Potential Improvement of the Parameter Identifiability in Ungauged Catchments

The objective of this study was to reduce the parameter uncertainty which has an effect on the identification of the relationship between the catchment characteristics and the catchment response dynamics in ungauged catchments. Model deficiencies influencing on the identification of the regional relationships were identified through analysing the non‐stationarity nature under different climate conditions. An advanced calibration approach was proposed to improve the identification of the regional relationships, according to the deficiencies on model structure suitability for the different flow regime. This study demonstrated the refined calibration strategy can improve the identification of the relationships between the catchment characteristics and the calibrated model parameters for the dry period. In the assessment of model structure suitability to represent the non‐stationary catchment response characteristics, there was a flow‐dependent bias in the runoff simulations. In particular, over‐prediction of the streamflow was dominant for the dry period. The poor model performance during the dry period was associated with the largely different impulse response estimates for the entire period and the dry period. Based on assessment of model deficiencies, the rainfall–runoff models were separately calibrated to different parts of the flow regime, and the calibrated models for the separated time series were used to establish the regional models of relevant parts of the flow regime (i.e. wet and dry periods). The effectiveness of the parameter values for the refined approach in regionalisation was evaluated through investigating the accuracy of predictions of the regional models. The regional models from the refined calibration approach clearly enhanced the hydrological behaviour for the dry period by improving the identification of the relationships between the catchment attributes and the catchment response dynamics representing the time constants in fitting recession parts of hydrograph (i.e. improving the parameter identifiability representing the different behaviour of the catchment) in regionalisation.