MizuRoute coupling with SUMMA

Motivation: SUMMA simulates land-surface hydrological processes and generates runoff at the HRU/GRU level but does not route this runoff through a river network. Currently, mizuRoute is commonly applied as a separate post-processing step using SUMMA NetCDF output. An optional online coupling would allow SUMMA runoff to be passed directly to mizuRoute during model execution, supporting efficient large-domain simulations and operational forecasting. This use case is very similar to the CESM use case.

Suggested code changes: The first step is to avoid duplicate module names. In the second step, a routing interface should be introduced to initialize mizuRoute, transfer runoff, advance routing, retrieve discharge, manage restart states, and finalize the model. SUMMA should calculate the total runoff entering the river system from all relevant surface and subsurface components and pass it to mizuRoute using documented units and time conventions. In the third step, duplicate input settings and parameters between SUMMA and mizuRoute should be removed.

Numerical testing: The initial implementation will support one-way coupling from SUMMA to mizuRoute. The coupled system will be tested using a hydrological simulation of the Bow River at Banff catchment. Runoff and streamflow generated by the coupled system will be compared with results from equivalent standalone SUMMA and mizuRoute simulations. The coupled and standalone results should agree within a defined numerical tolerance while maintaining water balance and consistent restart behavior.

Start by reviewing the proposed three-step coupling plan in this issue and the existing SUMMA and mizuRoute interfaces; no specific files or entry points are named. Define the one-way coupling boundary, then use the Bow River at Banff simulation to compare coupled and standalone runoff and streamflow, checking numerical tolerance, water balance, and restart behavior.

Written by the indexing model from the issue text.