Abstract
Abstract. The vadose zone is a zone sensitive to environmental changes and exerts a crucial control in ecosystem functioning and even more so in cold regions considering the rapid change in seasonally frozen ground under climate warming. While the way in representing the underlying physical process of the vadose zone differs among models, the effect of such differences on ecosystem functioning and its ecohydrological response to freeze–thaw cycles are seldom reported. Here, the detailed vadose zone process model STEMMUS (Simultaneous Transfer of Energy, Mass and Momentum in Unsaturated Soil) was coupled with the ecohydrological model Tethys–Chloris (T&C) to investigate the role of influential physical processes during freeze–thaw cycles. The physical representation is increased from using T&C coupling without STEMMUS enabling the simultaneous mass and energy transfer in the soil system (liquid, vapor, ice) – and with explicit consideration of the impact of soil ice content on energy and water transfer properties – to using T&C coupling with it. We tested model performance with the aid of a comprehensive observation dataset collected at a typical meadow ecosystem on the Tibetan Plateau. Results indicated that (i) explicitly considering the frozen soil process significantly improved the soil moisture/temperature profile simulations and facilitated our understanding of the water transfer processes within the soil–plant–atmosphere continuum; (ii) the difference among various representations of vadose zone physics have an impact on the vegetation dynamics mainly at the beginning of the growing season; and (iii) models with different vadose zone physics can predict similar interannual vegetation dynamics, as well as energy, water, and carbon exchanges, at the land surface. This research highlights the important role of vadose zone physics for ecosystem functioning in cold regions and can support the development and application of future Earth system models.
Highlights
Recent climatic changes have accelerated the dynamics of frozen soils in cold regions, for instance, favoring permafrost thawing and degradation (Cheng and Wu, 2007; Hinzman et al, 2013; Peng et al, 2017; Yao et al, 2019; Zhao et al, 2019)
We investigated the consequences of considering coupled water and heat transfer processes on land surface fluxes and ecosystem dynamics in the extreme environmental conditions of the Tibetan Plateau, relying on land surface and ecohydrological models confronted with multiple field observations
The detailed vadose zone process model STEMMUS and the ecohydrological model T&C were coupled to investigate the effect of various model representations in simulating water and energy transfer and seasonal ecohydrological dynamics over a typical Tibetan meadow
Summary
Recent climatic changes have accelerated the dynamics of frozen soils in cold regions, for instance, favoring permafrost thawing and degradation (Cheng and Wu, 2007; Hinzman et al, 2013; Peng et al, 2017; Yao et al, 2019; Zhao et al, 2019). In most of the current modeling research in cold region ecosystems, the water and heat transfer process in the vadose zone remains independent and not fully coupled Such considerations of vadose zone physics might result in unrealistic physical interpretations, especially for soil freezing and thawing processes (Hansson et al, 2004). Promising simulation results have been reported for the soil hydrothermal regimes While these efforts mainly focus on understanding the surface and subsurface soil water and heat transfer process (Yu et al, 2018, 2020) and stress the role of physical representations of the freezing and thawing processes (Boone et al, 2000; Wang et al, 2017; Zheng et al, 2017), they rarely take into account the interaction with vegetation and carbon dynamics. The frozen soil physics was explicitly taken into account, and soil water and heat transfer were fully coupled to further facilitate the model’s capability in dealing with complex vadose zone processes
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