Landscape structure, regimes, and the co-evolution of hydrologic systems

In this dissertation I discuss approaches to building hydrologic understanding in a changing world that go beyond the construction of models of ever-greater complexity. These approaches aim to develop insight into the relationship between catchment properties and hydrologic dynamics using reduced-complexity models, and by looking for patterns that reveal emergent relationships between hydrologic systems and the landscapes they are embedded in. The dissertation proposes a framework for thinking about hydrologic systems in a changing world based on seeking a synthesis between the search for mechanistic descriptions of landscape processes, and the search for explanations for emergent landscape patterns.;The dissertation is divided into two parts. The first describes a series of studies considering controls on the propagation of hydrologic variability through the landscape. One discusses the propagation of water and solutes through the vadose zone, another the lateral movement of water through a hillslope, and the third the accumulated effect of many hillslopes on the recession of flows at a watershed outlet. Each case builds on parsimonious representations of hydrologic processes to distill analytical results in terms of landscape and climate properties. These analytical results are used to define 'regimes' of hydrologic behavior in which particular properties play decisive roles in the hydrologic system. The studies demonstrate that the regime framework yields insight into controls on the aggregate behavior of hydrologic system that can be used to develop 'closure relations' capable of representing the effects of unresolved landscape structure on hydrologic fluxes without resolving them explicitly.;The second part of the dissertation is concerned with how the landscape structure controlling the hydrologic dynamics has come to be the way it is, and the role that hydrologic variability plays in its evolution. This question is pursued at a range of scales, using modeling and data analysis. Inter-annual water balance variability across the climates and geologies of the continental US are examined for patterns in the dynamics of co-evolved landscapes. A simple model is then used to illustrate how catchment water-balance is affected by feedbacks between the lateral redistribution of water and the spatial organization of vegetation along the network.;This work illustrates how insights into why and how landscape hydrology varies from place to place and through time can be built through a focus on the behavior that emerges from small-scale dynamics, conditioned by the over-arching climate, geology and the contingencies of history. These insights point the way to a new paradigm for hydrology that treats hydrologic systems as integrated wholes that have evolved through time, and will continue to change in the future.