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The overall study objective was to evaluate the applicability of different qualitative and quantitative methods forcomparing daily and monthly SWAT computer model hydrologic streamflow predictions to observed data, and to recommendstatistical methods for use in future model evaluations. Statistical methods were tested using daily streamflows and monthlyequivalent runoff depths. The statistical techniques included linear regression, Nash-Sutcliffe efficiency, nonparametrictests, t-test, objective functions, autocorrelation, and cross-correlation. None of the methods specifically applied to thenon-normal distribution and dependence between data points for the daily predicted and observed data. Of the testedmethods, median objective functions, sign test, autocorrelation, and cross-correlation were most applicable for the daily data.The robust coefficient of determination (CD*) and robust modeling efficiency (EF*) objective functions were the preferredmethods for daily model results due to the ease of comparing these values with a fixed ideal reference value of one. Predictedand observed monthly totals were more normally distributed, and there was less dependence between individual monthlytotals than was observed for the corresponding predicted and observed daily values. More statistical methods were availablefor comparing SWAT model-predicted and observed monthly totals. The 1995 monthly SWAT model predictions and observeddata had a regression Rr2 of 0.70, a Nash-Sutcliffe efficiency of 0.41, and the t-test failed to reject the equal data meanshypothesis. The Nash-Sutcliffe coefficient and the Rr2 coefficient were the preferred methods for monthly results due to theability to compare these coefficients to a set ideal value of one.
Coffey et al. (Thu,) studied this question.