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This is a review of two new and important developments in thermal science. First, there exist fundamental optima in the constitution and operation of flow (nonequilibrium) systems, man-made and natural. These optima can be identified based on the simplest models that still retain the essential features of the real systems. Examples are the spatial allocation of heat transfer area in a power plant, and the temporal optimization of on & off processes. The second development is that the engineering method of modeling and optimization has been extended to natural systems, animate and inanimate (e.g., tree networks). This step has been named constructal theory for the reasons given in Section 3. The objective of such work is to predict the macroscopic spatial and temporal structure (organization) that is everywhere. It is to inject a dose of determinsm (theory) in a field that until recently considered natural structures to be nondeterministic: results of chance and necessity. These developments bring to mind the advice left to us by J. W. Gibbs more than one hundred years ago: "One of the principal objects of theoretical research in any department of knowledge is to find the point of view from which the subject appears in its greatest simplicity."
Preferential flow in hillslope systems through subsurface networks developed from a range of botanical, faunal and geophysical processes have been observed and inferred for decades and may provide a large component of the bulk transport of water and solutes. However, our dominant paradigm for understanding and modelling hillslope hydrologic processes is still based on the Darcy–Richards matric flow framework, now with a set of additional methods to attempt to reproduce some of the aggregate function of the two‐phase system of network and matrix flow. We call for a community effort to design and implement a set of well planned experiments in different natural and constructed hillslopes, coupled with the development of new theory and methods to explicitly incorporate and couple the co‐evolution of subsurface flow networks as intrinsic components of hydrological, ecological and geomorphic systems. This is a major community challenge that can now benefit from new experimental infrastructure, renewal of older infrastructure and recent advances in sensor systems and computational capacity but will also require a sustained and organized interdisciplinary approach. Copyright © 2014 John Wiley & Sons, Ltd.