Comment on “Simple consistent models for water retention and hydraulic conductivity in the complete moisture range” by A. Peters

Abstract

Peters [2013] recently presented a new empirical model of soil hydraulic functions over the entire range of soil water potential. His model is based on established approaches for the retention and conductivity function, and assimilates recently gained knowledge about the shape of hydraulic functions in the medium and dry moisture range. Specifically, the retention function reaches a zero soil water content at a water potential corresponding to oven dryness and approaches this point by a linear decrease of water content with the logarithm of suction, which is in agreement with experimental data [Schneider and Goss, 2012] and physical models of water sorption on surfaces [Tokunaga, 2009]. The Peters [2013] retention function does not require more parameters than traditional models, i.e., it is fully parameterized by a saturated water content, a residual water content, and two parameters which describe the location and width of the pore-size distribution. The unsaturated hydraulic conductivity function represents the flow components capillary flow, film and corner flow, and isothermal vapor flow and requires one additional free parameter as compared to classic models. Peters' model hence offers great potential for modeling soil-water flow in the full moisture range while simultaneously keeping the number of model parameters at a minimum. The objective of this comment is to reference some shortcomings of the model formulation as published by Peters [2013] and to provide solutions to the following points which we regard as problematic: 2013] is not differentiable at suction h5h a [L], the suction below which X(h) is unity. As a result, the soil water capacity function is not continuous but has a discontinuity at h a . (8)). As X(h) is not differentiable at h a , the saturation function S cap is not continuously differentiable if the correction is applied. In cases where the capillary saturation function C(h)[ If the correction discussed under point 2 is applied, closed-form expressions of the conductivity functionbecome unavailable for the models of Kosugi and van Genuchten.4. It is possible that the correction of C(h) is turned on or off during parameter estimation depending on the possible combination of the two parameters describing the pore-size distribution. Unfortunately, the correction S cap ðhÞ5XðhÞCðhÞ changes a large portion of the retention curve and this may affect the performance of the iterative minimization algorithm by generating spurious secondary minima and discontinuities in the objective function [see Kavetski et al., 2007, for examples from rainfall-runoff modeling].In the following, we present solutions to these problems by proposing two modifications of Peters's [2013] retention function and present parameter estimation results for the 10 soils analyzed by him using the new model, which we refer to as Peters-Durner-Iden (PDI) model.

A pore-size classification for peat bogs derived from unsaturated hydraulic properties

“…The model is physically more comprehensive in that it ensures zero water content at oven dryness, and accounts for water flow in completely filled capillaries, partly filled capillaries, adsorbed water films, and isothermal vapour diffusion. We used the parameterization derived by Peters (2013), in the modified form published by Iden and Durner (2014), and Peters (2014). The PDI retention curve model has an identical number of free parameters as the VGM.…”

“…The WRC of the PDI model family (Peters, 2013(Peters, , 2014Iden and Durner, 2014) is given by the equation…”

“…Parameter b is a smoothing parameter for which empirical equations are given in Iden and Durner (2014). The HCC is defined as…”

Further tests of the HYPROP evaporation method for estimating the unsaturated soil hydraulic properties

“…Peters and Durner (2008) and Peters et al (2015) tested the HMS approach assuming synthetic (virtual) data for the two pressure heads within the sample, as well as for the evaporation rate (and hence the sample weight), as a function of time as generated with numerical solutions of the Richards equation. Peters and Durner (2008) used for this purpose the hydraulic functions of van Genuchten-Mualem (1980), while Peters et al (2015) used the equations of Kosugi (1996) as well as the PDI model of Peters and colleagues (Iden and Durner, 2014;Peters, 2013Peters, , 2014 to account for the effects of film, corner and isothermal vapor flow in very dry soils.…”

A Modified van Genuchten-Mualem Model of Hydraulic Conductivity in Korean Residual Soils

“…The samples belong to each group due to the presence of some fine-grained soil and residual formation environments. The low values of the tortuosity factor can be related to adsorptive water retention and film conductivity [20,21], which is beyond the scope of this study. For each group, the relative hydraulic conductivity, K r , for the modified VGM model was first compared with that of the VGM model and subsequently with the measured value.…”