Nutrient retention model, export maps
I have a few questions about the nutrient retention model. I'm using InVEST version 3.2. Can you clarify how the final export map has been created, i.e.
"output\n_export.tif" (kg/pixel): A pixel level map indicating how much load from each pixel eventually reaches the stream.
Is the "per pixel" value just the 'Outflow quantity from each cell' (from the table in the InVEST documentation, e.g. ALV1xG2+ALV2 for Cell2)? Or is the nutrient being retained downslope from the cell also accounted for?
Also in the Intermediate outputs, there is a N_load raster. Can I check how these loading values per pixel have been calculated? They don't seem to be based on export coefficients alone.
Lastly, I've been having some problems with gaps (NoData) in the output (export) map. There are no gaps in the retention map (or in any of the input layers). I saw another forum entry with a similar issue and I followed the advice, making sure that the hydrological sinks in the DEM were filled. This helped with some of the gaps but there are still a few present. If I reduce the flow accumulation value, the gaps get smaller. However, I've compared the InVEST stream layer with an actual water course layer for the area and if I reduce the Flow Accumulation value too much, the InVEST stream layer becomes unrealistic (too many stream branches).
Any advice would be appreciated.