Recharge mechanism of deep soil water and the response to land use change in the loess deposits

Abstract

Deep soil water (DSW) is of utmost importance for plant growth and groundwater recharge, but the depletion of DSW has been largely reported due to land use change, especially because of the conversion from shallow- to deep-rooted plants. To answer the question whether DSW can be replenished, it is important to reveal the recharge mechanism of DSW under different vegetation types. China’s Loess Plateau has experienced drastic land use change resulting in soil water depletion in deep vadose zones, which provides a good platform to explore these issues. We collected 380 soil samples from five boreholes up to 18 m deep covered by five plants (grassland, apricot, pine, poplar, and peashrub), and determined the contents, stable and radioactive isotopes (δ2H, δ18O, and 3H) of soil water. DSW, defined as soil water below 3 m in this study, was mainly recharged by the wet events in rainy season, especially those with intensities ≥ 50 mm/day, through piston flow. But they were significantly influenced by land use types. Specifically, the forestland and shrubland had lower evaporation effects and higher precipitation offset, and their soil water was recharged by wet events with higher intensities than those under grassland. Nevertheless, the deep-rooted plants substantially reduced recharge rates due to excessive root water uptake, and thus prevented rainfall from replenishing DSW. This study has implications to groundwater sustainability in those regions with limited water resources, drastic land use change, and thick unsaturated zones.