Curve number equation raster calculator

Question

I have a floating point raster of Curve Numbers (70-91) and I'm attempting to use the curve number equation to create a weight raster for flow accumulation, to determine runoff for a single rainfall event. I'm familiar with this curve number equation:

Q = (P - 0.2S)2 / (P + 0.2S)

where

Q = total runoff P = preciptation (1 inch in this case) 0.2S = initial abstractions 0.8 = potential maximum retention after runoff begins

First question, would the precipitation part of the equation be a raster (e.g a raster where all cells have a value of 1) or would it just be the number '1' representing 1inch of rainfall? Secondly, as the 2 outside represents 'to the power of 2' i am unsure as how to do this in raster calculator. If anyone has any experience using this or a similar curve number equation to measure determine runoff any help would be appreciated. Thanks

MS10 · Answer

Hi, I tried that below using a max retention raster with values from 2-5 (mm of retention), however i'm still getting an error in raster calculator. The syntax I used is below.

Appreciate the help.

Edit: changed to ** 2 and it worked

("Precip" - 0.2 * "Max_retention") ** 2 / ("Precip" + 0.2 * "Max_retention")

DanPatterson_Retired · Answer

(P - 0.2S)2 / (P + 0.2S)

P and S are rasters

(P - 0.2*S)**2 / (P + 0.2*S)

But this is simplified from the papers in the search

MS10 · Answer

Thanks for the replies, yes that is the curve number methodology i'm attempting Chris. I've just tried Equation 5 from that paper,(P - 0.3)2 / (p - 0.7), using Power(1 - 0.3, 2) / (1 - 0.7) in raster calcuator, which gives the error 'Error 000539 error running expression rcexec()'