slope of LMWL is low, and the intercept of LMWL decreases with the slope
Summary
Water in the soil environment plays a crucial role as a carrier of dissolved and solid species and as a reservoir in the hydrological cycle. Soil water represents a small proportion (only 0.05 %) of the hydrological cycle, but it is vital for ecosystems and affects spatial and temporal processes at different scales (Koeniger et al, 2016; Busari et al, 2013). Understanding soil water migration in the unsaturated zone is essential to describe the movement of salt, carbon, nitrogen and other nutrients. The traditional methods have been carried out to study the movement of soil water such as hydrologic experiments, intensive observations, modeling and remote sensing (Luo et al, 2013; Yang et al, 2013; Carucci et al, 2012).
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