Progressing pedotransfer functions for nation-wide mapping of soil hydraulic properties

As the measurement of soil hydraulic properties is time-consuming and expensive, they are often computed from easily measurable soil properties via pedotransfer functions (PTFs). There are plenty of existing PTFs which were mainly derived for specific regions or catchments.

In our study, new PTF’s for Austrian soils were developed to estimate soil hydraulic properties such as field capacity or permanent wilting point. The PTFs were built by applying the random forest method, including prediction of uncertainty measures.

The used database includes soil physical data from different project and monitoring campaigns all over Austria and represents many different landscapes and soil types within the country. It consists of 2300 samples from 518 agricultural sampling locations.

For the derivation of new PTF’s, several inputs were available. Since the model is to be applied to standard data from official Austrian soil mapping, we focused on the therein existing data as input variables. We tested four possible combinations for each target property, with particle size distribution and depth as fixed variables. Bulk density and soil organic matter were added in different compositions.

To ensure the applicability of the newly derived PTF’s, the available data set was randomly divided into two subsets. A fraction of 80 % of the samples were contained in training data set for the derivation of the prediction models, the validation set includes 20 % of the data for determining statistical parameters and for comparison with other PTFs. This comparison showed good applicability of the new PTFs in relation to the previous models with R² ranging from 0,73 to 0,83 per target variable.

Due to the fact that each prediction contains uncertainties, uncertainty maps for all agricultural areas in Austria were created. Therefore the 25% and 75% percentile and the respective interquartile range (IQR) of our predicted properties were determined. The results show mean values for the IQR between 6,8 % for field capacity at 60 hPa and 5,7 % for permanent wilting point.

As main outcome, new models for hydraulic properties of Austrian soils were produced. The comparison with established PTFs showed higher accuracy for the prediction of soil properties in Austrian soils than other established PTFs. With the implemented prediction of uncertainty, areas with good predictions can be identified as well as areas with high uncertainties.