A Comparative Study of Conceptual Model Complexity to Describe Water Flow and Nitrate Transport in Deep Unsaturated Loess
A Comparative Study of Conceptual Model Complexity to Describe Water Flow and Nitrate Transport in Deep Unsaturated Loess
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DOI：
10.1029/2020wr029250
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发表时间：
2021-08
影响因子：
5.4
通讯作者：
T. Turkeltaub;X. Jia;Yuanjun Zhu;M. Shao;A. Binley
中科院分区：
文献类型：
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作者：
T. Turkeltaub;X. Jia;Yuanjun Zhu;M. Shao;A. Binley
了解硝酸盐通过深层渗透带的迁移对含水层脆弱性评价至关重要。深渗透带物理性质的变化对硝酸盐运移的影响研究甚少。近年来，对中国黄土高原渗透带的深层硝酸盐储存剖面进行了研究。利用这些观测结果和实测的土壤性质，本研究探讨了黄土垂直非均质性对水分运动和硝酸盐在深层渗透带中运移的影响。建立并校准了不同复杂程度的模型。首先，将简单的活塞流量和硝酸盐质量平衡方法校准为观察到的硝酸盐储存。结果表明：总硝酸盐储量估算较好，而硝酸盐分布估算相对较差。随后，对Richards方程和平流-色散方程进行了计算。根据深层渗透带硝酸盐和含水量的观测，对数值模型的三种不同概念进行了校正：(1)假设黄土渗透带均质的单层模型；(2)考虑水力导率（Ks）衰减函数的模型；(3)规定Miller - Miller比例因子来解释水力函数随深度变化的模型。在数值模型中考虑垂直k衰减改善了水流性能。研究表明，采用水流和硝态氮输运数值模型以及简单的黄土垂直变异性来模拟黄土深渗透带环境中硝态氮的迁移是适当的。
Understanding nitrate migration through the deep vadose zone is essential for aquifer vulnerability assessments. The effect of variability of physical properties of the deep vadose zone on nitrate transport has been scarcely explored. Recently, deep nitrate storage profiles were determined in the vadose zone of the Loess Plateau of China. Using these observations along with measured soil properties, this study investigates the effect of loess vertical heterogeneity on water movement and nitrate transport through the deep vadose zone. Models of different complexity were established and calibrated. First, a simple piston flow and nitrate mass balance approach was calibrated to the observed nitrate storage. The results indicate that the total nitrate storage is estimated well, while the estimation of the distribution of nitrate is relatively poor. Subsequently, Richards' equation and the Advection‐Dispersion equation were evaluated. Three different conceptualizations of the numerical models were calibrated against deep vadose zone nitrate and water content observations: (1) one‐layer model assuming homogenous loess vadose zone; (2) a model that considers a hydraulic conductivity (Ks) decay function and (3) a model where the Miller‐Miller scaling factors are prescribed to account for changes of the hydraulic functions with depth. Accounting for the vertical Ks decay in the numerical models improved water flow performances. The study reveals the adequacy of implementing water flow and nitrate transport numerical models together with a simple representation of the vertical loess variability, for simulating nitrate migration in loess deep vadose zone environments.