The Mississippi River Basin is a vast near-planar surface, an area upon which sunlight falls and wind flows. Its gently banked geomorphology channels precipitation, sediment, biota, and human activity into a dynamic locus of regional Earth system interactions. This paper describes the major features of this region’s energy exchanges from a thermodynamic Earth systems perspective. This analysis is combined with descriptions of the historical and socio-political contexts that have helped shape energy use. In doing so, the paper contrasts the region’s available energy exchanges and flows with their anthropogenic diversion, providing an account of human impact at a regional scale. It also offers theoretical estimates of the potential availabilities of renewable energy. This is contrasted with a description of the geological formation of stocks of fossil energy in the region. On these bases, a number of maps are presented and an assessment of the region’s energy flows is offered. These exercises point to significant affordances for achieving regional de-fossilisation at the river basin scale.

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The present study confirms that a thermodynamic perspective on soil water is well suited to distinguishing the typical interplay of gravity and capillarity controls on soil water dynamics in different landscapes. To this end, we express the driving matric and gravity potentials by their energetic counterparts and characterize soil water by its free energy state. The latter is the key to defining a new system characteristic determining the possible range of energy states of soil water, reflecting the joint influences of soil physical properties and height over nearest drainage (HAND) in a stratified manner. As this characteristic defines the possible range of energy states of soil water in the root zone, it also allows an instructive comparison of top soil water dynamics observed in two distinctly different landscapes. This is because the local thermodynamic equilibrium at a given HAND and the related equilibrium storage allow a subdivision of the possible free energy states into two different regimes. Wetting of the soil in local equilibrium implies that free energy of soil water becomes positive, which in turn implies that the soil is in a state of storage excess, while further drying of the soil leads to a negative free energy and a state of storage deficit. We show that during 1 hydrological year the energy states of soil water visit distinctly different parts of their respective energy state spaces. The two study areas compared here exhibit furthermore a threshold-like relation between the observed free energy of soil water in the riparian zone and observed streamflow, while the tipping points coincide with the local equilibrium state of zero free energy. We found that the emergence of a potential energy excess/storage excess in the riparian zone coincides with the onset of storage-controlled direct streamflow generation. While such threshold behaviour is not unusual, it is remarkable that the tipping point is consistent with the underlying theoretical basis.@en