What is Calibration?

Each model that is included in the program has parameters. The value of each parameter must be specified to use the model for estimating runoff or routing hydrographs. Earlier chapters identified the parameters and described how they could be estimated from various watershed and channel properties. For example, the kinematic-wave direct runoff model described in Chapter 6 has a parameter N that represents overland roughness; this parameter can be estimated from knowledge of watershed land use.
However, as noted in Chapter 2, some of the models that are included have parameters that cannot be estimated by observation or measurement of channel or watershed characteristics. The parameter C_p in the Snyder UH model is an example; this parameter has no direct physical meaning. Likewise, the parameter x in the Muskingum routing model cannot be measured; it is simply a weight that indicates the relative importance of upstream and downstream flow in computing the storage in a channel reach. Equation 85 provides a method for estimating x from channel properties, but this is only approximate and is appropriate for limited cases.
How then can the appropriate values for the parameters be selected? If rainfall and streamflow observations are available, calibration is the answer. Calibration uses observed hydrometeorological data in a systematic search for parameters that yield the best fit of the computed results to the observed runoff. This search is often referred to as optimization.