Volume of groundwater whose surface is free, i.e. at atmospheric pressure. The surface of an unconfined groundwater fluctuates freely without any constraint. These aquifers are often shallow.
A groundwater sheet neighbouring a stream and whose hydraulic properties are closely related to those of the stream. The exploitation of such a sheet leads to a decrease in the low-flow of the river, either because the groundwater sheet provides less water to the stream, or because the river begins to feed the sheet.
Area of the subsoil in which the water completely occupies the interstices of the rock (aquifer) and is likely to move laterally under the effect of gravity and pressure gradients.
All of the water contained in a permeable fraction of the earth's crust which is completely soaked, as a result of the infiltration of water into the smallest interstices of the basement and its accumulation above an impermeable layer. Groundwater tables only form underground rivers in karstic areas. Groundwater corresponding to water infiltrated into the ground, circulating in the permeable rocks of the subsoil, form "reserves". Different types of aquifers are distinguished according to various criteria which can be: geological (alluvial aquifers - superficial porous media, aquifers in cracked environment - carbonate or eruptive, aquifers in karst environment - carbonate, aquifers in porous environment - sandstone, sand) or hydrodynamic ( alluvial aquifers, free aquifers, or captive aquifers The same aquifer may have a free part and a captive part.
The piezometric record is the collecting of the evolutions in time of the monitored groundwater level or piezometric level (see piezometric level). It consists of measurements that link the water level to a given date. Depending on the variability of groundwater level, the measurements are more or less close in time. The measurements obtained are positive or negative with respect to the measurement benchmark: they are positive when the groundwater level is below the benchmark measurement (the most common cases) and negative in the opposite case (artesian wells). Artesian: a well is artesian when the water is gushing. A record is represented by a curve which is time-slotted to be stored in a succession of points, each point representing a groundwater level at any given time. Points are linked to each other by means of a code that indicates whether a point is related to the preceding point in time. If the point is not related, it is then the initial point of a new part of the record…
Elevation defined either in NGF level (French levelling network) or in relative depth. The NGF level, established under the control of the National Geographic Institute (IGN), is the reference. NGF can indeed express altitudes in one and unique reference system, i.e. as compared to the same and unique fundamental point (or "origin-zero" or point zero). The "0" level is the average sea level in Marseilles. This point is used to determine the altitude of any other point. The elevation of the piezometric record can also be expressed in relative depth with respect to the benchmark of the