Hi, I think the comparison ...

Hi,

I think the comparison between inflow and outflow is quite large, hence with Qout >> Qin with Qout is estimated using this formula:

Qout=mu*A*sqrt(2*g*h)

you can neglect the inflow (Qin) and use the following formula:

t=[(2*A0)/(mu*A*sqrt(2*g))] * (sqrt(h1) - sqrt(h2))

where A0 is your tank cross section (which is constant with respect to tank height I assume), mu is orifice outlet coefficient or gate valve coefficient in this case, g is gravitational acceleration, h1 and h2 is the water level before and after (in this case h2 is 0 since it will be emptied).

In case you want to include the Qin, you need to use another approach since it involves tank/reservoir storage equation. Also another treatment is needed if the areal cross section of the tank is not constant e.g. cone shaped.