Pedotransfer functions for the estimation of soil water retention in Brazilian soils

Various studies have shown that micro-aggregated, strongly weathered tropical soils have different water retention properties than temperate-region soils because of differences in mineralogy and weathering history. Hence, pedotransfer functions (PTFs), derived from data from temperate-region soils (temperate PTFs) have limitations when applied to tropical soils. In the absence of PTFs specifically for tropical soils, temperate PTFs are being applied worldwide in global climate modeling exercises, regardless of their textural validity. We derived a PTF to predict the water retention parameters of the van Genuchten (1980) equation using data from more than 500 Brazilian soil horizons. A modified approach was adopted: Multiple regression was used to derive coefficients that relate the van Genuchten parameters to basic soil data, followed by re-optimization of those coefficients by fitting the individual water content estimations to the measured data simultaneously. Pedotransfer functions were developed for four levels of availability of basic soils data and validated using independent data from 113 Brazilian soil horizons. Water retention curves were better predicted by the new PTFs than by two temperate PTFs tested: The root mean square deviation (RMSD) ranged from 3.78 to 5.84, compared with 9.08 and 10.44, respectively. The new PTFs performed better even when the comparison was restricted to the range of textural validity of the temperate PTFs. For the proposed PTF, RMSDs increased with increasing silt content, but decreased for the temperate PTFs, This reflects the differences in silt content between temperate and strongly weathered tropical soils.