Long-term Simulations:Long-term Simulations
The GSSHA predecessor, CASC2D, was originally developed as an episodic model, and GSSHA can still be used to simulate single events with a specified simulation time. However, Senarath et al. (2000) showed the pitfalls of calibrating distributed parameter models like CASC2D to single events. CASC2D was subsequently modified to be able to simulate multiple rainfall events over an extended period. Typically, GSSHA is run in the long-term simulation mode.
There are no restrictions on the duration of the simulation or the number of rainfall events that can be simulated. Annual simulations have been successfully performed for a variety of watersheds. The only requirement is that hydrometeorological and rainfall data be available for the simulation period.
How GSSHA operates during a long-term simulation depends on the model options selected. When GAR is used to calculate infiltration, GSSHA operates in a dual mode. During rainfall events, GSSHA performs in the normal episodic rainfall/runoff mode. Once a user specified minimum discharge is reached on the recession limb of the hydrograph, the rainfall/runoff model is stopped and soil moisture calculations begin. At this point, GSSHA performs hourly evapotranspiration and soil moisture calculations until another precipitation event begins. If saturated groundwater calculations are chosen, then saturated groundwater and infiltration are calculated on a continual basis. See the GSSHA User’s Manual for more information.
When Richards’ equation is chosen as the infiltration option, Richards’ equation is continually solved so that infiltration, evapotranspiration, soil moisture accounting, and groundwater recharge are calculated whether or not rainfall events are occurring. When a rainfall event does occur, precipitation distribution, interception, retention, overland flow, and channel routing are initiated, assuming these processes have been selected for simulation. These processes continue as long as there is rainfall. After rainfall ceases, overland and channel flow continue until water on the overland flow plane and channels ceases to flow. As the decision is based on process conditions, any process may begin or end at any time. The minimum flow criteria discussed above is still used to describe events, but it is used for accounting purposes only and is not used to stop the execution of any processes.
- Long-term Simulations