Nutrient retention model, export maps

Hi,

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.



Comments

  • RichRich Administrator, NatCap Staff
    Hi KatShar,

    Yes the n_export raster accounts for the nutrient being retained downslope.  The value in the cell is the effective nutrient released from that cell that reaches the stream.

    "n_load" is the nitrogen loading density given in the lookup table multipled by the area of the pixel in hectares.

    Your issue with nodata gaps sounds like it might be related to hydrological pits in the DEM.  InVEST doesn't yet do anything other than trivial pit filling so you'll need to use something like ArcHydro to fill the DEM.  One way to check is to examine the flow accumulation raster in the intermediate output, if that has matching nodata blobs that's very likely the issue.

  • Hi Rich,

    Thanks for your reply. I tried filling the DEM in ArcHydro and I still have the gaps. Now the DEM is properly filled, they are much smaller than they previously were though.There aren't any similar gaps in the flow accumulation raster either.

    Can I also check, in the latest version on the InVEST manual, there is the following text on the NDR model:

    "Note 2: Load values may be expressed either as the amount of nutrient applied (e.g. fertilizer, livestock waste, atmospheric deposition); or as “extensive” measures of contaminants, which are empirical values representing the contribution of a parcel to the nutrient budget (e.g. nutrient export running off urban areas, crops, etc.) In the latter case, the load should be corrected for the nutrient retention from downstream pixels of the same LULC. For example, if the measured (or empirically derived) export value for forest is 3 kg.ha-1.yr-1 and the retention efficiency is 0.8, users should enter 15(kg.ha-1.yr-1) in the n_load column of the biophysical table; the model will calculate the nutrient running off the forest pixel (n_export) as 15*(1-0.8) = 3 kg.ha-1.yr-1."

    Does this apply to the export coefficients used in the nutrient retention model (version 3.2)  as well? I'm just wondering if I should be altering the export coefficients I've found in the literature before I put them into the biophysical table.

    Thanks again.




  • RichRich Administrator, NatCap Staff
    Hi KatShar, if you're no longer seeing gaps in the flow accumulation raster then I'm not sure what might be causing the other gaps.  It could be accidental nodata pixels in the input stack since the model stamps a nodata on the output if any pixel in the stack is nodata.  You could load all the intermediate outputs into a GIS browser and see if you have a pattern?

    As for your other question, I'll defer that to Perrine (our nutrient hydrologist) but she's travelling and may not be able to respond for a few days.

  • adrianvogladrianvogl Member, NatCap Staff
    Hi KatShar,
    Conceptually the nutrient load coefficient in the old nutrient retention model and the NDR model is the same. Therefore you should follow this guidance for updating your n_load values in the biophysical table when using older versions of the model.

    Cheers,
    Adrian
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