Flow Accumulation is a raster where: for current pixel
p_i the value of
p_i is the number of upstream pixels that enter into
p_i. This helps us compute the
contributing area by multiplying by the
cell_area. This is used in the
LS equation as shown in the Users Guide here: SDR: Sedim
Can it be the same raster as the one i use for the drainage layer?
No, these are separate and have different meanings. The drainage layer is just an etching of which pixels exit downstream and out of the watershed. And also, which pixels should not have SDR related outputs because it’s a waterway.
Have you any suggestions how to substitute the flow accumulation raster to the model?
I would first suggest using your drainage layer as input and setting the threshold value to unreasonably high so that you get a stream layer of all 0s. I would take a look at these results and see if how they fit your data, and to get a sense of what the model and what the drainage layer is doing. It might also be worth playing around with the flow accumulation threshold to see how the stream orders can change.
From there you could do a few things off the top of my head:
- Burn your drainage layer into the DEM by slightly raising DEM values around your drainage layer. Then use that DEM as input into the model and see if the stream layer then aligns better with your expected drainage layer. Note, there are consequences to this because
slope is a big factor in the model and by altering the DEM you are also altering slope values.
- Take the SDR produced Flow Accumulation and manually set values to 0 where you know pixels should not be contributing. Then edit source to use that raster specifically.
- Consider that this SDR model does not suit your needs. This is a general purpose model that is based on the USLE equation. Read the Limitation section of the User’s Guide: SDR: Sediment Delivery Ra
Do i have to make further changes on the source code or just copy the raster into the output folder were the regular flow accumulation raster is saved?
It’s complicated, but I think it would be easiest to just use the path of the modified flow accumulation raster in the code directly, commenting out the code that calls flow accumulation, and replacing the path variable that the model expects with the new modified path.
This is confusing and complicated because it is stretching the limits on what this model is intended to do and the problem it was designed to solve.