difference between sediment retention and sediment retention index in SDR model

Sediment retention index is calculated by
the equation in user guide: R*K*LS*(1-CP)*SDR, and it
emphasized that “this index underestimates retention since it does not
account for the retention from upstream sediment
flowing through the given pixel”. For sediment retention, “The value of the
retention service is then based on the difference between the sediment export
from this bare soil catchment and that of the scenario of interest”. There is
no equation for sediment retention. The equation for sediment retention index
can be changed to: (RKLS-USLE)*SDR=RKLS*SDR-USLE*SDR= RKLS*SDR-sediment export.
In my opinion, this is how to calculate sediment retention. So, I really
confused by this question, looking forward for your answer.
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  • PerrinePerrine Moderator, NatCap Staff
    Hi Crystal,

    Good question. The reason why  RKLS*SDR - sediment_export  underestimates the actual pixel retention is that this value does not account for the role of the pixel in retaining sediment from the upslope areas. 

    Remember that the SDR value for a given pixel depends on the LULC characteristics (the C value) of all pixels on the downslope flow path. So each downslope pixel contributes a tiny amount to this retention. Calculating this amount requires switching a pixel to bare soil and re-calculating the SDR for all pixels above, for each pixel, which would be computationally intensive.

    This is why we use the value of the retention at the subwatershed scale, by converting all pixels to bare soil in one shot.

    I hope this helps,


  • Hi Perrine,

          Thank you very much for your explicit reply, I can understand it now.

    Best wishes!


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