USLE output generated from InVEST SDR

NatCap Software Support
1

I have found the usle output generated from InVEST SDR model to be smaller than the one that I run directly in ArcGIS. Would there be any possible reason?

2

Hi @Habtamu_Assaye,
Do you mean the raster extent is smaller, or the values are smaller? What is your ArcGIS method for calculating USLE? Could you please upload the two versions? Thanks.

3

Dear esoth,
I have run the RUSLE using the raster calculator in ArcGIS using same input rasters of R, K, LS, C and P that I used for InVEST. However the usle output from InVEST was quite different from the one I found from calculated using the raster calculator. I have attache the two rasters. usle_SDR3 is the InVEST SDR output and A3 is using rastrer calculator. I was expecting similar result but there is big difference. I can send you all other files if needed.
Thank you for your support.
usle_SDR3.tif (146.7 KB)

4

Hi @Habtamu_Assaye -

It looks like only one of the rasters was attached (usle_SDR3.tif), would you try to upload the other one again? Thanks!

When you’re doing the RUSLE calculation using raster calculator, are you using the equations given in the InVEST User Guide, or different equations? In particular, we do the LS calculation a little differently. I don’t know if this explains your results, but might.

~ Stacie

1 Like
5

Hi@swolny,
I am sorry it is not uploading the other raster, a bit strange. I used the Dezmet and Govers 1996 method of LS factor determination which is integrated in the SAGA gis within the qgis environment which is similar to what InVEST SDR uses. My e-mail is habtamuasaye@gmail.com and I can send you all the factor files if you can help me figure it out. Thank so much indeed!

6

Hi @Habtamu_Assaye,
It would be helpful if you can send the R, K, LS, C, P files and the ArcGIS output as well so we can check if it’s working as expected. You can email them to me: esoth @ stanford.edu Thanks!

7

Hi @Habtamu_Assaye,
Thank you for sharing your data with me! I’m going to reply here in the forum in case this is useful to others. The output from InVEST looks correct to me.
There is a small mistake in the user’s guide that might have led you astray:

image
image940×193 11.2 KB

It incorrectly compares θ to percent values, which really should be radians: arctan(9/100) ≈ 0.08975, so this should say θ < 0.08975 and θ >= 0.08975. I will update it today.

However I doubt that would cause the large difference in USLE you are seeing. I assume you are using the same USLE equation in ArcGIS: USLE = R x K x LS x C x P.

  • For the LS, C, and P inputs in ArcGIS, are you using those intermediate outputs from InVEST (intermediate/ls.tif, intermediate/cp.tif), or are you calculating them independently?
  • If you are calculating LS independently, are you sure you’re using the same method (described here)? There are some different formulas for LS that could account for the difference.

Thanks for your patience working this out!

3 Likes
8

Dear Emily,
Thank you for your detailed response. I calculated LS, C and P independently. For the LS, I used Desmet and Govers 1996, directly from SAGA gis integrated with the Qgis environment which uses similar equations described in the InVEST guide. I will check with the updated version of the program and come back if I will still have further questions. Thank you so much indeed for your time, willingness and support!

With best regards,