Field capacity (FC) is an important soil hydraulic concept in soil science and irrigation management. It is generally determined from soil water content in a soil layer when soil profile reaches a steady pressure head or negligible drainage flux from an initially saturated soil. However, the proposed criteria are mainly tested for uniform soils and vary with soil textures. To quantify FC in layered soils, a Richards equation-based model was used to describe water flow in fine-textured soils with a coarse interlayer. With calibrated soil hydraulic parameters for loam and sand from infiltration measurements, drainage from saturation was simulated in the loam with a sand interlayer. A relative drainage rate (δ) was defined as a function of water storage and drainage flux to analyze soil water status at FC. Soil water content in the upper loam layer of layered profiles was improved compared with that in the uniform loam, which was negatively correlated with buried depth but positively correlated with thickness of the sand layer for a specified δ. Under different buried depths and thicknesses, soil water content decreased with the decline of δ and decreased rapidly as δ reduced to 1 % d−1. The drainage flux at δ = 1 % d−1 changed within a range of 0.056–0.26 cm d−1, and soil water content reached to 0.278–0.346 cm3 cm−3, which accounted for 70–87 % of the saturated water content of loam. Although the FC in the upper fine-textured soil layer varied for different buried depths and thicknesses of coarse interlayer, the proposed dynamic method is reliable and universal to estimate the FC in the above layered soils at δ = 1 % d−1. An empirical equation was also developed to calculate the FC in fine-textured soils with different buried depths and thicknesses of a coarse interlayer based on the critical δ value.

中文翻译：

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粗夹层细腻土田场容量估算的动态和经验方法


田间容量 （FC） 是土壤科学和灌溉管理中的一个重要土壤水力概念。它通常是根据土壤剖面达到稳定压力水头或初始饱和土壤的排水通量可以忽略不计时根据土壤层中的土壤含水量来确定的。然而，所提出的标准主要是针对均匀的土壤进行测试，并随土壤质地而变化。为了量化分层土壤中的 FC，使用基于 Richards 方程的模型来描述具有粗夹层的细纹理土壤中的水流。使用渗透测量中校准的壤土和沙土水力参数，在带有沙夹层的壤土中模拟饱和排水。相对排水速率 （δ） 定义为储水和排水通量的函数，以分析 FC 的土壤水分状况。与均匀壤土相比，层状剖面上壤土层的土壤含水量有所提高，在规定δ与埋深呈负相关，与砂层厚度呈正相关。在不同埋藏深度和厚度下，土壤含水量随δ的降低而降低，δ降低至 1 % d−1 时迅速降低。δ = 1 % d-1 处的排水通量在 0.056-0.26 cm d-1 范围内发生变化，土壤含水量达到 0.278-0.346 cm3 cm-3，占壤土饱和含水量的 70-87%。尽管上部细腻土层中的 FC 因埋深和粗夹层厚度不同而变化，但所提出的动力学方法可以可靠且普遍地估计上述层状土中 δ = 1 % d−1 的 FC。 还开发了一个经验方程，以基于临界δ值计算具有不同埋深和粗夹层厚度的细纹理土壤中的 FC。