 # A question about the key word "constant_neumann" in source term?

dear OGS delevopementers and users:
I have a question about the key word “constant_neumann” in source term?
I want to know the meaning of t"constant_neumann 1.6e-10" in the blow example. and is it contact with the length of the ployline?
#SOURCE_TERM
\$PCS_TYPE
LIQUID_FLOW
\$PRIMARY_VARIABLE
PRESSURE1
\$GEO_TYPE
POLYLINE WEST_bc
\$DIS_TYPE
CONSTANT_NEUMANN 1.6e-10

Hi there,

I only started to use OGS recently, but swamini.khurana forwarded me the following (also second hand) explanation on the keyowrd \$CONSTANT_NEUMANN which works fine for me also with other than Groundwater Flow:

" ONLY FOR GROUNDWATER_FLOW

Source Term:

CONSTANT: m3/s (or time unit which has been specified in the .tim). Apply that volumetric flow rate to node and every node will get the exact same value.
CONSTANT_NEUMANN: m/s ( “ ) . Apply that rate (?, don’t know how to call it) to a polyline or surface and it will be multiplied by the length or area, respectively.

Example:
Apply a recharge of 800 mm/year to a polyline of a 2D model:

800 mm/year —> 2.5*e^8 m/s

Take this value and set it to CONSTANT_NEUMANN and it will be distributed over the polyline, ensuring that you get a spatially constant recharge of 800 mm/year. This is independent of the discretization, meaning you will end up with the same amount of water entering your model for every cell size. When you double the length of your polyline, you will be pushing the double amount of water into your model but the value at a certain location/node will remain the same
"

You can also find some info here

Best, Erik

Dear erik, thank you very much.
But I have a problem about 800 mm/year. What is its physical meaning?
Apply a recharge of 800 mm/year to a polyline of a 2D mode. How does it equal to the CONSTANT: m3/s.
My model is a brine trench to extract brine, so I know the annual total pump output.

Hi again,
800 mm/year would be your recharge rate, also precipitation would be a relevant physical example. But also it’s a velocity, in the case of \$CONSTANT_NEUMANN the Darcy velocity as I understood.
You can compute the Darcy Velocity (q) from the volume flux in CONSTANT (Q) by accounting for the area (A) the volume flows through, i.e.:
q [m/s] = Q [m3/s] / A [m2].

Cheers, Erik