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Total catchment sediment load (E) vs. sed_export watershed output in SDR model

SPardoSPardo Member
edited June 2015 in Freshwater Models
Hi NatCappers,

I'm writing my final report for a study using the SDR model and as such I'm scrutinizing a lot of the documentation. I'm wondering, why is the total sediment export/load given at first as:

total catchment sediment load E (ton ha^-1 yr^-1) 

And then later the "Interpreting Results" section as:

sed_export (tons/watershed)

Does the model multiply E by the area of each watershed before exporting the results shapefile?
Post edited by SPardo on


  • SPardoSPardo Member
    I have one more related follow-up.

    Is it still the case that sediment is considered to have been exported and added to the watershed overland yield when it hits the stream network?

    The user guide defines the SDR as the "proportion of soil loss actually reaching the catchment outlet."

    I guess what this question boils down to is the use of the term "catchment outlet" instead of "stream network." 

    Please confirm that I'm just nitpicking here and that the SDR in fact DOES NOT account for instream process or anything else that may prevent sediment from being deposited before it reaches the watershed pour point.

  • PerrinePerrine Moderator, NatCap Staff
    Hi Sam, 

    The total sediment export is expressed in ton/ha in the model description (each pixel having the same size, it's possible to sum values in ton/ha).
    In the model outputs, sediment export is expressed in ton/watershed (in the .dbf table). So you are right, the model multiplies the values per ha by the catchment area.

    On instream processes: correct, the model does not represent instream processes, so the sediment export in the .dbf table represents the amount of sediment reaching the stream at any point in the stream network.

    Good luck with your report!


  • Hi Sam and Perrine,

    I am also puzzled about the result.If the size of my pixel is 30m. The result sed_retention.tif 's unit is t/pixel.So, if i want to express it at the unit of t/ha.So i can use the sed_retention.tif X 900/10000.Then the unit is t/ha. Are my thought right?


  • swolnyswolny Member, NatCap Staff
    Hi Lijuan -

    Yes, you are right that the result is in tons/pixel, and you can convert to tons/hectare using your equation.

    However, we don't recommend using sed_retention as absolute values. The User Guide says, "This index underestimates retention since it does not account for the retention from upstream sediment flowing through the given pixel. Therefore, this index should not be interpreted quantitatively." If we use the sed_retention output, we give it a qualitative ranking from low to high, we don't use the absolute values. 

    If you have scenarios, and want a quantitative result for retention, you can use the difference in sed_export between the two scenarios. That will show, for example, how much more sediment is being retained by the landscape if an area is reforested, therefore it is not making it to the stream to become export. Taking the difference in export does account for retention from upstream sediment.

     ~ Stacie
  • hi
    if I want to use sed_export between the two scenarios, so I want to express it at the unit of t/ha, the Sed_export also is X 900/10000?
  • swolnyswolny Member, NatCap Staff
    Yes, the units for sed_export are tons/pixel, so you can convert that to tons/ha using the cell size of the pixels in your sed_export raster.

    ~ Stacie

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