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SSURGO K factor with or without rocks?

SPardoSPardo Member
edited July 2014 in Terrestrial Models
I noticed that the SSURGO Soil Data Viewer provides 2 K factor options:
K factor, Rock Free
K factor, Whole Soil

Do I need to grab both and then mosaic in the rock free values for ag land cover and whole soil for the rest?
Post edited by kglowinski on


  • edited July 2014
    Hi Sam,

    For the RUSLE equation, the K factor considers the whole soil so you will want to use the equivalent. Unless a soil has substantial rock fragments, both values will be the same. I think it will be fine to just use the whole soil value for agricultural land as well.


    Post edited by BradEichelberger on
  • SPardoSPardo Member

    Thanks for the clarification Brad.

    I've got one more K factor question for you, how should we handle the "not rated or not available" areas?

    The land cover is almost always either water or medium/high intensity development, which makes sense in regards to those areas not be rated. I'm wondering if having NoData pixels in the K raster will eventually trip up the model run as the user guide states: "K is a GIS raster dataset, with a soil erodibility value for each cell."

    I guess I will find out soon enough when I finish preprocessing the data and give the new model run a shot...
  • SPardoSPardo Member

    I actually found an NRCS presentation online that says RUSLE2 uses Kf (K factor, rock free) because using Kw (K factor, whole soil) would be double counting.

    It's hard to know for sure because the following reference is only a powerpoint with sparse annotation, but it sounds like the rock fragments are somehow taken into account in the crop and support management parameters:,d.aWw&cad=rja

  • Hi Sam,

    RUSLE2 is a different model than InVEST, which is based on USLE/RUSLE(1). With InVEST, you will want to use K factor for whole soil.

    For the NoData values, I usually assign a value of 0 as there will be no sheet erosion within bodies of water or developed land.


  • SPardoSPardo Member
    Thanks Brad.

    I got the same feedback from an NRCS agronomist that said while RUSLE2 uses Kf, it does so because it was meant for analysis at the field level where % of rock fragments can be sampled.

    She went on to say exactly what you did, that for a watershed scale application Kw is the appropriate SSURGO attribute.
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