What makes Darwinian hydrology "Darwinian"? Asking a different kind of question about landscapes

Abstract

Abstract. There have been repeated calls for a Darwinian approach to hydrologic science, or for a synthesis of Darwinian and Newtonian approaches, to deepen understanding of the hydrologic system in the larger landscape context, and so develop a better basis for predictions now and in an uncertain future. But what exactly makes a Darwinian approach to hydrology "Darwinian"? While there have now been a number of discussions of Darwinian approaches, many referencing Harte (2002), the term is potentially a source of confusion because its connections to Darwin remain allusive rather than explicit.Here we suggest that the Darwinian approach to hydrology follows the example of Charles Darwin by focusing attention on the patterns of variation in populations and seeking hypotheses that explain these patterns in terms of the mechanisms and conditions that determine their historical development. These hypotheses do not simply catalog patterns or predict them statistically -they connect the present structure with processes operating in the past. Nor are they explanations presented without independent evidence or critical analysis -Darwin's hypotheses about the mechanisms underlying present-day variation could be independently tested and validated. With a Darwinian framework in mind, it is easy to see that a great deal of hydrologic research has already been done that contributes to a Darwinian hydrologywhether deliberately or not.We discuss some practical and philosophical issues with this approach to hydrologic science: how are explanatory hypotheses generated? What constitutes a good hypothesis? How are hypotheses tested? "Historical" sciences -including paleohydrology -have long grappled with these questions, as must a Darwinian hydrologic science. We can draw on Darwin's own example for some answers, though there are ongoing debates about the philosophical nature of his methods and reasoning. Darwin used a range of methods of historical reasoning to develop explanatory hypotheses: extrapolating mechanisms, space for time substitution, and looking for signatures of history. Some of these are already in use, while others are not and could be used to develop new insights. He sought explanatory hypotheses that intelligibly unified disparate facts, were testable against evidence, and had fertile implications for further research. He provided evidence to support his hypotheses by deducing corollary conditions ("if explanation A is true, then B will also be true") and comparing these to observations. While a synthesis of the Darwinian and Newtonian approaches remains a goal, the Darwinian approach to hydrologic science has significant value of its own. The Darwinian hydrology that has been conducted already has not been coordinated or linked into a general body of theory and knowledge, but the time is coming when this will be possible.

The evolution of process-based hydrologic models: historical challenges and the collective quest for physical realism

“…Addressing this challenge requires shifting focus from traditional approaches at short timescales where "properties define processes" towards approaches on longer timescales that focus on predicting how "processes define properties" (Rodríguez-Iturbe et al, 1992;Eagleson, 2002;Harman and Troch, 2014). Importantly, it requires the treatment of humans as an endogenous component of the Earth system Clark et al, 2015a).…”

“…A broader challenge is to simulate the myriad controls on catchment evolution, e.g., to predict how energy gradients dictate landscape evolution, how natural selection favors plants that make optimal use of available resources, and how the dynamic interactions between humans and the environment shapes the storage and transmission of water across the landscape (Rodríguez-Iturbe et al, 1992;Eagleson, 2002;Schymanski et al, 2009Schymanski et al, , 2010Sivapalan et al, 2012;Harman and Troch, 2014;Zehe et al, 2014;Clark et al, 2016;Grant and Dietrich, 2017). Addressing this challenge requires shifting focus from traditional approaches at short timescales where "properties define processes" towards approaches on longer timescales that focus on predicting how "processes define properties" (Rodríguez-Iturbe et al, 1992;Eagleson, 2002;Harman and Troch, 2014).…”

“…We need to substantially advance the development of new modeling approaches that simulate the temporal dynamics of environmental change. Key challenges include predicting how energy gradients dictate landscape evolution, how natural selection favors plants that make optimal use of the available resources, and how the dynamic interactions between humans and the environment shapes the storage and transmission of water across the landscape (Rodríguez-Iturbe et al, 1992;Eagleson, 2002;Schymanski et al, 2009Schymanski et al, , 2010Sivapalan et al, 2012;Harman and Troch, 2014;Zehe et al, 2014;Clark et al, 2016;Grant and Dietrich, 2017).…”

Streamflow recession patterns can help unravel the role of climate and humans in landscape co-evolution

“…Answering these questions means facing the complexity and multitude of interactions between soil, vegetation, atmosphere, and humans. Therefore, several recent opinion papers in hydrology called for the use of "Darwinian" approaches that try to summarise the effects of co-evolution between soil, vegetation, atmosphere, and humans on the hydrological cycle into general emergent patterns, and use these emergent patterns to explain the origin of the observed variations (Harman and Troch, 2014;Sivapalan et al, 2011;Savenije et al, 2014;Schaefli et al, 2011;Troch et al, 2013bTroch et al, , 2015.…”

A process-based diagnosis of catchment coevolution in volcanic landscapes: synthesis of Newtonian and Darwinian approaches

“…The results of coevolution that can be captured in the 35 spatial patterns of landscape features including vegetation, geometry of stream networks, soil profiles control the hydrological response of the catchments. Recognizing this interconections, hydrologists have recently proposed the idea of catchment coevolution (Sivapalan et al, 2012;Troch et al, 2013;Harman and Troch, 2014;Troch et al, 2015). In the framework of the catchment coevolution, one seeks to find an empirical relationship between landscape features and the hydrological response, 40 and to decipher the causality of the relationship from the historical development of catchments.…”