Hydropedology and pedotransfer functions

The emerging interdisciplinary research field of hydropedology attracts a substantial attention because of its promise to bridging pedology and hydrology. Pedotransfer functions (PTFs) emerged as relationships between soil hydraulic parameters and the easier measurable properties usually available from soil survey. One hypothetical explanation of current PTF shortcomings is that PTF inputs do not describe the structure of pore space per se and, therefore, do not represent relationships between structure and function of soil pore space. A possible direction for improvement is to look for PTF predictors that are better related to the structure of water-bearing pathways, in particular using the pedological soil structure description. The objective of this work was to develop and discuss an example of pedotransfer function relating soil structure and soil hydrologic parameters. We used the subset of 2149 samples from the US National Soil Characterization database that had values of water contents at - 33 kPa and bulk densities on clods, structure characterized with grade, size and shape, textural class determined in the field and from lab textural analysis. Classification and regression trees were used to group soil samples according to their water contents at - 33 kPa. The clay class was the best grouping parameter in all but loamy sand textural classes. The structural parameters served as important grouping variables to define groups of soil samples with distinctly different average water retention for the groups. Defining and quantifying soil structure at various scales, including pedon, hillslope and watershed scales, may contribute for the development scale-relevant PTFs at those scales.