Hydro-pedotransfer functions: A roadmap for future development

Hydro-pedotransfer functions (hyPTF) are important methods to relate available knowledge about soil properties to soil hydraulic properties and model parameters to be applied in process models. At least more than four decades have been invested to derive such relationships. However, while models, methods, data storage capacity, and computational efficiency have advanced, there are fundamental issues related to the scope and adequacy of current hyPTFs, particularly when applied to parameterise models at the field scale and beyond. Much of the hyPTF development process has focussed on refining and advancing the methods, while fundamental questions remain largely unanswered, namely i) how should hyPTFs be built for maximum prediction confidence, ii) which processes/properties need to be predicted to move beyond the van Genuchten-Mualem based parameterisation of the Richards equation, iii) which new datasets and data coverage are needed, iv) how does the measurement process of soil hydraulic properties determine the construction of hyPTFs and at which scale, iv) how can we incorporate diverging scales (scale of derivation vs scale of application), and v) what scaling/modulation/constraining strategies are affective to make hyPTF predictions at field-to-regional scale appropriate and physically meaningful? These questions have been addressed in a joint effort by the members of the International Soil Modelling Consortium (ISMC) Pedotransfer Functions Working Group with the aim to systematise hyPTF research and provide a roadmap guiding scientists, reviewers, and make users aware of the shortcomings.