Participatory Digital Earth Twin Hydrology systems (DARTHs) for everyone: a blueprint for hydrologists

The Digital Earth (DE) metaphor is very useful for both end users and for hydrological modellers. However, in literature it can promote the erroneous view of models as a commodity, that is a basic good used in commerce that is interchangeable with other goods of the same type, without any warning about the fact that some models work better than others, some models just work while others can be simply wrong. These distinctions are at the core of doing good science. This opinion contribution, on the one hand, tries to accept the challenge of adopting models as commodities but, on the other, it wants to show that this acceptance comes with some consequences as to how models must be structured.  The first reuirement is that Digital eARth Twin Hydrology system (called DARTH) need to be Open Source and built with Open Science rules in mind. We analyse different categories of models, with the view of making them part of a . We also stress the idea that DARTHs are not models in and of themselves, rather they need to be built on an appropriate infrastructure that provides some basic services for connection to input data and allows for a modelling-by-components strategy, which, we  argue, is the right one for accomplishing the requirements of the DE. The need to tie predictions to an estimated confidence interval is also supported. Finally, it is argued that DARTHs must promote a new participatory way of doing hydrological science, where researchers can contribute cooperatively to characterize and control model outcomes in various territories. Furthermore, this has consequences for the engineering of the systems.