[1] In the past two decades significant progress has been made toward the application of inverse modeling to estimate the water retention and hydraulic conductivity functions of the vadose zone at different spatial scales Many of these contributions have focused on estimating only a few soil hydraulic parameters, without recourse to appropriately capturing and addressing spatial variability The assumption of a homogeneous medium significantly simplifies the complexity of the resulting inverse problem, allowing the use of classical parameter estimation algorithms Here we present an inverse modeling study with a high degree of vertical complexity that involves calibration of a 25 parameter Richards'-based HYDRUS-1D model using in situ measurements of volumetric water content and pressure head from multiple depths in a heterogeneous vadose zone in New Zealand We first determine the trade-off in the fitting of both data types using the AMALGAM multiple objective evolutionary search algorithm Then we adopt a Bayesian framework and derive posterior probability density functions of parameter and model predictive uncertainty using the recently developed differential evolution adaptive metropolis, DREAMZS adaptive Markov chain Monte Carlo scheme We use four different formulations of the likelihood function each differing in their underlying assumption about the statistical properties of the error residual and data used for calibration We show that AMALGAM and DREAMZS can solve for the 25 hydraulic parameters describing the water retention and hydraulic conductivity functions of the multilayer heterogeneous vadose zone Our study clearly highlights that multiple data types are simultaneously required in the likelihood function to result in an accurate soil hydraulic characterization of the vadose zone of interest Remaining error residuals are most likely caused by model deficiencies that are not encapsulated by the multilayer model and can not be accessed by the statistics and likelihood function used The utilization of an explicit autoregressive error model of the remaining error residuals does not work well for the water content data with HYDRUS-1D prediction uncertainty bounds that become unrealistically large

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2010WR009265

Multiresponse multilayer vadose zone model calibration using Markov chain Monte Carlo simulation and field water retention data

https://abe.ufl.edu/faculty/carpena/files/pdf/research/reports/geoder_10759.pdf

Field-scale spatial variability of saturated hydraulic conductivity on a recently constructed artificial ecosystem

Examination of groundwater recharge with a calibrated/validated flow model of the deep vadose zone

https://escholarship.org/content/qt0jt9d92t/qt0jt9d92t.pdf?t=okav3y

Assessing the effectiveness of drywells as tools for stormwater management and aquifer recharge and their groundwater contamination potential

Can Electrical Resistance Tomography be used for imaging unsaturated moisture flow in cement-based materials with discrete cracks?

StaƟ sƟ cal analysis and interpretaƟ on of heterogeneous sediment hydraulic properƟ es is important to produce reliable forecasts of water and solute transport dynamics in the unsaturated zone. Most fi eld characterizaƟ ons to date have focused on the shallow 2-m root zone. We characterized the geologic and hydraulic properƟ es of a 16-m-deep, alluvial vadose zone consisƟ ng of unconsolidated sediments typical of the alluvial fans of the eastern San Joaquin Valley, California. The thickness of individual beds varies from 2.5 m for large sandy deposits associated with buried stream channels. Eight major geologic units (lithofacies) have been idenƟ fi ed at the site. Unsaturated hydraulic properƟ es were obtained from mulƟ step ouƞ low experiments on nearly 100 sediment cores. MulƟ variate analysis of variance and post hoc tesƟ ng show that lithofacies and other visual- and texture-based sediment classifi caƟ ons explain a signifi cant amount of the spaƟ al variability of hydraulic properƟ es within the unsaturated zone. GeostaƟ sƟ cal analysis of hydraulic parameters show spaƟ al conƟ nuity of within-lithofacies variability in the horizontal direcƟ on in the range of 5 to 8 m, which is approximately an order of magnitude larger than spaƟ al conƟ nuity in the verƟ cal direcƟ on. Low nugget/sill raƟ os suggest that 1- to 10-m sampling intervals are adequate for detecƟ on of horizontal spaƟ al structure. The existence of thin clay or silt layers within lithofacies units results in only moderate spaƟ al conƟ nuity in the verƟ cal direcƟ on, however, suggesƟ ng inadequate sampling frequency for hydraulic parameter variogram development in that direcƟ on.

Spatial Variability of Hydraulic Properties and Sediment Characteristics in a Deep Alluvial Unsaturated Zone

Heterogeneity in unsaturated soils and sediments is well known to exist at different scales, from microscopic scale to macroscopic scale. Characterization of different types of heterogeneity in deep vadose zones is challenging because of the usual lack of information at such sites. In this paper, we considered a site with detailed geological, chemical, and hydraulic properties measurements throughout an approximately 16-m deep vadose zone consisting of unconsolidated, alluvial deposits typical for the alluvial fans of the eastern San Joaquin Valley, California. At the agricultural site, data were also available for a 7-yr long field fertilization experiment that we used to independently estimate the amount of nitrate stored within the vadose zone at the end of the experiment. Simple mass balance calculations were performed and compared to six conceptually different two-dimensional and three-dimensional vadose zone numerical models that were implemented to represent varying degrees of hierarchical details of heterogeneity. Despite widely differing structure and heterogeneity of unsaturated flow, all models resulted in a narrow range of estimated nitrate storage in the deep vadose zone for near-cyclically repeated water and nitrogen fertilizer applications. Simulated nitrate storage was found to be approximately six to eight times larger than the measured storage at the field. Simulated nitrate variability, while qualitatively similar in pattern, was considerably lower than measured, despite the large simulated hydraulic variability. This study underscores that physical heterogeneity of deep vadose zones may have limited effects on the transfer of conservative contaminants applied repeatedly to the land surface. It also raises questions about our understanding of the chemical fate of nitrate in the vadose zone; and suggests the presence of a significant immobile moisture domain within the deep vadose zone that is not explainable by heterogeneity of Richards equation parameters, yet needs to be considered for simulating nitrate transport under conditions of cyclical infiltration with gravity dominated convective flux.

https://acsess.onlinelibrary.wiley.com/doi/pdf/10.2136/vzj2011.0145

Farag E. Botros

Papers

Spatial Variability of Hydraulic Properties and Sediment Characteristics in a Deep Alluvial Unsaturated Zone

Richards Equation–Based Modeling to Estimate Flow and Nitrate Transport in a Deep Alluvial Vadose Zone