What are the units of the SDR outputs

Hey all,
Can somebody help me with what the units are of the outputs ‘avoided export’ and ‘avoided erosion’ in the Sediment Delivery Ratio model? I know that they represent the valuation of vegetation, but what exactly do the numbers indicate? Thanks!

Hi @Willem, and welcome to the forum!

As noted in the InVEST User Guide, the units of avoided export and avoided erosion are tons/pixel/year. The User Guide also contains more information about these outputs in the section Evaluating Sediment Retention Services.

~ Stacie


Dear Swolny,
Thank you for your helpful comment. Upon converting the unit of soil loss (usle) tonnes/pixel/year into tonnes/hectare/year (by dividing it by 25 since the pixels are 500x500m each), I come on an average soil loss (usle) value of around 22 ton/hectare/year. Usually, these values should barely exceed 10. Do you know what could have gone wrong?

I checked:

  • The inputs have the right units
  • The inputs have realistic values
  • The output map shows higher avoided erosion rates in realistic/expected locations (e.g. at steep slopes).

So there seems to be something wrong with the total values. Did you ever had a similar problem? I have attached the my inVEST output files in this link may that prove useful: SDR output - Google Drive

Hi @Willem -

It is often the case that model results are significantly different than reality, especially with a model as simplified as SDR. We always recommend calibrating the model with real-world data before using the absolute values in any sort of decision-making context. If it’s not calibrated, we feel much more confident in the relative high-low values and spatial patterns, not so confident in absolute values.

~ Stacie

I see, that makes sense. Thanks a lot once again :slight_smile:

EDIT: For those interested, I solved the issue. I had to update the usle_c values in the biophysical table, they were too high (hence, erosion rate was also too high)

Quick question: Are avoided export and avoided erosion outputs the amount of erosion/export that vegetation currently is avoiding, or is it the potential when there would be vegetation on those locations?

The avoided erosion equation is

RKLS does not include vegetation factors. USLE = RKLS plus the C and P factors, which do add the effect of the vegetation in the LULC map. So the difference between them is showing the effect of current vegetation on erosion from that pixel.

~ Stacie

Thank you! It is interesting to see that a changed P factor has neglectable impact on the avoided erosion but has a significant impact on the avoided export.

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