Analyzing dual porosity in soil hydraulic properties using soil databases for pedotransfer function development

Current databases of soil hydraulic properties (SHPs) have typically been used to develop pedotransfer functions (PTFs) to estimate water retention [theta(h)] assuming a unimodal pore-size distribution. However, natural soils often show the presence of bimodal to multimodal pore-size distributions. Here, we used three widely spread databases for PTF development: UNsaturated SOil hydraulic DAtabase (UNSODA) 2.0, Vereecken, and European hydropedological data inventory (EU-HYDI), to analyze the presence of structural effects in both theta(h) and hydraulic conductivity [K(h)]. Only undisturbed samples were included in the analysis that contained enough datapoints for both theta(h) and K(h) properties, especially in the wet range. One-hundred ninety-two samples were suitable for our analysis, which is only 1% of the total samples in the three databases. Results showed that 65% of the samples exhibited a bimodal pore-size distribution, and bimodality was not limited to fine-textured but also coarser-textured soils. The Mualem-van Genuchten (MvG) expression for both unimodal and bimodal soils was not able to predict the observed unsaturated K. Only a joint fitting of measured theta(h) and K(h) functions provided parameter estimates that were able to describe unsaturated K for uni- and bimodal soils. In addition, we observed a negative relationship between alpha and n in the case of low sand content (<52%) for both unimodal and bimodal matrix domain properties, contradicting the classical notion. The ratio of alpha for the macropore and matrix domain was positively correlated with the fraction of macropores and sand content. We anticipate that the results will contribute to deriving PTF for structured soils and avoid unrealistic combinations of MvG parameters.