Hydrology and change

Abstract

Since "panta rhei" was pronounced by Heraclitus, hydrology and the objects it studies, such as rivers and lakes, have offered grounds to observe and understand change and flux. Change occurs on all time scales, from minute to geological, but our limited senses and life span, as well as the short time window of instrumental observations, restrict our perception to the most apparent daily to yearly variations. As a result, our typical modelling practices assume that natural changes are just a short-term "noise" superimposed on the daily and annual cycles in a scene that is static and invariant in the long run. According to this perception, only an exceptional and extraordinary forcing can produce a long-term change. The hydrologist H.E. Hurst, studying the long flow records of the Nile and other geophysical time series, was the first to observe a natural behaviour, named after him, related to multi-scale change, as well as its implications in engineering designs. Essentially, this behaviour manifests that long-term changes are much more frequent and intense than commonly perceived and, simultaneously, that the future states are much more uncertain and unpredictable on long time horizons than implied by standard approaches. Surprisingly, however, the implications of multi-scale change have not been assimilated in geophysical sciences. A change of perspective is thus needed, in which change and uncertainty are essential parts.Key words hydrology; change; turbulence; Hurst-Kolmogorov behaviour; uncertainty Hydrologie et changementRésumé Depuis que "panta rhei" fut prononcé par Héraclite, l'hydrologie et les objets qu'elle étudie, comme les rivières et les lacs, offrent des champs d'observation et de compréhension du changement et des flux. Le changement se produit à toutes les échelles de temps, de la minute aux durées géologiques, mais la limitation de nos sens et de la durée de nos vies, de même que l'étroitesse de la fenêtre des observations instrumentales, restreignent notre perception aux variations les plus évidentes, quotidiennes et annuelles. En conséquence, nos pratiques habituelles en matière de modélisation supposent que les changements naturels ne sont qu'un "bruit" à court terme se superposant aux cycles quotidiens ou annuels sur une scène statique et invariable à long terme. Selon cette perception, seul un forçage exceptionnel et extraordinaire peut produire un changement à long terme. L'hydrologue H.E. Hurst, en étudiant les longues séries de débit du Nil et d'autres séries chronologiques géophysiques, fut le premier à observer un comportement naturel, auquel son nom a été donné, lié à des changements d'échelles multiples, ainsi qu'à en tirer des conséquences au niveau de la conception des ouvrages. Essentiellement, ce comportement montre que des changements à long terme sont beaucoup plus fréquents et plus intenses qu'on ne le perçoit généralement et, également, que l'avenir à long terme est beaucoup plus incertain et imprévisible que ce que supposent les approches classiques. Étonnamment, ce...

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