Avoiding the redistribution of ponded water

In the standard version of the Simplified Overland Flow solver, the solver cal­culates a mean water depth for the entire flow zone using the available over­land water from all of the cells in the flow zone. During the Overland flow time step, ET and infiltration are calculated for each cell and lateral flows to and from the zone are calculated. At the end of the time step, a new. average water depth is calculated, which is assigned to all cells in the flow zone.

In practice, this results in a redistribution of water from cells with ponded water (e.g. due to high rainfall or low infiltration) to the rest of the flow zone where cells potentially have a higher infiltration capacity. To avoid this redistri­bution, an option has been added where the solver only calculates overland flow for the cells that can potentially produce runoff, that is, only in the cells for which the water depth exceeds the detention storage depth.

Example application

To illustrate the effect of this option, it was applied to a model with a 10 x 10 square domain, one subcatchment and 3 different soil types in the unsatu­rated zone with the following saturated hydraulic conductivities

         coarse   1e-5 m/s

         medium    1e-7 m/s

         fine    1e-9 m/s

For rainfall, a synthetic time series with alternating daily values of 50 and 0 mm/day was used. The simulation period was 2 weeks. Thus, the cumulative rainfall input was 350 mm.

For the case, where the ponding water was not redistributed, the cumulative runoff was 96 mm. Whereas, when the ponded water was redistributed, the cumulative runoff was essentially zero.

Activating the option

This option is activated by means of the boolean Extra Parameter, Only Sim­ple OL from ponded, set to On. For more information on the use of extra parameters, see Extra Parameters (V1 p. 733).