Land use layer in SDR model

I have a trouble when using SDR model.
I imported 2 periods of land use layers to SDR model to see the differences from land transition.
However I got just the same retention results from these two layers.
Is it possible that those 2 layers I imported didn't have enough transition so I got just the same retention?

Hope someone can help me.


  • swolnyswolny Member, NatCap Staff
    Hi @ansineileen -

    It is possible that there is not enough transition to cause a difference. Are the retention values exactly the same, or are they just close? What about the export values? Really, we recommend only using the absolute values for export, not retention, so it is better to compare the export values when evaluating results.

    ~ Stacie
  • Thanks for your reply.

    I compared the sed_retention maps of three periods on GIS. All of them showed no change or very very few changes.
    I also added up all the cells (retention) in the same community, but three LULC show totally the same amount.
    Moreover, my minimum value of sed_retention is a minus quantity. I checked the location of all the minus cells. All of them are around the contour of the prefecture (my study area).
    However, fallowing your advice, I check the sed_export. Three periods of LULC show different results, and the minimum value is 0. It matches to my expectation.
    If I can't solve the discomforts of retention, I'll only use the result of export.
  • swolnyswolny Member, NatCap Staff
    The minimum of sed_retention should not be a minus quantity - is it a large negative number, or a very tiny negative number that is close to zero?

    Also, what are your values for usle_c and usle_p in the biophysical table? As the User Guide says, sed_retention is calculated as 
    R * K * LS(1-C*P) * SDR

    so if (1-C*P) is negative, your result will be as well. In general, if you have a doubt about the result, it is always good to go back through the intermediate rasters and see if they help explain things. Here, you could look at ls.tif and sdr_factor.tif and see if there are any negative values as well. (Likely not, or you'd see them in the export results too.)

    ~ Stacie
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