Fitting of hydrologic models: A close look at the Nash-Sutcliffe index

Quantitative assessments of the degree to which the modeled behavior of a system matches with the observations provide an evaluation of the model's predictive abilities. In this context, the Nash-Sutcliffe efficiency index is widely used in water resources sector to assess the performance of a hydrologic model. Through a series of results, this technical note demonstrates that this index alone is not adequate in describing the performance of a model. It is shown that relatively poor models can give a high value of the index and vice-versa. Thus, it is advisable to employ the other statistical measures before arriving at a definite conclusion about the performance of a hydrologic model.