Hydrology

Study of the physical, chemical and biological properties of waters located on the surface of the Earth and under ground, especially in terms of their formation, movement, distribution in time and space and their interaction with the inert and living environment. Continental hydrology studies the streams, wetlands and bodies of water, groundwater and solid water bodies on land masses, while marine hydrology is identified with oceanography.

Cumulative difference between potential evapotranspiration (soil evaporation and transpiration from vegetation) and precipitation during a certain period in which the precipitation is the smaller of the two.

A 12-month period that begins after the usual month of the lowest waters. Depending on local weather patterns, the water year may begin on dates different from the regular calendar, but in France, it usually starts in September.

Ratio between the total length of the watercourse in a river basin and the surface area of the latter. Drainage density is expressed in km/km2.

Statistics on characteristic flows. We can express the value of the classified flow not exceeded over n days per year on the average (DCNn) or the value of the classified flow exceeded over n days per year (DCXn).

Time needed to complete the theoretical water renewal of a body of water, on average, over a calendar year. The residence time of water, expressed in days, can be calculated by the following formula: total capacity / annual discharge of rivers that feed the body of water.

Average water depth in metres. The average depth of a body of water is calculated by dividing the volume by the surface area, both determined at the average elevation for a natural body of water, and at the normal operating elevation for an artificial body of water.