What are the units of the SDR outputs

Software Support
1

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!

2

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

2 Likes
4

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

5

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

6

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)

7

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?

8

The avoided erosion equation is
RKLS - USLE

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

9

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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