We have some questions about the results of the invest’s urban floodrisk mitigation model (3.8.5).
We do an analysis of area(fua) of Amsterdam. For this study, the following files are used in the model; Landcover data from urban atlas, the Soils Hydrological Group Grid (HYSOGs250m), a biophysical table with the land cover classes with pre-defined Curve number (CN) values for each LULC type with the corresponding hydrologic soillogic group(1-4) for the following column names: CN_A, CN_B, CN_C, CN_D. For the ‘depth of rainfall’ we have filled in with a value of 100mm . (see screenshot of the invest form below), raster cells are 20 x 20 m.
If we look at the model results of this study, the model has created the following data.
If we look closer at the map and we select a specific spot in the dunes near Amsterdam (normally a spot where there is high retention and low runoff levels), we see the following values which we don’t quite understand:
We understand ‘Runoff’ as draining water and ‘retention as water retention. So, in our opinion ‘Runoff_retention’ contradicts itself.
If we look at the data, we see that Runoff_retention is 0,68 (i.e. retention is 68%, what matches the dunes), and Runoff_retention_m3_NL002L3_AMS is 27 m3 which equals 68% of its total of 40m3, but in value also equals to Q _m3_NL002L3_AMS. Is this correct? You would expect the opposite, that from the total amount of water, the runoff is deducted from it and what remains is allocated to retention. The Q_mm matches that idea, because it is 31% of the total depth of rainfall. So, it would be nice if you could clarify the four outcomes as we see in the Identify view above.
Also it doesn’t help that the fourth map in the results (Q_m3) can’t be found anywhere in the documentation!