Building a Model:Calibration and Verification

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The complete model should be calibrated and verified to an extended period of data while operating in the LONG_TERM mode. For flow models, the model should be calibrated to observed discharges at the outlet and any interior points. The OPTIMIZE project card can be used to provide peak discharge and discharge volume for individual events and the entire simulation at the watershed outlet and at any desired internal locations. These can be used to calculate a cost function, and parameter sets that produce the smallest cost function, minimum error as defined by the cost function, can be determined either manually or preferably with an automated method, such as the SCE method. Typical calibration parameters listed in order of importance for each process include:

Overland Flow
Surface roughness
Retention depth
Saturated hydraulic conductivity (all methods)
Suction head (GA, multi-layer GA, GAR) or bubbling pressure (RE)
Initial moisture
Channel Flow
Roughness coefficient
Root depth
Canopy resistance
Soil Moisture
Pore distribution index
Wilting-point soil moisture
Groundwater Flow
Saturated hydraulic conductivity

To use the SCE method, the number of parameters to be calibrated should be kept to a minimum, typically less than 16 (Senarath et al., 2000). To reduce the number of parameters, the proportions of initial estimates of parameters for different index types can be adjusted as a set, i.e. adjust all values of some parameter, saturated hydraulic conductivity for example, by the same fractional amount.

The model with the calibrated parameter set should be tested against independent verification period, such as a split-sample test (Klemes, 1986). Once the model demonstrates the ability to predict discharge, or other variables of interest, for the verification period, it can be used with confidence to analyze model scenarios and make predictions under varying hydrologic conditions.

GSSHA User's Manual

16 Building a Model
16.1     Delineating the Watershed
16.2     Selecting a Grid Size
16.3     Overland Flow Routing
16.4     Infiltration
16.5     Channel Routing
16.6     Single Event Calibration
16.7     Long-term Simulations
16.8     Saturated Groundwater Modeling
16.9     Calibration and Verification
16.10    Sediment Transport
16.11    Contaminant Transport