Interpretation of SDR results (P factor)

What is the issue or question you have?

I just ran the SDR model for the northern part of Tunisia, initially the results were good, but when we incorporate the P factor in the biophysical table we don’t understand the results.

Since we expected that in the USLE map the areas with P values, the erosion should be less than the rest of the area, and yet the opposite happens, the areas where soil protection practices are carried out, the erosion is greater than the places where no practice takes place.
I share the link with the inputs and outputs to see if someone can help us.

What do you expect to happen?

One specialist reviews the inputs and outputs and mentionable if are corrects or if is necessary to adjust the inputs.

What have you tried so far?

I have reviewed the biophysical table and I don´t know if the results are correct, this is my first time using factor P. I don´t have other results to know if the model generates these results.

Attach the logfile here:
InVEST-natcap.invest.sdr.sdr-log-2022-10-26–22_28_59.txt (67.4 KB)

Hi @Gerardo,
It appears that you are using the P factor incorrectly. The default value representing no support practices should be 1, not 0. Please see the explanation here: SDR results all '0' - #6 by esoth and let me know if you have further questions.

Hi @esoth,
Thank you for your indications and I apologize for the delay to answer.

In my tries, I discovered when I used value 1 in the factor P, the results were correct. But I did not find how representative the value of practices.

So to confirm:
In the area that doesn’t have practices, I put 1.0, right?
For the sites where I have practices, I must put 1.x right?
If this is correct what is the range that I can use for the sites with practices?, of 1.0 to 2.0? (with decimals) or 1 to 10 (or whole numbers), which is the scale that I need to use?

Hi @Gerardo -

For places you have practices that reduce erosion, the value of P will be < 1. This might help visualize why…

The soil loss equation is
USLE = R * K * LS * C * P

So if your value of P is 1, it has no influence on the result. If the value of P is > 1, it increases soil loss. If the value of P is < 1, it decreases soil loss.

~ Stacie


Hi Stacie for you comments

I have run the model again using comments Emily and yours and I think that I have obtained good results.

I share the inputs and output if you have some comments