An isotope study (δ18O and δD) of water movements on the Loess Plateau of China in arid and semiarid climates

Abstract

Precipitation infiltration and evaporation are the main controlling factors on soil water content (SWC) in the Chinese Loess Plateau (CLP). However, the temporal and spatial variations of soil water in the CLP are still unclear. Here, we investigate the stable isotope compositions (δ18O and δD) of soil water for five different vegetation cover types in the central CLP, to trace the dynamics and movement mechanisms of soil water. Our results show that the depth of precipitation infiltration is approximately 120cm in five different vegetation cover types under natural rainfall conditions throughout the year. The rapid 18O-enrichment of shallow (<30cm depth) soil water, which is observed in all profiles, indicates that the evaporation effect mainly occurs in the shallow layer. The δ18O isotope dynamic pattern between 30 and 120cm depth is probably controlled by the precipitation infiltration characteristics at a mean annual precipitation of 572.4mm. In contrast, deep (>120cm depth) soil water is in a steady state in our study period, which suggest that the residence times of this water can be several months or more. Although the vegetation cover types can affect the profile dynamics of δ18O, we find that variations in seasonal precipitation are the key factor that influences the profile dynamics of δ18O, which is attributed to the large differences in the climate parameters and the frequency of rainfall. We suggest that δ18O is more sensitive in tracing the precipitation infiltration depth and recharge mechanisms of soil water than the soil water content. Further observation over a much longer time scale and an combination of both the oxygen and hydrogen isotope compositions of soil water in the CLP would provide more insight into role of isotopic techniques in tracing the soil water cycle.