Studying Unimodal, Bimodal, PDI and Bimodal-PDI Variants of Multiple Soil Water Retention Models: II. Evaluation of Parametric Pedotransfer Functions Against Direct Fits

Amir Haghverdi,Wolfgang Durner,Hasan Sabri Öztürk

doi:10.3390/w12030896

Abstract

A high-resolution soil water retention data set (81 repacked soil samples with 7729 observations) measured by the HYPROP system was used to develop and evaluate the performance of regression parametric pedotransfer functions (PTFs). A total of sixteen soil hydraulic models were evaluated including five unimodal water retention expressions of Brooks and Corey (BC model), Fredlund and Xing (FX model), Kosugi (K model), van Genuchten with four free parameters (VG model) and van Genuchten with five free parameters (VGm model). In addition, eleven bimodal, Peters–Durner–Iden (PDI) and bimodal-PDI variants of the original expressions were studied. Six modeling scenarios (S1 to S6) were examined with different combinations of the following input predictors: soil texture (percentages of sand, silt and clay), soil bulk density, organic matter content, percent of stable aggregates and saturated water content (θs). Although a majority of the model parameters showed low correlations with basic soil properties, most of the parametric PTFs provided reasonable water content estimations. The VGm parametric PTF with an RMSE of 0.034 cm3 cm−3 was the best PTF when all input predictors were considered. When averaged across modeling scenarios, the PDI variant of the K model with an RMSE of 0.045 cm3 cm−3 showed the highest performance. The best performance of all models occurred at S6 when θs was considered as an additional input predictor. The second-best performance for 11 out of the 16 models belonged to S1 with soil textural components as the only inputs. Our results do not recommend the development of parametric PTFs using bimodal variants because of their poor performance, which is attributed to their high number of free parameters.

Highlights

The term pedotransfer function (PTF) was first proposed by Bouma and van Lanen [1] to describe regression models formulating relationships between soil properties
We developed and evaluated a set of parametric PTFs for a total of 16 soil water retention models, including five original unimodal models and their PDI, bimodal and bimodal-PDI variants
Our results showed that considering saturated water content as an additional input predictor enhanced the performance of the parametric PTFs for all models

Summary

Introduction

The term pedotransfer function (PTF) was first proposed by Bouma and van Lanen [1] to describe regression models formulating relationships between soil properties. Following their proposition, a majority of the published research in this area has focused on estimating soil hydraulic properties, mainly on individual water retention points and the soil water retention curve (SWRC). Point pedotransfer functions (Group 1 PTFs, [2]) are developed to estimate the soil water content at one or several predefined soil tensions (typically field capacity and permanent wilting point). Parametric PTFs (Group 2 PTFs, [2]) are the most widely used models for continuous estimation of the soil water content over a wide range of soil tensions. Haghverdi et al [3]

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