# Mean water

In nautical science , oceanography and hydrology, mean water (abbreviated MW ; English Mean Sea Level , MSL) is the mean height of the water level (water level) at a selected point in a body of water over a long period of time . For a reliable calculation, the level of this measuring point has to be observed or automatically registered over a number of years or even decades, which is now done with digitized writing levels .

## calculation

However, there are several procedures for calculating the mean water:

1. Arithmetic mean of the highest and lowest water levels observed in each year j ,
MW = 0.5 · sum ( high water j + low water j ) / number of years
2. Averaging of all readings of the level - which is usually done daily at a certain hour - over the entire observed period.
In order to avoid systematic influences of the annual cycle, the beginning and end of this period must, if possible, fall on the same calendar day and not during the usual time of high or low water .
3. Calculation from the mean frequency of the water levels in this period.
4. In the case of rivers, the calculation of the water level that corresponds to the mean mean discharge (MQ) .

The second method is most common (see also integral mean ), while the third method roughly corresponds to the median of the measured values. The fourth is important for technology and power plants .

## Along the river

The fluctuations in the water level of rivers are mainly based on the annual snowmelt and the seasonal distribution of precipitation . However, topographical features also play a role, for example when a mountain river or stream has to overcome a long breakthrough valley. For example, the Danube flow in the narrow stretch of the Banat Mountains can be so severely restricted that the water accumulation is communicated to the upstream tributaries , especially the flat lower reaches of the Tisza .

## Sea level

The fluctuations in the sea ​​level depend on the one hand on the daily tides and on the other hand on oceanographic and meteorological conditions such as changing sea ​​currents and the drift currents caused by the wind .

The mean water level is used in waters with a tide less than 30 cm to define the nautical chart zero , for example on the Baltic Sea .