13: Pattern, Process and Function: Elements of a Unified Theory of Hydrology at the Catchment Scale

Catchment hydrology is presently operating under an essentially reductionist paradigm, dominated by small-scale process theories. Yet, hydrology is full of examples of highly complex behavior, including strong nonlinearities and thresholds, and paradoxes that defy causal explanation through these small-scale process theories. There are strong interactions and feedbacks between processes, leading to apparent simplicities in the overall catchment response, yet the laws governing these feedbacks are not well understood. Routine measurements and specialized field experiments have been valuable for observing catchment responses and understanding the underlying process controls, but there has been little progress in extrapolating the local knowledge and understanding gained from these well studied (or gauged) catchments to ungauged catchments. Efforts at generalization are hampered by the lack of an appropriate quantitative framework, for example, a classification system, to help identify interesting and useful patterns in the observations. There are many theories governing different elements of catchment hydrology, but not a unified theory that connects these seemingly disparate elements. This article presents the broad outlines of an emerging new, unified theory of hydrology at the catchment scale, and the approaches being used to develop it. The new theory embraces multiscale heterogeneities as a natural and intrinsic part of catchment hydrology. Instead of relying solely on current process theories, it seeks to discover new catchment-scale process theories that embed within them the effects of natural heterogeneities. Instea