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The choice of any particular objective function will depend on the intended application. Each of the pre-defined objective functions are formulated to put emphasis (reproduce as closely as possible) on different flow characteristics (Vaze et al, 2011).
The discussion below assumes that the objective functions are being applied to streamflow data but they can be applied to any time series data.
Missing Data
It is common for observed time series of hydrological processes to contain missing values. Also, the observed and modelled time series may have different start and end dates. The Source calibration tool calculates the objective function values using only data from those time steps for which both observed and modelled data is available.
The descriptions of the objective function equations assume that the observed and modelled data has been filtered to include only:
- data from within the calibration period, and
- data for time steps with complete data pairs.
Nash Sutcliffe Coefficient of Efficiency (NSE)
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This formulation obviates the necessity to calculate the average of the observed flows before evaluating the denominator in the traditional version.
Flow Duration
The flow duration objective function sorts the observed and modelled data values in increasing order and then calculates the NSE of the sorted data.
Absolute Bias
Combined Objective Functions
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