Runoff Raster Layer Calculator 10.4

[SarahDawson]

I'm trying to create a runoff layer using this equation:

Q = (P – 0.2S)2 / (P + 0.8S)

The S in this equation is a soil retention layer that I made using this equation with curve numbers:

S = (1000.0 / CN) – 10.0  

P, in this instance, is rainfall total for the storm-- in this case, a one-year storm for the watershed I am studying. I only have one value for P (3.01).

The soil retention layer is working, but the runoff layer isn't going through. I keep getting this error message when I type out the equation: "Error 000989: Python syntax error; parsing error; SyntaxError: invalid syntax (Line 1).

The instructions I have are part of a lab, and they also have this addendum:

"Use the S layer along with the P value to compute a Q layer, a cell by cell runoff layer from the 1-year storm. Use the standard equation below (assumes “normal soil moisture at the beginning of the storm”): Q runoff (inches) P rainfall total for the storm (inches) VERY IMPORTANT: Runoff does not start until P is greater than 0.2S. If P < 0.2S, then Q=0. You will need to set runoff to “0” in cells where P < 0.2S. Note the raster calculator Con function is helpful here."

So i'm not sure where to go from here, since everything I have tried thus far, including the usage of the Con function, have not worked. Any suggestions are welcome!

[SarahDawson]

Did NOT mean to post this three times, oops

[JayantaPoddar]

Using Raster Calculator, just write the expression on the right, i.e.,

Con(((0.2)*S)>(3.01),0,((((3.01)–((0.2)*S))**2)/((3.01)+((0.8)*S))))

Replace S with the Soil_Retension Raster, in the above equation. 

[XanderBakker]

I guess the solution that Jayanta Poddar provided will work, although you could probably reduce the number of brackets.

I ended up with:

Con(3.01 < 0.2*S, 0, (3.01 – 0.2*S)**2 / (3.01 + 0.8*S)

... assuming that the 2 in Q = (P – 0.2S)2 / (P + 0.8S) is a square: Q = (P – 0.2S)² / (P + 0.8S)

And if you don't want to calculate the soil retention before, you could do this:

Con(3.01 < 0.2*((1000.0 / CN) – 10.0), 0, (3.01 – 0.2*((1000.0 / CN) – 10.0))**2 / (3.01 + 0.8*((1000.0 / CN) – 10.0))

Please note that you are dividing by CN. In case CN is 0, this will result in NoData values.