In order to advance our ability to predict the future availability of water resources and the risk of water related disasters under a changing climate, it is necessary to bring together the land surface and hydrological modelling communities. 

Historically land surface models (LSMs) have focused primarily on the interactions between the land surface and the atmosphere, whereas most hydrological models (HMs) have focused on the flows of surface and groundwater. Each class of model has made strong simplifications in the other’s domain in order to make the numerical and computational problems more tractable and in response to different scientific issues.  

LSMs have been designed to serve atmospheric models and thus concentrate on the vertical movement of water, heat and carbon. They have historically been bound by the spatial and temporal discretization of the atmospheric model, with their aim being to close the water, energy and carbon budgets at the surface.  Many of the assumptions that have been made in their representation of surface processes can be traced back to this origin. By contrast, HMs have focused on the horizontal movements of water at the surface as well as underground, often with the aim of making accurate predictions of river flows assessed by comparison with gauged observations. Their spatial discretization is often driven by topography and landscapes and time scales cover a much larger spectrum than LSM. They have served a very wide community within and outside of the geophysical sciences. The complementarity of both types of model is obvious but neither provides a complete description of terrestrial hydrological processes. While it is often recognized that hydrology and land-surface science require excellence in both components of the system, little has been done up to now to achieve this.  

The Hydro-JULES vision is to bring together both communities in one modelling environment which can couple the most advanced hydrological models, to the most advanced land surface models and thus to the atmospheric component. In some configurations for some scientific questions, the land surface models will provide the lower boundary condition to the atmosphere and the upper boundary condition for the hydrology. It is possible that for some scientific questions, the hydrology and land-surface models may operate separately and adopt the best-available representations of the processes appropriate. Hydro-JULES will enable the different options to be explored and evaluated for a range of case-studies, and will be made available to the wider LSM and HM community. Developing this LSM to HM interface coupler will lead the international terrestrial modelling strategy, enabling cutting-edge developments in both communities to be available to all.

It is to be hoped that defining such an interface between land surface and hydrological processes will create the opportunity to bring the respective scientific communities together, in pursuit of the shared goal of improving land surface and hydrological predictions. Moreover, the careful definition of physically-based interfaces permits the maintenance of a plurality of model subcomponents and allows innovation and interaction between the research and academic communities.