Zonal Statistics and Precipitation values

[eberga04]

Dear Bastian & Fast-Flood-Community,

I am a new and happy Fast Flood user and i am freaking glad about that tool.
However, i have some questions about the results.

Here are the steps of my analysis - so I ...

1. ... imported an 3x3m DTM
2. ... computed the water depths for a precipitation event of 66.6 mm/h (60 minutes) - default settings + simulation
3. ... exported the flood map (3x3 m raster)
4. ... computed the zonal statistics for the flood map in QGIS (Sum) within my subbasin (= 5.665.792 m²)
5. ... to get the total volume from the water depths (m) I multiplied the zonal-statistics-result (Sum) by 9 (because of my 3x3m Pixel size equals 9 m²)
6. ... The result is 1.250.580 m³ (but the total precipitation volume for the whole area is only 66.6 * 5.665.792 / 1000 = 377.342 m³)

So actually the Volume of the zonal statistics should either be as high as the precipitation volume (which is actually not logic unless we dont have a huge sink with no outflow) or lower, but not higher?
And even if the max. water depth is shown for each pixel, this is still quite high though...

The order of magnitude doesn’t really make sense or am I making a huge mistake in my thinking?
Please help!

Thank you and best wishes,
Gisi