GSSHA is a physically based, distributed-parameter, structured grid, hydrologic model that simulates the hydrologic response of a watershed subject to given hydrometeorological inputs. Major components of the model include spatially and temporally varying precipitation, snowfall accumulation and melting, precipitation interception, infiltration, evapotranspiration, surface runoff routing, simple lake storage and routing, unsaturated zone soil moisture accounting, saturated groundwater flow, wetland peat layer hydraulics, overland sediment erosion, transport and deposition, in-stream sediment transport, and overland contaminant transport, and uptake. In GSSHA, each process has its own time-step and an associated update time. During each time-step, the update time of each process selected by the user is checked against the current model time. When they coincide, the process is updated, and updated information from that process is transferred to dependent processes. The update time or time-step of dependent processes may be modified as part of the process update. This formulation permits the efficient simultaneous simulation of processes that have dissimilar response times, such as overland flow, evapotranspiration (ET), and lateral groundwater flow. This scheme also allows an integrated solution of processes coupled through boundary conditions and flux exchanges.