NDR Output: Interpreting n_export.tif

Hello, fellow InVEST users! :seedling:

I am using the NDR model to estimate nutrient retention in riparian habitat for an M.S thesis.

I am exploring the model output and trying to understand the logical connection between two outputs: the n_export.tif and the stream.tif.

I understand that the stream.tif is the stream network computed from the DEM.
In the InVEST documentation, the n_export.tif is defined as " A pixel-level map showing how much load from each pixel eventually reaches the stream."

In my n_export.tif output, the highest values of n export are the pixels that are also a part of the stream network. Photos 1 and 2 visualize how the highest n_export values match up perfectly with the stream.tif.


(photo 1: n_export map with red representing the highest n export.)

Could not upload photo 2 because I am a new user.
photo 2: stream.tif with streams in green and land with no color, overlaid on n_export)

My Question: How should I interpret the n_export.tif?
Does n_export.tif simply show the accumulation and export of N in streams?
Iā€™m confused because I interrupted the n_export.tif to show how much load from each pixel eventually reaches the stream. I assumed that only land pixels would contribute n loads that would reach the stream so that land pixels would have the highest N export values. My sense is that n_export.tif is indeed a simple display of stream pixels that have high values of n export, but I wanted to check here for clarification.

Thank you!

photo 2: stream.tif with streams in green and land with no color, overlaid on n_export)

Hi @atershy ,

Welcome to the Forum!

Your interpretation on the ā€œn_export_{SUFFIX}.tifā€ output is correct in that it should display spatial patterns of nitrogen ā€œload from each pixel that eventually reach the stream.ā€ As such, values >0 should appear on the landscape, and not actually in any streams, so something went wrong with this model run.

From looking at your screenshots, one thought is that perhaps your spatial extent may somehow be limited to only the streams themselves. What does your Watersheds input vector look like? It should consist of polygon(s) that cover your entire area of interest, like the beige underlying polygon in your screenshots; it should not be a linear stream layer.
Please upload your log file (ending in .txt) here too. That will help us to further diagnose what went wrong.

Thanks,
Jesse

2 Likes

Hi Jesse,

Thank you for your insight! Iā€™m not quite sure what the issue was, but I think it likely could have been a faulty DEM reconditioning that I did. I fixed the reconditioned DEM, re-ran my model, and got the correct output for n_export

Thanks again for your time and expertise!