Pressure is a measure of how much force is acting over an area. The si unit of pressure is pascal (Pa), but is in water supply typically used in the unit bar or mH2O (meters of water column). 1 bar is exactly 100.000 Pa, which is sligtly less than the current average atmospheric pressure on Earth at sea level (101.325 PA). Commonly 1 bar is roughly equal to 10 mH2O, but in reality it is a little bit more (10.1974 mH2O).
in water distribution pressure is produced by pumps in a waterworks which is transferred to the distribution network. when you open your tap the pressure causes the water to run out of your tap. if there were no pressure there would be no water in your tap. if there is a high pressure your water will run out with force and with low pressure, you will be upset on how long it takes to fill you bathroom tub, kitchen sink or a glass of water.
A key concept in water distribution is pressure loss. Pressure loss happens when there is a high resistance in the pipe and in the analysis of a distribution network, it is very important to know the pressure loss throughout the network and analyzing if a consumer has to low pressure.
Pressure loss cannot be directly determined by just looking at the difference in pressure from the waterworks and a measured pressure at a consumer. In order to determine pressure loss we need to introduce head also known as the hydraulic grade line.
Hydraulic head is liquid pressure above a vertical datum. Typically this refers to the elevation of the pipe plus its pressure.
Imagine a pressure of 50 mH2O from a waterworks at elevation 0 m.a.s.l (Metres Above Sea Level). Some distance down the pipe path, there is a hill where the elevation rises to 20 m.a.sl. and the pressure is here measured to 30 mH2O. THis does not mean the pressure loss is 20 mH2O, since the pressure would increase again on the other side of the hill. In this example there is no pressure loss since the sum of the elevation and the pressure for both measuring points are 50 m.a.s.l.
In reality there will be a pressure loss along the pipe path depending on the pipe resistance and pipe length. A more realistic pressure measurement on the top of the hill would be slightly lower than 30 mH2O and the hydraulic head would be lower than 50 mH2O and the pressure loss could easily be determined by subtracting the two values from each other.