Soil structure and pedotransfer functions Y. A. P ACHEPSKY a & W. J. R AWLS b a USDA Animal Waste Pathogen Laboratory, Building 173, BARC-EAST, Beltsville, MD 20705, and b USDA Hydrology and Remote Sensing Laboratory, Building 007, Room 104, BARC-WEST, Beltsville, MD 20705, USA Summary Accurate estimates of soil hydraulic properties from other soil characteristics using pedotransfer func- tions (PTFs) are in demand in many applications, and soil structural characteristics are natural candi- dates for improving PTFs. Soil survey provides mostly categorical data about soil structure. Many available characteristics such as bulk density, aggregate distribution, and penetration resistance reflect not only structural but also other soil properties. Our objective here is to provoke a discussion of the value of structural information in modelling water transport in soils. Two case studies are presented. Data from the US National Pedon Characterization database are used to estimate soil water retention from categorical field-determined structural and textural classes. Regression-tree estimates have the same accuracy as those from textural class as determined in the laboratory. Grade of structure appears to be a strong predictor of water retention at 33 kPa and 1500 kPa. Data from the UNSODA database are used to compare field and laboratory soil water retention. The field-measured retention is significantly less than that measured in the laboratory for soils with a sand content of less than 50%. This could be explained by Rieu and Sposito’s theory of scaling in soil structure. Our results suggest a close relationship between structure observed at the soil horizon scale and structure at finer scales affecting water retention of soil clods. Finally we indicate research needs, including (i) quantitative characterization of the field soil structure, (ii) an across-scale modelling of soil structure to use fine-scale data for coarse-scale PTFs, (iii) the need to understand the effects of soil structure on the performance of various methods available to measure soil hydraulic properties, and (iv) further studies of ways to use soil–landscape relationships to estimate variations of soil hydraulic properties across large areas of land. Introduction Soil hydraulic properties have many potential users. In hydrol- ogy, soil water retention and transport characteristics are used to partition precipitation into runoff and infiltration and to assess evapotranspiration. In agronomy, the same data are used to schedule management practices, especially irrigation and chemical application. In meteorology, surface soil mois- ture is used to establish components of the heat balance. In contaminant hydrology and geochemistry, estimates of hydraulic properties in the vadose zone are essential for esti- mating contaminant transport. Soil hydraulic measurements are time-consuming and become impractical when hydro- logical estimates are needed for large areas. Generations of researchers have quantified and interpreted relationships between soil hydraulic properties and data available from soil survey. Terms such as ‘predicting soil properties’, ‘estimat- ing soil properties’, and ‘correlating soil properties’ have been used interchangeably to name contents, procedures and results of such studies (van Genuchten & Leij, 1992; Pachepsky et al., 1999). Bouma (1989) introduced the term pedotransfer func- tions (PTFs) for statistical regression equations, expressing relationships between soil properties. Pedotransfer functions are most commonly used to predict soil hydraulic character- istics, but soil chemical and biological characteristics have also been estimated. Several reviews on the development of PTFs and their use have been published (e.g. van Genuchten & Leij, 1992; Pachepsky et al., 1999; Wo¨sten et al., 2001). Soil texture has long been used to predict soil hydraulic properties. Clapp & Hornberger (1978) used soil textural classes to estimate hydraulic properties, but more detailed particle-size distributions have been shown to increase the accuracy of predictions (Schaap et al., 1998). The accuracy of texture-based estimation is, however, limited. Table 1 shows the variation of water retention within textural classes in the Paper given at the Michel Rieu Memorial Colloquium, 8–10 October 2001, in Paris. Correspondence: Y. A. Pachepsky. E-mail: ypachepsky@anri.barc. usda.gov Received 13 November 2001; revised version accepted 14 May 2002 European Journal of Soil Science, September 2003, 54, 443–451 # 2003 Blackwell Publishing Ltd 443