Combining Dissimilar Hydrologic Model Formulations Using a Catchments/Nexus Interface: Building the Next Gen National Water Model

The next generation National Water Model (NWM) will be based on a framework that allows application of dissimilar catchment-scale hydrologic models within a unified set of continental-scale computations. A key goal of the next gen NWM is the creation of a model system that allows heterogeneous modeling approaches to be applied simultaneously. It allows evidence-based evaluation of different model formulations in different locations and at different times and will enable an ensemble-of-model-formulations approach . In order to achieve this, we are utilizing a catchment/nexus interface. Finally, in the larger sense, the framework enabled through the application of the HY_Features standard catchment-nexus definition creates the formulation flexibility required to extend the framework to different classes of prediction problems including sediment transport, water quality, water supply, and earth-systems scale process modeling.

The WaterML2 HY_Features standard defines the concepts of catchment and nexus. Catchments represent regions that are mostly surrounded by no flow boundaries allowing application of independent model structures in catchments without affecting adjacent catchments. Note that surface catchments and related groundwater systems need not be coincident in this definition. Nexuses represent upstream-downstream connections between catchments where information exchange occurs by passing boundary conditions between catchments, and from catchments to water bodies (streams, lakes, or estuaries). Depending on the model representation used in a catchment, a nexus realization represents a boundary condition with zero to three spatial dimensions. This dimensionality depends on the formulation differences between the receiving and contributing side of a nexus. We present designs for the currently implemented sets of nexus interfaces, discuss some of the potential complications with connecting different model types, and show example outputs from initial tests of catchment/nexus systems.