L'unicité de lieu, d'action et de temps

Beven, Keith and Musy, André and Higy, Christophe (2001) L'unicité de lieu, d'action et de temps. Revue des Sciences de l'Eau, 14 (4). pp. 525-533. ISSN 0992-7158

Abstract

Tribune Libre: The uniqueness of place, action and time We recently had the pleasure of re-reading the Tribunes Libres of Ghislain de MARSILY (1994) and Jacques GANOULIS (1996), especially their discussions of a new typology for hydrological models and the analysis of uncertainty. It appears, however, that some confusion and alternative interpretations of hydrological modelling still persist. It is therefore important, notwithstanding our agreement with many of the authors' points, to re-examine some aspects of hydrological modelling in order to clarify certain ambiguities. A distinction made by de MARSILY, between models conditioned by observable phenomena and the physically-based models employed when no phenomena have been observed, invites criticism in terms of the practices to which it leads. GANOULIS' argument, that physically-based models can provide a viable description of processes if differing spatial and temporal empirical coefficients are used, does not stand up to a detailed analysis of the effects of scale. In other words, the issues addressed by these authors arise from the impossibility of using purely physically-based modelling in practical applications due to the difficulty of taking into account and transcribing the characteristics and unique behaviour of each unit of landscape or sub-catchment. To this we can now respond that there are now other lines of thought concerning what are known as physically-based models. Where distributed modelling is concerned, that all places have unique characteristics is a geographical aphorism. The fact remains that the limitations of modelling, expressed by de MARSILY (1994) as the three principles of uniqueness of place, action and time, can be better defined by performing more detailed analysis in the context of uniqueness. Uniqueness limitations partly explain the wide-ranging developments in modelling in respect of both the theory and tools specific to particular applications. One cannot help but notice that expectations of quantitative prediction in hydrology have increased in parallel with the availability and power of computers. This evolution, however, is essentially due to technological advances rather than real scientific progress. Why? Principally, because of the unique characteristics of catchments: in our view, catchments transcend all available theories concerned with hydrological modelling. Moreover, this does not change if better physical hypotheses are proposed, nor if predictions are made for the variables or "non-observable phenomena" discussed by de MARSILY. In this paper, we address these questions and suggest a relevant approach to hydrological modelling for taking into account the unique character of catchments.