Hydro-pedotransfer functions: A roadmap for future development

Hydro-pedotransfer functions (hyPTF) are used to relate available knowledge about soil properties to soil hydraulic properties and parameters of interest for applications 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 (methods) 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) what is the implication of diverging scales (lab measurements/field to regional scale of application), v) what scaling/modulation/constraining strategies are required to make hyPTF predictions at field-to-regional scale appropriate and physically meaningful, and vi) what is the spatial representativeness? 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 both scientists and reviewers.