Classic soil physics relies heavily on the concept of representative elementary volume (REV), which is necessary to perform upscaling from the studied soil samples and parameterize continuum scale hydrological models (e.g., based on Richards or Darcy equations). In this paper, we explore the boundaries of the classic REV concept and conventional representativity studies that claim REV for any given physical property, and that its values converge to a steady value with increasing sample volume. We chose two undisturbed soil samples of standard size from Ah and B horizons, subcropped two subvolumes within each of them and performed pore‐scale flow simulations using binarized X‐ray microtomography scans as input data. The volume of the simulation domains was 900³ voxels, with a physical volume within two orders of magnitude of the whole soil core. Based on the 3D pore geometry images and the resulting flow velocity and pressure fields, we performed REV analysis for the saturated hydraulic conductivity and porosity. Although in general subvolumes showed classical REV behaviour (convergence of porosity and saturated hydraulic conductance (K_sat)to plateau‐like behaviour), their flow properties converged to different REV values. We also evaluated the stationarity of pore structures by computing the directional correlation functions to explain the observed non‐unique behaviour. We concluded that neither of the studied samples can be considered to be representative due to their structural non‐stationarity. We extensively discussed the implications of these results for the upscaling and parameterization of continuum‐scale flow models. We argued that the plateau in K_sat (or any other physical property) is a necessary, but insufficient, condition for the REV, which requires (at least) pore structure stationarity as an additional criterion. REV as a concept is much broader than previously anticipated and the possibility of establishing REVs in structured soils will require an analysis of tensorial flow properties with correct boundary conditions, multiscale soil structure imaging with stationarity analysis, and pore‐scale simulations on fused multiscale images.

中文翻译：

---

孔隙结构的非平稳性如何影响土壤样品的流动性表征性（REV）：孔隙度模型和平稳性分析

经典土壤物理学在很大程度上依赖于代表性基本体积（REV）的概念，这对于从研究的土壤样本进行放大和参数化连续尺度水文模型（例如，基于Richards或Darcy方程）是必要的。在本文中，我们探索了经典REV概念和传统代表性研究的边界，这些研究声称REV具有任何给定的物理特性，并且随着样本量的增加，其值收敛于稳定值。我们从Ah和B层中选择了两个标准尺寸的原状土壤样品，在每个样品中细分了两个子体积，并使用二值化X射线显微断层扫描作为输入数据进行了孔尺度流动模拟。模拟域的数量为900 ³体素在整个土壤核心的两个数量级内的体素。基于3D孔隙几何图像以及由此产生的流速和压力场，我们对饱和的水力传导率和孔隙度进行了REV分析。尽管总的来说，子体积显示出经典的REV行为（孔隙度和饱和水力传导率的收敛（K _sat到高原样的行为），它们的流动特性收敛到不同的REV值。我们还通过计算方向相关函数来解释观察到的非唯一行为，从而评估了孔隙结构的平稳性。我们得出的结论是，由于结构不稳定，这两个研究样本均不能被认为具有代表性。我们广泛讨论了这些结果对连续尺度流模型的升级和参数化的影响。我们认为K的高原_坐着（或任何其他物理特性）对于REV是必要的但不充分的条件，它至少需要孔隙结构的平稳性作为附加标准。REV作为一个概念比以前预期的要广泛得多，并且在结构化土壤中建立REV的可能性将需要分析具有正确边界条件的张量流动特性，具有平稳性分析的多尺度土壤结构成像以及融合多尺度图像上的孔隙尺度模拟。