Accuracy of the saturated hydraulic conductivity prediction by pedo-transfer functions compared to the variability within FAO textural classes

Abstract

Saturated hydraulic conductivity is one of the key parameters for water transport models in the unsaturated zone. Several attempts have been made to estimate this parameter from available soil data as e.g. particle size distribution, bulk density, and organic matter content. All of these estimation methods (pedo-transfer functions) exhibit large differences between the predicted and the measured saturated hydraulic conductivities, because they calculate the saturated hydraulic conductivity as a deterministic physical parameter. In this study the saturated hydraulic conductivity is defined as a lognormally distributed random variable, the (geometric) mean and standard deviation of which depend on the texture. The confidence limits of the geometric mean and standard deviation of the measured saturated hydraulic conductivity within the FAO textural classification scheme are presented. Due to the lognormal distribution the ratio of the predicted and the measured saturated hydraulic conductivity (error ratio) is statistically analyzed within the FAO textural classes using the database of the Lower Saxony Soil Information System. For some pedo-transfer functions the geometric mean error ratio is near one, but the geometric standard deviation of the error ratio is generally large and within the investigated textural classes has nearly the same value as the geometric standard deviation of the measured saturated hydraulic conductivity. This indicates that the pedo-transfer function approach is useful to predict saturated hydraulic conductivity, if mean and standard deviation are considered.