Hydrology and Earth System Sciences www.hydrol-earth-syst-sci.net 1027-5606 1607-7938 14 3 2010 10.5194/hess-14-585-2010 http://www.hydrol-earth-syst-sci.net/14/585/2010/ http://www.hydrol-earth-syst-sci.net/14/585/2010/hess-14-585-2010.html http://www.hydrol-earth-syst-sci.net/14/585/2010/hess-14-585-2010.pdf 585 601 2010-03-25 HESS Opinions "A random walk on water" D. Koutsoyiannis dk@itia.ntua.gr Department of Water Resources and Environmental Engineering, School of Civil Engineering, National Technical University of Athens, Greece Invited contribution by D. Koutsoyiannis, recipient of the EGU Henry Darcy Medal 2009. According to the traditional notion of randomness and uncertainty, natural phenomena are separated into two mutually exclusive components, random (or stochastic) and deterministic. Within this dichotomous logic, the deterministic part supposedly represents cause-effect relationships and, thus, is physics and science (the "good"), whereas randomness has little relationship with science and no relationship with understanding (the "evil"). Here I argue that such views should be reconsidered by admitting that uncertainty is an intrinsic property of nature, that causality implies dependence of natural processes in time, thus suggesting predictability, but even the tiniest uncertainty (e.g. in initial conditions) may result in unpredictability after a certain time horizon. On these premises it is possible to shape a consistent stochastic representation of natural processes, in which predictability (suggested by deterministic laws) and unpredictability (randomness) coexist and are not separable or additive components. Deciding which of the two dominates is simply a matter of specifying the time horizon and scale of the prediction. 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