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Abstract

Anthropogenic activities such as climate change and forest harvesting have amplified magnitudes and frequencies of a range of hydroclimatic extremes, with continued increases expected throughout the 21st Century. Extreme events have already caused deaths, adverse health impacts, infrastructure damage, lawsuits, and major economic losses worldwide, underscoring the urgent need for effective risk management. Reliable projections and effective solutions are essential for managing risk, and both depend on a sound understanding of hydroclimatic phenomena. This thesis synthesizes insights from across hydroclimatology literature to argue that a causal, stochastic approach is the most reliable framework for understanding extremes. Stochastic physics embeds physical understanding within probabilistic distributions, accounting for the phenomenon’s natural and anthropogenic controls. It allows for identification of change between a system’s natural and anthropogenically influenced states, especially large frequency changes which surpass those of magnitude. A review of changing hydroclimatic regimes, core stochastic physics principles, and finance literature on system fragility suggests that such large frequency changes under anthropogenic influence may stem, at least in part, from natural regimes which are inherently sensitive (or fragile) to disturbance. The strengths of a causal, stochastic approach are highlighted with a forest hydrology example, where the question of whether forests mitigate large floods remains a decades-long scientific controversy. The traditional deterministic approach uses regression analysis to compare floods between control (e.g., forested) and treatment (e.g., harvested) catchments to study the effects of changing forest cover. This approach evokes less relevant research questions, an improper hypothesis, and a non-causal experiment, indicating low rigour in its conclusions. In contrast, a causal, stochastic approach using probabilistic concepts to understand the flood response to changing forest cover evokes relevant research questions, falsifiable hypotheses, a controlled experiment, and sound physical insight. Its robust conclusion that forests can mitigate large floods thus informs harvesting practices which minimize increases in downstream floods and advance nature-based solutions for flood management. In this way, use of rigorous scientific methods best position hydroclimatology to motivate strong policy and support the design of innovative solutions in the Anthropocene.