Summary:

## Numerical Implementation of MOSART

The numerical implementation of the MOSART (Model for Scale Adaptive River Transport) model is based on a subcycling scheme and a local time-stepping algorithm.

Subcycling:
- There are two levels of subcycling:
1. Between the runoff inputs (from CLM to MOSART) and the MOSART routing. If the input time step is 10800s and the MOSART routing time step is 3600s, three MOSART time steps are invoked per input update.
2. Between the hillslope routing and the channel routing, to account for the slower water velocity across hillslopes compared to channels. The hillslope routing time step is usually the same as the MOSART time step, but it includes several channel routing time steps.

Local Time-Stepping:
- The local time-stepping algorithm is used to account for the faster water travel velocity in some river channels (e.g., with steeper bed slope, narrower width) compared to others.
- The final time step of local channel routing is calculated as T_local = T_channel / D_local, where D_local is empirically estimated based on the local channel slope, width, length, and upstream drainage area.

If MOSART crashes due to numerical issues, the recommendation is to increase the D_levelH2R parameter and, if the issue persists, reduce the T_mosart time step.