Continuous:Sequence of Events During Long-Term Simulations

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At the beginning of a long-term simulation, GSSHA opens the hydrometeorology data file, and determines the date/time of the beginning and end of the hydrometeorology data. GSSHA begins at the date/time specified as the beginning of the hydrometeorology data file. If the first rainfall event in the rainfall input file does not begin during the period of hydrometeorology data, then GSSHA aborts with a warning.

In LONG_TERM mode control of process activation switches from rainfall events to the physical conditions in the watershed. When there is no moving water on the overland flow plane or in the channels, ET demand and unsaturated water movement are updated at an hourly bases. This is possible because, altough the overall model timestep is used to loop through time in the model, each process has it's own individual update time. Therefore, computations for any process can be started and stopped at any time, and any process updates can occur at multiple overall timesteps. This allows signficant time savings by eliminating needless calculations. Altough the internal control is complicated, it is generally described below.

When water appears on the overland flow plane due to rainfall, exfiltration or channel overbank flow, overland flow routing and unsaturated zone calculations begin at the overall model time step, typically around 60s or less. Channel routing begins as water in the channel accumulates, also at the overall model time step. Overland flow calculations continue until all water on the overland flow plane runs off, infiltrates, evaporates, or stops moving. Channel routing continues as long as a minimum amount of water is in the channel and the water is moving. If groundwater/channel interactions are specified, these fluxes continue to be computed and the volume in the channel is updated each groundwater time step. If exchange with the groundwater results in the minimum amount of water required for channel routing to begin, then channel routing will resume, but at the larger groundwater time step. Anytime rainfall is occurring, overland flow, ET, channel flow, unsaturated zone water movement, and saturated groundwater flow are updated each overall model time step. Events are still recognized, but are only used for accounting purposes.

When continuous simulations are performed GSSHA also outputs a file called evap_out which list basin wide average hourly values of: soil moisture, potential evaporation (cm hr-1), actual ET (cm hr-1), infiltration rate (cm hr-1), groundwater recharge (cm hr-1), continuous frozen ground index, temperature (C), gridded temperature (C), surface soil temperature (C).

GSSHA User's Manual

9 Continuous
9.1     Computation of Evaporation and Evapo-transpiration
9.2     Computation of Soil Moisture
9.3     Hydrometeorological Data
9.4     Snowfall Accumulation and Melting
9.5     Sequence of Events During Long-Term Simulations