Tuflow and estry manual Version 3

6.3.3Mass Balance Output

Mass balance information is generated by setting Mass Balance Output to ON. The cumulative mass error (CE) appears at the far right of the display lines on the DOS Window for each timestep displayed to the screen. A _MB.csv file is also created in the results folder. The file contains information at each display time (see Screen/Log Display Interval) on the inflows and outflows, volume, predicted volume error and the mass and cumulative mass errors as a percentage, for all 2D domains and each individual 2D domain.

The mass error values are based on dividing the estimated volume error by the average inflow/outflow through the domains. At the startup of a model, particularly when there is little or no flow, and or the model rapidly becomes wet, the cumulative mass error can appear high, however, this should drop away as the model “settles” down. The mass error is displayed as zero while there is less than 1m³/s of water moving through the model.

The majority of models should fall within +/-1% cumulative mass error. If a model experiences higher mass errors this maybe due to using too large a timestep and/or areas of the model are sensitive or slightly unstable. Models with significant areas of complex, steep flows and/or rapid wetting and drying usually experience higher mass errors than those with predominantly more benign, sub-critical flows.

Note that the estimation of mass errors is in itself a prediction and has errors associated with the estimation process. It is also recommended that conventional mass balance checks be carried out to cross-check (see Section 7.1).

For 1D domains see Section 6.4.4.