Fitting of Hydrologic Models: A Close Look at the Nash–Sutcliffe Index

Abstract

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.