Acting as a limitation factors for the vegetation growth, soil water is crucial to the hydrological cycles in semiarid environments on the Loess Plateau of China (LPC). The Loess Plateau has the greatest erosion rate in the world. One of the causes of soil erosion is excessive use of land [1
]. With the large-scale “Grain for Green” implementation on the Loess Plateau, many of the sloping croplands were converted to forestland, shrubland, and grassland. The world is trying to achieve the sustainable goals for development by the United Nations and achieve the land degradation neutrality via the use of restoration [3
]. Land use type is one of the main driving forces affecting the evolution of soil and water resources in unsaturated zones [5
]. Understanding the movement of soil water in unsaturated zones in different land use types is important for understanding the distribution and cycling of water, salt, and other nutrients, and for improving land management for controlling erosion and runoff in revegetation and ecological construction [9
]. However, the influence of land use for the soil water movement through unsaturated zones on the Loess Plateau has been reported in a few papers. Accordingly, research on soil water with land use types play an important role in the mechanism of soil water movement and soil water supply system on the Loess Plateau. The key research of ecohydrological started to delve into the relationship between soil water and subsurface mixing, as well as its interaction and feedback with the ecosystem [11
]. Little attention, however, has been paid to the effect of revegetation on soil water movement in unsaturated zones on the Loess Plateau.
The stable hydrogen and oxygen isotopes of soil water have been used to study the infiltration, evaporation, and mixing processes and quantitatively evaluates the groundwater recharge and evaporation rates, which is hard to get by other technologies [14
]. As isotope ratios of oxygen and hydrogen in soil water impact these ratios differently, they can be used to distinguish between transpiration and evaporation. The evaporation enriches the soil water of oxygen and hydrogen isotopes, and the transpiration process does not fractionate isotopes [17
]. Many early studies have reported oxygen isotope- and hydrogen isotope-enrichment in soil water near the surface [21
]. A seminal study by Landwehr and Coplen developed a new method that is helpful in deriving isotope fractionation due to soil evaporation [23
]. A review on groundwater recharge estimates via stable isotope of the unsaturated zone was provided by Koeniger et al. [24
]. The movement of soil water in unsaturated zone under various climate conditions, inclusive of arid and semiarid, has been studied to contrast movement in various processes such as the infiltration of precipitation, mixing, and evapotranspiration in a profile of soil [25
]. The stable hydrogen and oxygen isotope have been used for the identification of spring and stream water sources in high mountain regions, for the identification of flow properties, and hydrodynamic parameters of springs in karstic environments [26
]. The stable isotopes have also been identified for the contribution of rainfall to groundwater discharge in San Vittorino Plain, for the identification of processes, structure, and status of the groundwater system [30
]. The flow of soil water in unsaturated zones, however, is complex and affected by vegetation cover, wetness, structure, and soil texture. Infiltration of precipitation and subsequent downward percolation are described to be piston and preferential flow [32
]. For piston flow, the soil water produced from the recent precipitation will reduce the amount of residual soil water that is older, and is normally combined with it [35
]. Preferential flows occur in unsaturated zones through macropores attributed to forces, inclusive of earthworm burrows, decayed plant roots, and cracks [36
]. Different flow mechanisms result in different isotope profile. The use of oxygen and hydrogen stable isotopic compositions for tracing the soil water movement in unsaturated zones and to determine the characteristics of infiltration in a soil profile, however, has rarely been applied on the Loess Plateau under revegetation and ecological construction as a function of time.
We examined the seasonal variations in stable oxygen isotope of soil water in unsaturated zones and precipitation for four typical land uses on the Loess Plateau to identify the factors controlling soil water movement within the unsaturated zone. The objectives of the present study are: (1) To determine the temporal and spatial variations of soil water content; (2) to analyze stable oxygen isotopes of the precipitation and differences in profile soil water δ18O for four land use types under revegetation and ecological construction; and (3) to study the infiltration mechanisms and influence factors in unsaturated zones based on δ18O value characteristics in the study area.