HESS Opinions: Participatory Digital eARth Twin Hydrology systems (DARTHs) for everyone - a blueprint for hydrologists

The "Digital Earth" (DE) metaphor is very use- ful for both end users and hydrological modelers (i.e., the coders). In this opinion paper, we analyze different cate- gories of models with the view of making them part of Digi- tal eARth Twin Hydrology systems (DARTHs). We stress the idea that DARTHs are not models, rather they are an appro- priate infrastructure that hosts (certain types of) models and provides some basic services for connecting to input data. We also argue that a modeling-by-component strategy is the right one for accomplishing the requirements of the DE. Five technological steps are envisioned to move from the current state of the art of modeling. In step 1, models are decom- posed into interacting modules with, for instance, the agnos- tic parts dealing with inputs and outputs separated from the model-specific parts that contain the algorithms. In steps 2 to 4, the appropriate software layers are added to gain transpar- ent model execution in the cloud, independently of the hard- ware and the operating system of computer, without human intervention. Finally, step 5 allows models to be selected as if they were interchangeable with others without giving decep- tive answers. This step includes the use of hypothesis test- ing, the inclusion of error of estimates, the adoption of liter- ate programming and guidelines to obtain informative clean code.The urgency for DARTHs to be open source is supported here in light of the open-science movement and its ideas. Therefore, it is argued that DARTHs must promote a new participatory way of performing hydrological science, in which researchers can contribute cooperatively to character- ize and control model outcomes in various territories. Finally, three enabling technologies are also discussed in the context of DARTHs - Earth observations (EOs), high-performance computing (HPC) and machine learning (ML) - as well as how these technologies can be integrated in the overall sys- tem to both boost the research activity of scientists and gen- erate knowledge.