Building a Model:Long-term Simulations

Long-term simulations with ET calculations, as described in Section 9, are needed to model longer periods with multiple storm events. Long-term simulations must also be performed to properly simulate soil moistures in the unsaturated zone, saturated groundwater movement and stream interaction. As described by Senarath et al (2000) long-term simulations are also necessary to properly calibrate any GSSHA model, even if it to be used only to simulate single events. ET parameters are assigned with the Mapping Table, and related to the combination land use/soil-texture/vegetation index map. The selection of appropriate root depths for such crude indexes can be difficult (Downer and Ogden, 2003a) and these values are most properly thought of as effective values that are determined through calibration. Although the method to simulate the seasonal effects in GSSHA is crude, it has been shown to be effective (Downer and Ogden, 2003a) and the SEASONAL_RS card should be included in the project file if simulations are to be conducted outside the summer growing season, May-September.

The rainfall file and hourly hydro-met data file should be constructed to cover the calibration, verification, and simulation scenarios period. This period may be weeks to years, depending on the available record. To properly calibrate a model, a period with overlapping rainfall and streamflow measurements from several storm events that produce stream flow should be selected. The hydro-met data should start just after the last rainfall event before the simulation or calibration period, and saturated or near saturated initial moistures are assumed. The model should be run in continuous mode with the entire calibration period to locate and fix problems in the input files and assure the model will run for the entire period with the initial parameters, time step and grid size.