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

Abstract

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. Instead of attempting to prescribe in detail the actual patterns of heterogeneity in every catchment, it will seek to incorporate the geomorphic or landforming processes that may have generated them in the first place, and their ecological, pedological, and geomorphological functions. Instead of using our rather meagre observations to calibrate complex models that are based on small-scale theories, the theory will emphasize the use of patterns in the observations to formulate and test alternative hypotheses about the underlying process controls. Instead of using field measurements to learn more and more about individual catchments, it will seek to find connections between observations in different catchments, to identify broad-scale or general patterns. The defining feature of the new theory of catchment hydrology will be a sharp focus on the interconnection and feedbacks between pattern and process, over a range of scales, and their interpretation in terms of their "function", that is, the reason that these connections arise. The renewed focus on pattern, process, and function will revolutionize hydrology, elevate its place within the earth system sciences, and strengthen the scientific foundations of its practice.

Air temperature variability over three glaciers in the Ortles–Cevedale (Italian Alps): effects of glacier fragmentation, comparison of calculation methods, and impacts on mass balance modeling

“…These models rely on accurate spatial calculation of input data, in particular air temperature, which affects not only their final performance but also the calibration of parameters and model generalizability. Indeed, wrong temperature estimates lead to wrong calibration and/or distortion of parameters, possibly hampering the applicability of models to ungauged catchments, despite the good knowledge achieved for individual processes (Savenije, 2001;Sivapalan, 2006). Charbonneau et al (1981), for example, highlighted that issues in extrapolating meteorological input data are much more crucial than the possible choice between different approaches for modeling snow yields from a well-equipped catchment in the French Alps.…”

Distributed mass-balance modelling on two neighbouring glaciers in Ortles-Cevedale, Italy, from 2004 to 2009

“…This is even more evident in fully distributed applications, which require a realistic distribution of input variables in space and time (Charbonneau and others, 1981;Machguth and others, 2006a). Scaling issues, over-parameterization, lack of adequate data for internal validation and equifinality may eventually hamper the applicability of models to ungauged catchments, despite the good knowledge achieved for individual processes (Blö schl, 1999;Refsgaard, 2001;Savenije, 2001;Sivapalan, 2006).…”

Entropy production of soil hydrological processes and its maximisation

“…Although single processes (e.g. infiltration or bare soil evaporation) are well understood, a unifying quantitative framework to describe hydrological Correspondence to: P. Porada (pporad@bgc-jena.mpg.de) behaviour at catchment or larger scales is still missing (Sivapalan, 2005). It is therefore in general not possible to make correct predictions about a certain catchment or region based on a model that has been designed for another catchment.…”