This study focuses on the reliable parametrization of the full Soil Water Retention Curve (SWRC) from saturation to oven-dryness using high resolution but limited range measured water retention data by the Hydraulic Property Analyzer (HYPROP) system. We studied the performance of five unimodal water retention models including the Brooks and Corey model (BC model), the Fredlund and Xing model (FX model), the Kosugi model (K model), the van Genuchten constrained model with four free parameters (VG model), and the van Genuchten unconstrained model with five free parameters (VGm model). In addition, eleven alternative expressions including Peters–Durner–Iden (PDI), bimodal, and bimodal-PDI variants of the original models were evaluated. We used a data set consisting of 94 soil samples from Turkey and the United States with high-resolution measured data (a total of 9264 measured water retention data pairs) mainly via the HYPROP system and supplemented for some samples with measured dry-end data using the WP4C instrument. Among unimodal expressions, the FX and the K models with the Mean Absolute Error (MAE) values equal to 0.005 cm3 cm−3 and 0.015 cm3 cm−3 have the highest and the lowest accuracy, respectively. Overall, the alternative variants provided a better fit than the unimodal expressions. The unimodal models, except for the FX model, fail to provide reliable dry-end estimations using HYPROP data (average MAE: 0.041 cm3 cm−3, average r: 0.52). Our results suggested that only models that account for the zero water content at the oven dryness and properly shift from the middle range to dry-end (i.e., the FX model and PDI variants) can adequately represent the full SWRC using typical data obtained via the HYPROP system.

中文翻译：

---

研究多种土壤保水模型的单峰、双峰、PDI 和双峰-PDI 变体：I. 使用扩展蒸发和露点方法的直接模型拟合

本研究的重点是使用由水力特性分析仪 (HYPROP) 系统测量的高分辨率但范围有限的保水数据，对从饱和到烘箱的完整土壤保水曲线 (SWRC) 进行可靠的参数化。我们研究了五种单峰保水模型的性能，包括 Brooks and Corey 模型（BC 模型）、Fredlund 和 Xing 模型（FX 模型）、Kosugi 模型（K 模型）、具有四个自由参数的 van Genuchten 约束模型（VG模型），以及具有五个自由参数的 van Genuchten 无约束模型（VGm 模型）。此外，还评估了 11 个替代表达式，包括原始模型的 Peters-Durner-Iden (PDI)、双峰和双峰-PDI 变体。我们使用了一个由来自土耳其和美国的 94 个土壤样品组成的数据集，其高分辨率测量数据（共 9264 个测量保水数据对）主要通过 HYPROP 系统，并补充了一些具有测量干端数据的样品，使用WP4C 仪器。在单峰表达式中，平均绝对误差 (MAE) 值等于 0.005 cm3 cm-3 和 0.015 cm3 cm-3 的 FX 和 K 模型分别具有最高和最低准确度。总体而言，替代变体提供了比单峰表达式更好的拟合。除 FX 模型外，单峰模型无法使用 HYPROP 数据（平均 MAE：0.041 cm3 cm-3，平均 r：0.52）提供可靠的干端估计。