Metres of H2O (mH2O)

Meters of water gauge or column is a metric unit for measuring liquid level. 1 metre of water column at 4 degrees celsius equals 9806.65 pascals.

The hydrostatic pressure generated by a certain liquid level is typically represented by the equivalent height of a water column.
Since the pressure exerted by one metre of water is dependent on its density and the local gravity it is not a fundamental unit of measure for pressure but a derived one which is called a manometric unit.

Metres of water column or water gauge are used throughout the world but mostly in Europe to measure the level of fresh water in boreholes, rivers and reservoirs.

Metres of water pressure units are expressed in mostly three different ways which are metres of H2O (mH2O), metres of water gauge (mWG) or metres of water column (mWC).

Since the density of a liquid is affected by changes in temperature, metres of water column should be accompanied by the temperature of the liquid that the units were derived from. A pure water density of 1000 kg/m3 at 4 deg C and standard gravity of 9.80665 m/s2 is used in the calculation of this pressure unit. The significance of 4 degrees Celsius (39.2 degrees Fahrenheit) is that it is very close to the temperature that water reaches its maximum density.

It is conventional practice to use 1000 kg/m3 as the density of pure water at 4 deg C which is very close to the precise density and for most measurements this does not introduce any significant error. In fact, since the temperature can vary significantly, measuring pressure in metres of water is never going to be a precise representation of the true liquid height. Local gravity also varies at different geological locations, which also adds some minor uncertainties to the use of metres of water gauge as an indication of exact water level in different parts of the world.