When comparing results (Modeled) to historical gauge data (Observed) on the Gauge Comparison charts -- for streamflow (both quantity and water quality), snow accumulation, and reservoir storage -- WEAP will calculate and display various statistics to aid you in calibrating your model. For the Streamflow Gauge Comparison, River Water Quality Gauge Comparison, Reservoir Storage Gauge Comparison and Snow Gauge Comparison charts, calibration statistics are shown on the right of the chart underneath the legend, and also on their own tab ("Statistics"). From the Statistics tab, you can export to Excel or CSV files. You may select one or more of the following twelve statistics to view:
|
N |
Count of non-missing data |
|
Missing |
Count of missing data |
|
NSE |
Nash-Sutcliffe Efficiency |
|
KGE |
Kling-Gupta Efficiency |
|
NRMSE |
Normalized root mean square error |
|
PBIAS |
Percent bias |
|
RSR |
RMSE-observation standard deviation ratio |
|
LNS |
Log Nash-Sutcliffe Efficiency (log transformed) |
|
RMSE |
Root mean square error |
|
MAE |
Mean absolute error |
|
r |
Pearson's correlation coefficient |
|
r^2 |
Pearson's coefficient of determination |
WEAP calculates the statistics for whatever data are selected for display in the chart. For example, to see how well calibrated the model is on an annual basis, choose Annual Total. To see how well each month's results matches the observations, choose Monthly Average. Or you could investigate the calibration for a subset of months, such as for the summer months, or for a subset of years. Missing data are excluded from the statistics, except for the "Missing" statistic, which reports how many data points were missing.
If is beyond the scope of this document to explain how each statistic is calculated, which statistics you should use, or what values of each represent a "good" calibration. For a good general background in these statistics and how to use them in calibration, please refer to the following two papers:
"Model Evaluation Guidelines for Systematic Quantification of Accuracy in Watershed Simulations," D. N. Moriasi, J. G. Arnold, M. W. Van Liew, R. L. Bingner, R. D. Harmel and T. L. Veith, Transactions of the American Society of Agricultural and Biological Engineers, Vol. 50(3): 885-900, 2007. https://swat.tamu.edu/media/1312/moriasimodeleval.pdf
"Objective functions used as performance metrics for hydrological models: state-of-the-art and critical analysis," Paloma Mara de Lima Ferreira, Adriano Rolim da Paz and Juan Martín Bravo, Brazilian Journal of Water Resources, v. 25, e42, 2020. https://doi.org/10.1590/2318-0331.252020190155