The Nash-Sutcliffe Efficiency (NSE) is a statistical measure widely used in hydrology to evaluate the predictive performance of models. It is a dimensionless value that ranges from negative infinity to 1, with values closer to 1 indicating better model performance. The VBA code provided above calculates the NSE using two input ranges, one for observed values and the other for simulated values. The function first calculates the mean of the observed values and then uses it to compute the numerator and denominator of the NSE formula. The numerator sums the squared differences between the observed and simulated values, while the denominator sums the squared differences between the observed values and their mean. The function then subtracts the quotient of the numerator and denominator from 1 to obtain the NSE value. This VBA code can be used to calculate the NSE for a wide range of hydrological models in Microsoft Excel. It is a useful tool for model calibration and validation, as it allow...