There must be a minor difference between flow and pressure.
Water Flow :
Water Flow is a measurement of how much water is delivered at a particular outlet over a set period of time. For example, if you place a 10 litres bucket under the tap in a sink and it takes 10 seconds to fill the bucket you can see that the flow rate is 1 litre per second.
Water Pressure :
Water Pressure is a measurement of the force exerted by the water.
We understand it by one example a cold water storage cistern in the attic may be used to supply water to a basin in a bathroom and a basin in a downstairs cloakroom. Assuming everything else is equal you will notice that the pressure at the downstairs tap is considerably more than that at the one upstairs. The increased pressure is due to the height of the cistern in relation to the tap.
Higher pressure will cause greater flow through any given pipe size, but as the flow increases, the pressure will decrease downstream due to friction loss because water velocities increase as well.
For any flow to happen, there is a requirement of pressure gradient and not the pressure. Higher the pressure gradient, keeping all other things (fluid, pipe diameter, length) constant, higher is the mass flow rate. But higher pressure does not reveal anything about the flow.
Water Flow :
Water Flow is a measurement of how much water is delivered at a particular outlet over a set period of time. For example, if you place a 10 litres bucket under the tap in a sink and it takes 10 seconds to fill the bucket you can see that the flow rate is 1 litre per second.
Water Pressure :
Water Pressure is a measurement of the force exerted by the water.
We understand it by one example a cold water storage cistern in the attic may be used to supply water to a basin in a bathroom and a basin in a downstairs cloakroom. Assuming everything else is equal you will notice that the pressure at the downstairs tap is considerably more than that at the one upstairs. The increased pressure is due to the height of the cistern in relation to the tap.
Higher pressure will cause greater flow through any given pipe size, but as the flow increases, the pressure will decrease downstream due to friction loss because water velocities increase as well.
For any flow to happen, there is a requirement of pressure gradient and not the pressure. Higher the pressure gradient, keeping all other things (fluid, pipe diameter, length) constant, higher is the mass flow rate. But higher pressure does not reveal anything about the flow.
 The pressure is defined as the force acting perpendicular to the surface of an object per unit area over which the force is distributed.
 In the case of gases, this force is because of the collision of the gas particles with the surface. So, the pressure exerted by the gases in a given environmental condition is more of a statistical average than average value.
 In terms of liquids, the pressure exerted is the weight of the liquid over a surface, acting on that surface.
 The definition of flow is subjective depending on the time and length scales considered. Since we defined flow as the bulk movement of fluid particles.
If we need more water, so increase the pipe size so we don't lose more pressure to friction loss.
PSI:
Pounds per square inch, the standard measurement of pressure in the United States.
Water Velocity:
The accepted standard for water velocity in piping systems is 5 feet per second or less. As flow increases in any given pipe size, the velocity of that water also increases. As velocity and/or flow increases in any given pipe size, the PSI loss also increases. The means of decreasing pressure loss for a given flow is to increase pipe size. (diameter)
Friction Loss:
The PSI loss which results from friction against the interior walls of pipes, directional fittings, valves or any other obstruction to the irrigation water. Once again, as flow increases so do friction loss. Friction loss is synonymous with PSI loss.
GPM, GPH, GPD:
Gallons Per Minute, the standard measure of flow; Gallons Per Hour, often used for lowvolume flow such as drip irrigation; Gallons Per Day, a measure of overall water use on a daily basis.
Feet of Head:
Another term for water pressure. The pressure is directly affected by elevation change, and every 2.31 vertical feet of change upwards will decrease pressure by 1 psi in a holding tank. That is why such enormous pressures exist in the deep ocean; enough to crush a submarine as depth increases. Another way to look at it: each 1 foot elevation change equals .433 "feet of head".
Total Dynamic Head:
TDH is a measure of overall head (pressure) loss in a water system. When an irrigation designer or engineer calculates all of the friction (pressure) losses and outlet pressure required for an irrigation system, they will express the number as TDH. If an irrigation system has a maximum TDH of 250, that means that just over 108 PSI will be required to power the system.
Pounds per square inch, the standard measurement of pressure in the United States.
Water Velocity:
The accepted standard for water velocity in piping systems is 5 feet per second or less. As flow increases in any given pipe size, the velocity of that water also increases. As velocity and/or flow increases in any given pipe size, the PSI loss also increases. The means of decreasing pressure loss for a given flow is to increase pipe size. (diameter)
Friction Loss:
The PSI loss which results from friction against the interior walls of pipes, directional fittings, valves or any other obstruction to the irrigation water. Once again, as flow increases so do friction loss. Friction loss is synonymous with PSI loss.
GPM, GPH, GPD:
Gallons Per Minute, the standard measure of flow; Gallons Per Hour, often used for lowvolume flow such as drip irrigation; Gallons Per Day, a measure of overall water use on a daily basis.
Feet of Head:
Another term for water pressure. The pressure is directly affected by elevation change, and every 2.31 vertical feet of change upwards will decrease pressure by 1 psi in a holding tank. That is why such enormous pressures exist in the deep ocean; enough to crush a submarine as depth increases. Another way to look at it: each 1 foot elevation change equals .433 "feet of head".
Total Dynamic Head:
TDH is a measure of overall head (pressure) loss in a water system. When an irrigation designer or engineer calculates all of the friction (pressure) losses and outlet pressure required for an irrigation system, they will express the number as TDH. If an irrigation system has a maximum TDH of 250, that means that just over 108 PSI will be required to power the system.
Conversion Factors:
Pressure  
To Convert From: 
Into:
 Multiply By: 
PSI

Feet of Water

2.307

PSI

Pounds/Sq.Foot

144

PSI

Atmospheres

.06805

PSI

Bars (metric)

.06895

PSI

Inches Water @ 39.2 F

27.681

PSI
 Millimeters Mercury @ 0 C 
51.715

Feet of Head

PSI

.433501

Bars (metric)

PSI

14.5038

Bars

Feet of Head

33.4883

Bars

Pounds/Square Foot

2089

Bars

Atmospheres

.98692

Bars

Centimeters Mercury @ 0 C

75.0062

Bars

Inches Mercury @ 32 F

29.53

Flow
 
To Convert From:

Into:

Multiply by:

GPM

Gallons/Hour

60

GPM

Cu. Feet/Second

.002228

GPM

Cu. Feet/hour

8.0208

GPM

Cu. Meters/Second (metric)

.000063

GPM

Cu. Meters/Hour

.2268

GPM

Liters/Second (metric)

.06308

GPM

Liters/Minute

3.7853

GPM

AcreFeet/Day

.0044192

GPM

Millions Gallons/Day

694.444

Millions Gallons/Day

AcreFeet/Day

3.0689

Millions Gallons/Day

AcreInches/Day

36.8266

Millions Gallons/Day

Gallons/Hour

41,666.667

Millions Gallons/Day

GPM

.00144

Liters/Minute (metric)

GPM

.26418

Liters/Minute

Gallons/Second

.004403

Liters/Minute

Cu. Feet/Second

.000588

Liters/Minute

Cu. Feet/Minute

.0353
