Does a higher water volume increase pressure? I am constructing a gravity flow water system. I have 100ft point where I can put my tank. My question is does the size of my tank matter? I am using a 1" pipe. Will I get more pressure if I use a bigger tank? For example what is the difference in pressure if I use a 10 gallon tank or a 50 gallon tank?
 A: 
The static pressure $p$ depends solely on the height $h$. If $p_0$ is the atmospheric pressure, then:
$$p=p_0+\rho gh$$
Where $\rho$ is the liquid's density and $g\approx10\:\mathrm{ms^{-2}}$.
So for the left hand side tank the pressure would be slightly higher because the tank is taller (not because it has higher or lower capacity).
Flow through the pipe always causes some viscous pressure loss though.

Answering OP's concern (comment section):

Do I need a tank and secondly as far as I can gather from you guys it is the pressure that the water leaves the bottom of the tank is the pressure at the end of the pipe?

If you have easy access to the stream, a tank is not required.
The pressure discussed in my answer is called hydrostatic pressure and for good reason: if one blocks the end of the pipe, the pressure $p$ at that point would be exactly as indicated above ($p=p_0+\rho gh$, where $h$ is the height difference between the water's open surface and the pipe's end).
But when flow is allowed a certain amount of pressure is lost due to friction in the pipe. In that case the corrected pressure can be noted as:
$$p'=p_0+\rho gh-\Delta p$$
Without going into full details of pipe flow theory we can say with certainty that $\Delta p$:


*

*increases with pipe length,

*decreases with pipe diameter,

*increases with volumetric throughput (flow speed),

*increases with pipe inside roughness: smooth, uncorroded, new pipes reduce pressure less,

*increases in the presence of local resistances like bends, kinks, valves, sudden diameter changes and such like.


A short, wide, smooth, straight pipe operated at low flow speed will deliver pressure almost identical to the hydrostatic pressure $p$.
But using a long, narrow (etc) pipe can lead to almost all of the hydrostatic pressure being lost to friction ($\Delta p \approx p_0+\rho gh$).
Further reading: Darcy-Weisbach.
A: If you have a tank with an outlet at the bottom then the pressure in the outlet is the same as the pressure in the water at the bottom of the tank.
The pressure $P$ at the bottom of the tank is $P=\rho g h$ where $\rho$ is the constant density of water, $g$ is the acceleration due to gravity, and $h$ is the height of your tank.
This shows that for your tank the pressure at the outlet depends only on the height of your tank.
That sounds like an odd result but think about this:
If you swim to the bottom of a 2m deep pool you can feel some pressure. If you're in a small back-yard pool it feels like the same pressure as an olympic sized swimming pool. The volume of the pool isn't affecting the pressure 2 metres down.
A: The pressure experienced at the bottom of the pipe depends on the diameter of the pipe, the flow rate, the total height difference between the surface of the water and the point where you measure the pressure, and the density of the liquid.
When the flow rate is zero (no liquid flows: before you open the valve) the only thing left is the density and the height difference, and the pressure at the bottom will be higher than atmospheric pressure according to
$$\Delta P = \rho g \Delta h$$
Where $\rho$ is the density of water (1000 kg/m$^3$), $g$ is the gravitational acceleration (9.8 m/s$^2$), and $\Delta h$ is the height difference (100 ft).
Now if you have a small container at the top, then when water starts to flow the water level is likely to drop quickly - and this will give a small drop in pressure (small, because with 100 ft initial height there just isn't a lot of height to drop in a 10 gallon container). But otherwise there will be no difference between the two containers.
A: Gravity flow pressure can be figured, or measured, simply by two methods:
1) divide the drop in elevation (in feet) by 2.31 OR........2.31 feet of drop = 1# of pressure
2) if you have a system already established, screw an inexpensive water pressure gauge on a hydrant, or hose bibb and read the pressure
NOTE: the volume of water in a tank does not increase the water pressure. Height, or amount of drop, is what creates water pressure. CAVEAT: I am a lay person, not an engineer, so can only explain this in simple terms. And, being a lay person, I have found that it is difficult to wrap your head around the fact that the volume of a tank does not impact water pressure. Only the height of the tank plays into the amount of water pressure you'll have.
Friction will reduce water pressure. The more elbows and the distance/length of the pipe will change the pressure.
