Marked differences between van Genuchten soil water-retention parameters for temperate and tropical soils: a new water-retention pedo-transfer functions developed for tropical soils

All of the physical, chemical and soil water-retention data suitable for the derivation of a Pedo-Transfer Functions (PTF) for water retention for tropical soils (771 suitable horizons) were extracted from the IGBP-DIS soil database. The parameters θs, θr, α and n of the van Genuchten (vG) [Soil Sci. Soc. Am. J. 44 (1980) 892] equation were derived and compared with parameter values from two published data sets of temperate region soils. Thirty-five percent of the soils in the tropical (IGBP/T) database were classified as clays, compared with only 4% and 7% in the temperate databases. The IGBP/T soil bulk densities were significantly lower (p<0.001) for nine textural classes. For the more clayey textural classes, the value of α was higher for the IGBP/T soils, implying more large pores and more structure. For kaolinitic soils in the IGBP/T database, α was typically around 0.4 kPa−1 compared with 0.04 kPa−1 for montmorillonitic soils. Mineralogy is clearly important and should be included in PTFs (if available). The value of n for clay soils in the IGBP/T data set was significantly higher than for temperate soils, and θs was significantly higher for nine textural classes (p<0.001) reflecting their lower bulk density. θr was also significantly higher for all textural classes except sand. For clay, the mean value of θr was 0.27 m3 m−3, compared to only 0.11 m3 m−3 for temperate soils. For 17% of the IGBP/T data, θr exceeded 0.3 m3 m−3, a value often used as a fitting constraint. A tropical soil PTF was developed using multiple regression techniques. This predicted water-retention curves more reliably than either class PTFs or “soil class” PTFs, but was not reliable for low-density soils such as Andosols. The IGBP/T data set includes some groups of soils, e.g., Andosols and Ferralsols, whose properties are extremely different from those of most temperate soils. These differences emphasise the need to develop separate PTFs for tropical soils, or perhaps for specific groups of soils. There were few water-retention data from below 1.5-m depth and a complete lack of hydraulic conductivity data in the IGBP/T database. This is a major impediment to the modelling of water movement and uptake in deep-rooted ecosystems, for example, tropical forest and savannah.