Regional calibration of a watershed model

As watershed models become increasingly sophisticated and useful, there is a need to extend their applicability to locations where they cannot be cali brated or validated. A new methodology for the regionalization of a watersh ed model is introduced and evaluated. The approach involves calibration of a watershed model to many sites in a region, concurrently. Previous researc h that has sought to relate the parameters of monthly water balance models to physical drainage basin characteristics in a region has met with limited success. Previous studies have taken the two-step approach: (a) estimation of watershed model parameters at each site, followed by (b) attempts to re late model parameters to drainage basin characteristics. Instead of treatin g these two steps as independent, birth steps are implemented concurrently. All watershed models in a region are calibrated simultaneously, with the d ual objective of reproducing the behaviour of observed monthly streamflows and, additionally, to obtain good relationships between watershed model par ameters and basin characteristics. The approach is evaluated using 33 basin s in the southeastern region of the United States by comparing simulations using the regional models for three catchments which were not used to devel op the regional regression equations. Although the regional calibration app roach led to nearly perfect regional relationships between watershed model parameters and basin characteristics, these "improved" regional relationshi ps did not result in improvements in the ability to model streamflow at ung auged sites. This experiment reveals that improvements in regional relation ships between watershed model parameters and basin characteristics will not necessarily lead to improvements in the ability to calibrate a watershed m odel at an ungauged site.