Hi, I want to use the SDR model and have read the user guide, but feel confused about the results.
My question is, what’s the difference between sed_retention_[Suffix].tif and sed_retention_index_[Suffix].tif, and how does sed_retention_[Suffix].tif come from? It seems that I can’t find the equation.
Thank you!
Hi @Tookie ,
I’ve reached out to one of the model developers and agree that it is a little confusing. It sounds like you’ve already checked out the Users Guide, but the equation under Evaluating Sediment Retention Services is what is used for the sed_retention_index.tif output. The sediment_retention.tif output gives retention based on using SDR calculated for bare soil.
More soon!
Best,
Doug
Thanks Doug! Hope it will come soon. I’m wondering which result I should use. I have read some paper in our country context using Invest. It seems that most of them used rkls.tif minus usle.tif to evaluate soil retention, not even mentioned SDR. And in the User Guide it says sed_retention_index_.tif is NOT the sediment retained on each pixel. So is sed_retention.tif the sediment retained on each pixel? I found it says sediment_deposition.tif is the total amount of sediment deposited on the pixel due to retention from upstream sources. So is it right to use rkls.tif minus usle.tif plus sediment_deposition.tif for the sediment retained on each pixel?
Hi Doug, I wonder if there is more explanation now. Currently I used the SDR for scenario simulation, and shift the C factor to be larger, but got larger sed_retention_index.tif results, and smaller sed_retention.tif. Generally, the sediment retention should be smaller when the C factor becomes larger. So I wonder how to interpret sed_retention_index results, and how to use it?
Hi @Tookie -
While I’m also not sure about the difference between sed_retention.tif and sed_retention_index.tif (and would love clarification as well, especially since I’d think that an “index” would have values between, say, 0 and 1, but sed_retention_index.tif does not), I’ll say a few things about some of your other points.
Using rkls.tif minus usle.tif only considers the erosion produced on a particular pixel, it does not take into account retention of sediment coming to that pixel from upslope. So it depends what you’re trying to model, soil loss by pixel, or amount of soil loss reaching streams. Using the SDR-related results will also take into account what happens upslope/downslope of each pixel, and help evaluate soil loss to streams.
The only thing I’ve ever (rarely) used sed_retention.tif for is looking at the patterns (not quantity) of how soil loss would be affected if the natural vegetation was removed and turned to bare ground. Really, it’s just creating a quick scenario for us by comparing our input landscape with bare ground.
Usually, we evaluate sediment retention by comparing a “current” or “baseline” with some user-created scenario of change, and use the difference in sed_export.tif to show how the scenario changes the amount of erosion reaching streams, and where those changes occur. If you don’t have a scenario, and you’re looking at the watershed or sub-watershed scale, you can look at the ratio between sed_export and USLE in the watersheds output shapefile to see what percentage of erosion is being retained in the watershed.
To your last point, when the C factor is larger, it means that more erosion is being created, which also means that there is more erosion to retain. So it’s not surprising that sed_retention would also increase in that case.
Once all of this gets worked out, I’ll update the User Guide, hopefully clarifying retention a bit.
~ Stacie
Thank you for your prompt reply！ I still have two things to confirm.
In the user guide it says sed_retention_index is calculated with this equation: Ri⋅Ki⋅LSi(1−CiPi)×SDRi. So when C factor becomes larger the sed_retention_index.tif will be smaller. However when I used larger C factor and got larger sed_retention_index.tif. But sed_retention.tif is getting smaller. What’s the equation of sed_retention.tif?
If I want to get the pixel value of sediment retention, which result should I use?
Thank you so much!
Thank you for your prompt reply！ I still have two things to confirm.
Thank you so much!
Hi @Tookie ,
So when C factor becomes larger the sed_retention_index.tif will be smaller. However when I used larger C factor and got larger sed_retention_index.tif.
The Sediment Retention Index should be smaller with larger C values. If you’re seeing the opposite I’d love to take a look at your data and see what is going on.
If I want to get the pixel value of sediment retention, which result should I use?
You should use sed_retention.tif. This calculates the difference of sediment export on the real landscape and a bare soil landscape using calculated SDR for the bare soil. The equation used is:
(rkls * sdr_factor_bare_soil - usle * sdr_factor)
I interpret this as: “Compared to bare soils here is how my current landscape helps retain sediment”. @swolny can correct me here but, I think if you had another scenario you could compare those two sed_retention.tif rasters and say something like: “Scenario 1 has higher retention values than scenario 2 and thus maybe those vegetation LULC would be ‘better’ somehow”. To me, this is basically how Stacie describes using sed_export.tif but maybe less intuitive or useful.
Again, pixel level outputs are fine to look at but these models were designed at the Watershed and Sub Watershed scale.
As for sediment_retention_index:
(rkls - usle) * sdr_factor / sdr_max
I can’t say much more than what the Users Guide points out about it since I’m not the developer or science lead behind this model.
Cheers,
Doug
Thank you so much for your reply! It is so helpful!
Hi All and especially @Tookie. Reading through this thread, I suspect you actually want the downslope sediment deposition raster. It gives a good estimate of where sediment that has run off from a pixel has been retained downstream by the vegetation on the landscape: SDR Sediment Deposition User’s Guide Section.
The sed_retention_index.tif and sediment_retention.tif analyses were left over from very old versions of InVEST that was our only “what is the benefit” scenario at the time and before we derived the sediment deposition analysis. In reality, comparing the current state to a state where the entire landscape is bare exposed dirt isn’t really useful. I suspect in all practical cases folks, and you too, will want to use sediment_deposition_[Suffix].tif in lieu of either of the sed_retention rasters.
This topic was automatically closed 7 days after the last reply. New replies are no longer allowed.