Nash–Sutcliffe model efficiency coefficient
The Nash–Sutcliffe model efficiency coefficient is used to assess the predictive power of hydrological models. It is defined as:
where Qo is the mean of observed discharges, and Qm is modeled discharge. Qo is observed discharge at time t.
Nash–Sutcliffe efficiency can range from −∞ to 1. An efficiency of 1 (E = 1) corresponds to a perfect match of modeled discharge to the observed data. An efficiency of 0 (E = 0) indicates that the model predictions are as accurate as the mean of the observed data, whereas an efficiency less than zero (E < 0) occurs when the observed mean is a better predictor than the model or, in other words, when the residual variance (described by the numerator in the expression above), is larger than the data variance (described by the denominator). Essentially, the closer the model efficiency is to 1, the more accurate the model is.