Building a Model:Single Event Calibration

A GSSHA model with overland flow, infiltration, and channel routing represents a fairly complete model, and this model can be used to determine appropriate time steps, RE cell sizes, and channel routing parameters, i.e. channel roughness. While this step is not essential, it is useful. For the single event calibration the user should select one storm event from the observed data that provides a reasonably well defined outlet hydro-graph. Overland flow and infiltration parameters can be adjusted to produce the approximately correct volume of flow at the watershed outlet. The in-stream channel roughness is tuned to match the hydrograph peak and shape. This initial single event calibration can either be done manually, or with an automated calibration process, such as the SCE method. With a single event the SCE method will converge in a short period of time, likely overnight. While the overland flow and infiltration parameters from this effort are of limited value (Senarath et al., 2000), the values of in-stream roughness should be approximately correct. Also, this calibrated model can be used to determine what model time step can be used and the appropriate cell size for RE solutions.

Assuming the model has been calibrated at some small time step, say 10s, the calibrated simulation can be repeated with increasingly larger time steps until either a) the model crashes b) the outlet hydrograph begins to oscillate, or c) the shape of the outlet hydrograph begins to significantly change in shape. When any of these occur, the time step is too large and should be reduced until the problem disappears. It is possible that very large time steps, several minutes, can be used in the simulation without significantly affecting the results. This will significantly reduce execution times and may be especially important when using an automated calibration process over an extended simulation period. As this exercise will demonstrate, the model will produce almost exactly the same results for time steps below some critical value, so that using time steps much smaller than this critical value will not result in improved results, only longer simulation times. This optimal time step is then used in subsequent calibrations and simulations.

The same procedure can be used to determine the appropriate cell size to use in the RE solution (Downer, 2002a). Starting with very small cell sizes in the top 10 cm of the soil column, 1 – 10 mm, the cell size is increased until the volume of runoff begins to significantly deviate from the original results. This theshold cell size is used in subsequent calibrations and simulations.