Excluding organic matter content from pedotransfer predictors of soil water retention

The majority of pedotransfer functions (PTFs) published for estimating water retention characteristics (WRC) use data on soil texture, bulk density, and organic matter content (OM) as predictors. For soil hydrological modeling on a regional scale, in particular the derivation of appropriate values for a PTF parameterization can be difficult where organic C data are missing. Assuming the indirect interdependency between OM and bulk density, a new PTF has been developed that estimates the WRC using only soil texture and bulk density data. To achieve a regression-based reproduction of the correlations, a calibration was chosen that connects the parameters of the van Genuchten equation with the data on bulk density and soil texture, using linear and nonlinear relationships. More than 90% of the variability in measured soil water contents was explained by the new model. The validity of the PTF was tested with a data set of 147 measured WRCs (r(2) = 0.94). Compared with another frequently used PTF model, which uses the organic C content as an additional predictor, the new model provided comparable or slightly better predictions of the WRCs.