This principle has not yet actually been established, but there are several theories currently. Martijn Westhoff, has focussed on a particular case, the maximum power principle, which implies that nature, despite its numerous complex responses, tends towards a state of maximum power transfer according to the energy and resistance present in the system. “Therefore, in the case of water flow, there is an optimal state in accordance with temperature, precipitation, soil etc. In response to certain variations, the river basin adapts and evolves until it reaches this state”. This principle has not been chosen by chance, but because it has already worked in other contexts. In the case of hydrology, the principle remains a theory. It is a theory that has not yet been demonstrated in practice, but which provides a focus for the researcher. By coupling analyses of data from river basins across the world and the study of samples in the laboratory, the researcher verifies whether or not nature responds to the predictions that can be made by using this principle.

The maximum power principle in thermodynamics

A good way to understand this maximum power principle is to return to its source. In 1979, the Australian atmospheric physicist Garth Paltridge suggested a theory to explain heat transfer in the atmosphere. His model can be reduced to two sets, two simplified blocks. The equator on one side, the poles on the other. The equator is more exposed to solar radiation and gathers more heat than the poles. Part of this heat at the Equator returns to space. If, in fact, this energy input was conserved in the system, the temperature of the Earth would soar, continually. The remaining heat, is redistributed to the poles by an entire series of mechanisms, the winds in particular. Paltridge did not try to understand these mechanisms in detail, but rather to quantify the transfer of energy that resulted from it. This measure would enable him to verify his principle which stated that the power associated with the transfer of heat from the equator to the poles was naturally and continually optimal. And what else?

“On a graph”, explains Benjamin Dewals, “the x axis represents the facility of the heat transfer”, and the y axis the power of the transfer. In the extreme case of a very strong resistance, at point zero, there would be no transfer of heat, therefore no deployment of energy. The two blocks, equator and poles, would keep a very high temperature differential and would not communicate. Conversely, if there was no resistance, the heat would be instantly transferred to the poles. It is because of the temperature difference that a continuous transfer is possible. If the temperature was equal everywhere, there would also not be any transfer of energy, a state which corresponds to the right limit of the x axis. Between zero and infinity, an entire series of possibilities condition the power of this heat transfer”. Somewhere among these intermediaries, a resistance linked to a certain temperature differential will enable a maximum power transfer of heat. In theory, the principle states that the atmosphere will naturally tend towards this balance which conditions the maximal deployment of energy. And the measurements taken since then corroborate this prediction.

This is not the only natural phenomenon that has been predicted using this principle. There have been others, such as the vertical circulation of air from the ground surface to the atmosphere. Martijn Westhoff continues, “Let’s imagine a room with a basin of water. When the water evaporates, the temperature of the room drops proportionately. Because this transformation from the liquid state to the gaseous state requires energy and is translated by heat transfer. This loss of heat rebalances the distribution of heat vertically. This observation creates the possibility of predicting the evaporation from the soil surface by taking very few measurements”. The principle therefore works for the atmosphere and for other systems too, but how to verify its capacity to predict water flow in river basins?