I try to calculate the critical channel slope (J) for each pixel of a channel raster layer by use of the raster calculator in QGIS. The formula is J=(A/(1+(B/"CatchmentArea")^(C)))+D and A,B,C,D are parameters with known values. For "CatchmentArea" I generated a raster and each pixel of the raster (masked to channel layer) has a specific value. If I use the formula like written before it works really well and I get a raster with correct values for J. But to obtain the right J of a channel pixel I have to calculate this with the catchment area value of the channel pixel located above.
Is there any opportunity to take this into account in the formula of the raster calculator? Or to assign for the whole raster the (first) calculated value of J afterwarts to the raster pixel located below?
The aim is to compare the critical channel slope of each pixel with the real channel slope to get outline areas for debris flows.