Examples of imbalance relative to the piezometric balance. A : the level of the river is higher than that of the web. A flox of water (with possible transfer of calories, cold water, salts, pollutants, viral or bacterial propagules ..) occurs from the river to the underground water (for example due to a flood), until the balance recovers, or we go to the opposite situation (B): B: the level of the river is lower than that of the level of underground water (at low water period, for example). In this case, ther is a flow from underground water to the stream, bringing in it heat or cold or potential contaminants (if the water or sediments are polluted) . Darcy's law and others allow to model these two types of flows and exchanges.