Scale, Complexity, Variability in Catchment Hydrology: The Need for Classification

The goal of science is to organise knowledge. Clearly then, the role of scientific hydrology is to organise hydrological knowledge. Catchments are complex natural entities, and so there is a great deal of knowledge, much of which is difficult to organise. The notions of scale, complexity and variability have
helped to impose some order on the great variety of observed catchment behaviours. This paper reviews some of that research, summarising progress to date. Although elegant concepts and detailed findings have emerged from that research, the results do not usually support transfer of knowledge to other locations or times. Typically the findings are either specific to the study site, or have too many free parameters, to be reliably transferred. This is an inevitable consequence of studying diverse natural systems such as catchments. I suggest that the absence of a widely agreed catchment classification scheme is a key factor inhibiting the transfer of hydrological research results. The paper concludes with a suggestion for the development of a more appropriate and widely accepted classification of catchments, permitting an efficient and rational sub-division of catchment hydrology into manageable, interacting sub-disciplines.