A Comparative Study of Conceptual Model Complexity to Describe Water Flow and Nitrate Transport in Deep Unsaturated Loess

• Published in: Water resources research 2021-08, Vol.57 (8), p.n/a
• Main Authors: Turkeltaub, Tuvia, Jia, Xiaoxu, Zhu, Yuanjun, Shao, Ming‐An, Binley, Andrew
• Format: Article
• Language: English
• Subjects: Adequacy; Advection; Agricultural practices; Aquifers; Calibration; Comparative analysis; Comparative studies; Complexity; Decay; deep loess vadose zone; Depth profiling; Fertilizers; Groundwater; Groundwater flow; Groundwater table; Heterogeneity; Hydraulic conductivity; Land management; Land use; Land use management; Loess; loess vertical variability; Mass balance; Mathematical analysis; Mathematical models; Modelling; Moisture content; nitrate transport; nitrate travel time; Nitrates; Numerical models; Physical properties; Representations; Risk assessment; Scaling; Scaling factors; Soil investigations; Soil properties; Storage; Transport; Unconfined aquifers; Vadose water; Variability; Vulnerability; Water content; Water flow; Water table
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2020WR029250

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. Plain Language Summary Enhanced concentration of nitrate in groundwater is a global problem. The source of such nitrate is commonly linked to agricultural practices, in particular the use of fertilizers. The intensive development of China in the last few decades has put at risk many groundwater systems, such as the unconfined aquifer of the Loess Plateau of China (LPC). Understanding the fate and travel times of nitrate through the LPC vadose zone before its arrival at the water table would help sustain groundwater systems and identify areas requiring changes in land use management. The current study examines various modeling approaches for nitrate transport estimations in the LPC vadose zone. Four modeling approaches with different complexities are tested: A simple mass‐balance model and more sophistic