How does the "average" method work for pond infiltration/seepage?
Why is the average infiltration method not supported with an elevation-volume based pond?
The Average Infiltration Method specifies an average drop in the pond water level based on a distance per time, such as inches per hour. This works off of the water surface area at a given timestep. As an example, if the water surface area at a timestep is 1 acre and the seepage rate is set to 1 in/hr, then the seepage flow rate at that time would be 1 acre-inch per hour.
Note that the conversion from an Elevation-Volume curve to an Elevation-Area curve is not straightforward and can result in imperfect results. So, the only way to get an accurate infiltration rate with the average infiltration method is to use an Elevation-Area table for your pond.
In PondPack, you may be able to right click on your elevation-volume based pond, choose "Pond Volume Results Table", click the data table tab, select elevation and area columns, press CTRL+C to copy, then change the pond type to elevation-area and paste in the data. However, as mentioned above, this may be error prone so should be used with caution. You will also need to check the area values to make sure they are expected.
Note that in some releases of SewerGEMS and CivilStorm, seepage results may be included when using an Elevation-Volume pond type (without warning). These results are not valid and you should use an Elevation-Area pond type. Defect 894559 will address this.