Evaluation of pedotransfer functions for predicting soil hydraulic properties: A voyage from regional to field scales across Europe

Study region: Europe. A total of 660, 522, and 4940 soil samples belonging to GRIZZLY, HYPRES,
and EU-HYDI databases, respectively, were used for parametric evaluation.
Study focus: The soil water retention and hydraulic conductivity functions are crucial input information
for land surface models. Determining these functions by using direct methods is
hampered by excessive time and unaffordable costs required for field activities and laboratory
analyses. Pedotransfer functions (PTFs) are widely-used indirect techniques enabling soil hydraulic
properties to be predicted by using easily-retrievable soil information. In a parametric
evaluation, the predictive capability of PTFs is examined by comparing measured and estimated
soil water retention parameters and saturated hydraulic conductivity. Yet information about the
performance of PTFs for specific modeling applications is mandatory to evaluate PTF effectiveness
in greater depth. This approach is commonly defined as functional evaluation.
New hydrological insights for the region: The best performing four PTFs selected in the parametric
evaluations are tested under two functional evaluations. The first encompasses a spatial interpolation
with a geostatistical technique, whereas the second employs Hydrus-1D to simulate the
water balance components along an experimental transect. Our results reinforce and integrate the
insights of previous studies about the use of a PTF, and highlight the ability, or inability, of this
technique to adequately reproduce the observed spatial variability of soil hydraulic properties
and simulated water fluxes.