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The identification of a conceptual hydrologic model involves two stages: selection of a suitable model structure and parameter estimation. These two stages cannot be treated as distinct, and it is important that the limitations of available parameter estimation methodologies be given serious consideration during the procedure of model structure selection. This paper focuses on the concepts and tools necessary for the evaluation of whether the structure of a hydrologic model is such that its parameters can be successfully identified. A definition of structural identifiability is advanced that is based purely on model properties and is independent of the stochastic nature of the output observation errors. The approach is used to develop a measure of local structural identifiability which is useful in the analysis of parameter sensitivity. A measure, called the sensitivity ratio, is introduced which is suitable for studying parameter interactions in the multidimensional parameter space. Also, the use of allowable reparameterizations to improve identifiability is discussed. The utility of the concepts discussed in this paper are demonstrated by a simulation example using a conceptual rainfall‐runoff model.
Sorooshian et al. (1985) studied this question.