[Invest-forum] Calculating erodibility

[noreply at ecoinformatics.org]

Calculating erodibility

http://invest.ecoinformatics.org/discuss/tier-1-sediment-retention/514745455/227033636

    Hello, I am about to use the avoided sedimentation model. It seems
   the biggest road block will be calculating the soil erodibility. The
   description in the InVEST appendix (1.005) is somewhat confusing for
   me. I'm hoping there might be a more detailed tutorial or explanation
   out there or perhaps someone confirming what I write below will
   suffice. Based on reading the appendix it seems that I calculate soil
   erodibility (K) for each soil horizon and then take a weighted average
   for all horizons. So, lets assume I have 1 homogeneous soil polygon
   that contains, lets say, 3 horizons. Based on the soil data available
   to me from STATSGO or SURGO I am able to calculate K for my polygon
   for horizon O. This was fairly simple because all the variables used
   to calculate K are in my soil data (and hopefully easy to find -
   please confirm this, well that they are in the soil data). So in this
   polygon for horizon O I can first create a new field, K, use the field
   calculator and plug in the K equation. I think this will work, the
   alternative is to make a raster of each variable in the equation and
   use raster calculator. Now I repeat this process for horizon A and
   come up with some other value of K. I repeat again for Horizon B and
   come up with a third value of K. Here is where I do the weighting (I
   don't know much about soils, but I assume the top layer in the one I'm
   really interested in????) So I would include that K value 3 times and
   the other K values 1 each and take the average by dividing by 5 (or
   something similar). Now I have my weighted K for 1 soil polygon across
   3 horizons. I repeat this for all other polygons and now I have my K
   for the entire study area. But, somehow I am suppose to weight K based
   on the soil type component. I don't understand why this part is
   necessary as I just calculated K independently for each soil type
   component. Why is this necessary or am I not understanding this part?
   Any help clarifying this process would be much appreciated. Thanks!
   Kevin