# Compute discharge through an internal zone
**URL:** https://forum.itascainternational.com/t/compute-discharge-through-an-internal-zone/2015
**Category:** FLAC3D
**Tags:** fish, tips
**Created:** [April 1, 2025, 10:02am UTC](https://forum.itascainternational.com/t/compute-discharge-through-an-internal-zone/2015 "2025-04-01T10:02:35Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@stephaneBeaussier](https://forum.itascainternational.com/u/stephaneBeaussier)
#### Post date: [April 1, 2025, 10:02am UTC](https://forum.itascainternational.com/t/compute-discharge-through-an-internal-zone/2015/1 "2025-04-01T10:02:36Z")
Hi,
I want to compute the fluid flow through an internal limit in the model.
For example the flow that pass a certain given depth. Therefore the computation can be on zones or gridpoint depending of what is the simplest.
Because it isn’t a boundary with a fixed pore pressure I cannot use the fish function gp.flow.
My guess is that I could use the zone.flow function but I would need to compute the area of the zone section normal to the discharge vector to get a volume per time.
Will that give me an accurate estimation of a different approach computing all the source and sink terms is needed?
Thank you in advance!
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### Author:  [@jhazzard](https://forum.itascainternational.com/u/jhazzard)
#### Post date: [April 15, 2025, 6:54pm UTC](https://forum.itascainternational.com/t/compute-discharge-through-an-internal-zone/2015/2 "2025-04-15T18:54:25Z")
Yes - this is correct. You could take the zone.flow vector and multiply this by the cross-sectional area of some plane intersecting the zone. I did a simple test on [this example](https://docs.itascacg.com/itasca930/flac3d/zone/test3d/Fluid/FreeSurfaceSteadyStateFluidFlow/freesurfacesteadystatefluidflow.html) assuming a vertical plane and it worked fine. The tricky part would be calculating the cross-sectional area for an arbitrary plane …
```
fish def test_discharge
total_flow_z = 0
xarea = 0.15*0.3 ; y * z
loop foreach zp zone.list
if zone.pos(zp)->x < 0.3
total_flow_z += zone.flow(zp)->x * xarea
endif
end_loop
total_flow_gp = 0
loop foreach gp gp.list
if gp.pos(gp)->x < 0.01
total_flow_gp += gp.flow(gp)
endif
end_loop
end
[test_discharge]
```