Submitted to: European Journal of Soil Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/13/2002
Publication Date: 5/9/2003
Citation: Pachepsky, Y.A., Rawls, W.J. 2003. Soil structure and pedotransfer functions. European Journal of Soil Science. 54:443-452. Interpretive Summary: Accurate estimates of soil hydraulic properties from other soil parameters with pedotransfer functions (PTF) are in demand in many applications. Soil structural parameters, albeit rarely used, are natural candidates for improving PTFs. Our objective was to outline and illustrate possible avenues of using data on soil structure for that purpose. First, we show that soil structural data, which are reported by soil surveys by classes rather with numbers, can be successfully used to develop and improve PTF with a novel regression technique of regression trees. Second, we demonstrate an inherent scale-dependence in soil hydraulic properties, i.e. possibility of values of the same soil hydraulic parameter to depend on the size of the sample and measurement method. Corrections in PTFs developed from lab samples appear to be necessary to use the PTFs in file-scale projects. Finally, we show that the soil water transport equations can be modified to include the effects of soil structure on water transport with no explicit structure characterization needed. Research needs are discussed, including quantitative characterization of the field soil structure, an across-scale modeling of soil structure to use fine-scale data for coarse-scale PTFs, understanding effects of soil structure on the performance of differences methods to measure soil hydraulic properties, and using soil-landscape relationships to estimate variations of soil hydraulic properties across large spatial units.
Technical Abstract: Accurate estimates of soil hydraulic properties from other soil parameters with pedotransfer functions (PTF) are in demand in many applications, and soil structural parameters are natural candidates for improving PTFs. Soil survey provides mostly categorical data about soil structure, soil structure is inherently hierarchical, many available parameters, i.e., bulk kdensity, aggregate distribution, and penetration resistance, reflect not only structural but also other soil properties. Our objective was to outline and illustrate ways of using both structural information and the notion about the existence of structural hierarchy per se in modeling water transport in soils. Three case studies are presented. The observed combined effect of structure grade and size on soil water retention indicates that models of scaling in aggregate and particle distribution may introduce new useful parameters to use in PTFs. Differences in field and laboratory soil water retention have a deterministic component that can be explained using Rieu and Sposito theory to simulate scaling in soil structure. A scale- dependence in hydraulic diffusivity found in infiltration data can be viewed as the consequence of the hierarchy in structure and simulated with a fractional Fokker-Plank equation. Research needs are discussed, including quantitative characterization of the field soil structure, an across-scale modeling of soil structure to use fine-scale data for coarse-scale PTFs, and using soil-landscape relationships to estimate variations of soil hydraulic properties across large spatial units