Hi there,
I am working on establishing C and P factors for all my LULC types. For water, ice, and urban areas (essentially not applicable), I have seen some literature which categorizes these as 1, and some as 0, to indicate that they don’t have a value. Based on my limited understanding of the model, choosing either 0 or 1 for these LULC types could have a significant impact on my results. What have other users of the SDR model used for these LULC types to indicate their lack of relevance (0 or 1)?
Any insight is appreciated! Thanks in advance.
Kind regards,
Michelle van Hilten
For water and ice, I generally give a value of 0. If these are lakes, they’re much more likely to trap sediment than produce it. Rivers generally have a complexity of in-stream erosion, but our model does not include in-stream processes at all, so I still give them a value of 0, which again indicates that they themselves are not contributing to the specifically overland erosion that the model does include.
For urban areas, I’d say that it depends on what they are like. In some places, what’s called “urban” in a land cover map might actually be mostly unpaved, dirt roads, maybe some vegetation, etc, so I tend to give these a value that represents bare soil, or an average of paved, bare soil, vegetation, etc, depending on the proportion of these. If it really is just a big, solid concrete jungle, then yes, a value of 0 or close to it is appropriate.
Can I just carry on the thought experiment here, to make sure I understand…
Is it correct to say:
c = 0 0% of incoming sediment is exported (a permanent sediment trap)
c=1 100% of incoming sediment is exported
c=0.2 20% of incoming sediment is trapped
e.g., an empty in-ground swimming pool might be c=0, a piece of concrete on a 50% slope might be 1
The C factor has a direct multiplier effect on USLE, as you noted. But its relationship with sediment export is more complex. If you look at the equations for sediment export in the User Guide, you’ll see that the C factor is used in different ways in the Dup and Ddown calculations that go into the SDR value.