Citation:

He, J., Mohamed M. Hantush, L. Kalin, AND S. Isik. Two-Layer numerical model of soil moisture dynamics: Model assessment and Bayesian uncertainty estimation. JOURNAL OF HYDROLOGY. Elsevier Science Ltd, New York, NY, 613 part A:128327, (2022). https://doi.org/10.1016/j.jhydrol.2022.128327

Impact/Purpose:

Modeling water movement in variably saturated soils with relatively shallow water table is important in land system models. A two-layer approximation of Richards’ equation (RE) was recently developed by the authors for this purpose and its utility was shown for few simple cases. This study presents a comprehensive assessment of the two-layer RE model. First, the two-layer model was evaluated for 231 soil textures under varying soil layer thicknesses, with a prescribed upper boundary and two bottom boundary conditions. The vertical soil profile was assumed to be uniform. Second, the two-layer model was tested for conditions where the top and bottom soil layers have contrasting hydraulic characteristics. For this, we tested two extreme conditions with a highly permeable soil overlying a low permeability soil and vice versa. For the first two cases, the performance of the two-layer mod 23 el was evaluated by comparing the results of volumetric soil moisture contents and fluxes from the two-layer model and HYDRUS. Last, a case study of model application at a Soil Climate Analysis Network (SCAN) site was presented. The application was combined with the Bayesian Monte Carlo (BMC) method for model calibration and uncertainty analysis. Computed vertically averaged soil moistures of two delineated soil layers were compared with observed data. Results showed that when dealing with a homogeneous soil profile, the two-layer model had excellent performance. 99.8% and 87.5% of the 92,400 simulations had root mean square error (RMSE) of moisture contents smaller than 0.015 for free-drainage and zero-pressure head bottom boundary conditions, respectively. With heterogeneous soil profiles, the soil moisture contents and fluxes from the two-layer model agreed well with those from HYDRUS (Šim?nek et al., 2008). The two-layer model combined with the BMC method showed good agreement with the observed average soil moisture of the root zone and the vadose zone below. Soil moisture observation data, hydroclimate data, and model structural uncertainties contributed to the overall model uncertainty the most. The model input parameters had minimal contribution to the overall model uncertainty. The posterior parameter space and their likelihood values obtained by BMC in the calibration process were used for model validation. During model validation, the overall RMSE was smaller than 0.02340 and ENS was greater than 0.72 for both soil layers.

Description:

A two-layer model based on the integrated form of Richards’ equation (RE) was recently developed to simulate the soil water movement in the roots layer and the vadose zone with a relatively shallow and dynamic water table. The model simulates thickness-averaged volumetric water content and matric suction as opposed to point values and was numerically verified for three soil textures using HYDRUS as a benchmark. However, the strengths and limitations of the two-layer model and its performance in stratified soils and under actual field conditions have not been tested. This study further examined the two-layer model using two numerical verification experiments and, most importantly, tested its performance at site-level under actual, highly variable hydroclimate conditions. Moreover, model parameters were estimated and uncertainty and sources of errors were quantified using a Bayesian framework. First, the two-layer model was evaluated for 231 soil textures under varying soil layer thicknesses with a uniform soil profile. Second, the two-layer model was assessed for stratified conditions where the top and bottom soil layers have contrasting hydraulic conductivities. 

URLs/Downloads:

Record Details: