A web facility made by a Water Agency for educating children in the good management of water includes a scheme of the water cycle. In it, the water cycle starts in the stream / reservoir.

This is the real world of water management: we only manage ‘blue’ water (with some exceptions most of them from South Africa). But it is amazing to stress that the two ‘black swans’ cited above by Alberto are examples where changes in land cover induced the increase of ‘blue’ water at the expense of ‘green’ water irrespectively of precipitation forcing. Gil Mahé provided another example from the African Sahel.

Yet, land cover changes are not only a result of direct man’s activity, but severe vegetation changes have been anticipated in many areas of the World for the next decades as a result of changing climatic water balances (1).

This is very recently that foresters (2) and the ‘watershed management’ community (3) assimilated and decided to put in practice the findings (paradigm change) obtained after decades of research on forest hydrology. But we (hydrologists) can not help them so much because, as Peter Troch commented, most hydrological models ‘are not restricted when it comes to assigning soil and vegetation parameterizations’.
This is not just a modelling issue; we must take into account that hydrology is also a Natural Science and rather ‘soft’ approaches such as classifications and comparisons between basins or in gradients (Siva, Peter Troch and Pierre Gentine comments) are necessary.

I think therefore that land cover/use issues should therefore be more explicitly included in the schedule.

All the best
Francesc

Links:
1: (http://www.ipcc.ch/publications_and_data/ar4/wg2/en/figure-4-3.html)
2: (http://www.efimed.efi.int/portal/events/mfw2011/portal/1642)
3: (http://www.fao.org/docrep/009/a0644e/a0644e00.htm)

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Quantifying, characterising, and eventually reducing uncertainty in hydrologic predictions has been recognised by many contributors to this blog as one of the key objectives for the next hydrologic decade. This objective is inherently difficult because of the so-called “unknown unknowns” – those events or processes for which we have not quantified or observed yet which have the potential to fundamentally change our parameterisation or conceptualization of a particular hydrologic phenomenon. One could argue that this very intriguing scientific issue, i.e., to somehow incorporate unknown unknowns in our theories and models is marginal in practice (there are of course many things not incorporated into models simply because we believe they have marginal impact on the predictions). But what if that is not the case? What if the unknown unknowns are indeed “Black Swans”, i.e., they also have a great impact? Nassim Nicholas Taleb defines in his book Black Swans as surprising events, which have a great impact and that can be explained easily after they manifest themselves but have not been predicted a priori [Taleb, 2007]. The idea of Black Swans in hydrology was explored during the 2012 Vienna Catchment Science Science Symposium, held at the Vienna University of Technology on May 28, 2012.
The discussion focused on two questions: (1) Are there Black Swans in hydrology? and (2) How can we deal with Black Swans in our theories/models? A summary of this discussion is presented below.

To address the question of whether there are Black Swans in hydrology, the discussion focused on providing examples of previous Black Swan hydrologic events throughout history.
Most of the examples from the discussion were flood events. One such example given was a 2002 flood in Arizona. A long-term policy of extinguishing small fires led to the build-up of massive amounts of fuel that, in turn, resulted in a massive wildfire. The large wildfire resulted in damaged soils that were no longer able to absorb water. A seemingly higher than normal precipitation event (although not an extreme precipitation event) came through the area and resulted in a 500-year flood due to the hydrophobicity of the soils.
A non-flood example was given that related the problem of salinity in Western Australia. Since European settlement, perennial vegetation had been cleared for farmland. This triggered a complete change in the water balance in the area and the loss of evapotranspiration caused the groundwater table to rise to the surface and displace the salt contained in the soils from the bottom to the top layers. Once the salt came to the surface, the vegetation started dying, resulting in a complete and irreversible loss of the farmland. These were Black Swan events: surprising, with major impact, and, although easy to explain once they have happened, could not have been predicted prior to their occurrence. In discussing and contrasting various examples of Black Swans in hydrology several observations and questions were raised:
• It was noted that a “black swan” is defined relative to a body of knowledge. What may be considered a Black Swan event for people in one part of the globe may not be considered a Black Swan elsewhere. In the Arizona wildfire example, the problem of hydrophobicity of soils from wildfires is a well-known phenomenon in other parts of the western United States. Is there a spatial or locational element to considering Black Swans in hydrology?
• We recognized that many catastrophic events are Black Swans because the societies in which they occur are unprepared for them. An event that is catastrophic in one place may not be elsewhere, where society is better prepared, or more resilient. This highlights the fact that Black Swans arise because of our (lack of) knowledge and expectations.
• Do Black Swans occur naturally in hydrology or are they a result of human interaction with the hydrologic cycle. For example, does an extreme flood only become a Black Swan when a dam fails or people and property are located in a flood plain?
• Can we categorize hydrologic Black Swans into several classes? The discussion raised four possible categories of Black Swan events: (1) Black Swan – a discrete single sighting, (2) Flocks of black swans – a cascade of events in which one black swan produces another, (3) an ugly duckling – a black swan produced by two white swans (two seemingly innocuous events), and (4) Stupid swan – an event we know could be catastrophic but choose as a society to ignore.
• Are there Black Swans events for which we have already recorded but have discarded as outliers or equipment error? For example, there was a delay in the discovery of the hole in the ozone layer because the observations were believed to be outliers due to equipment malfunction.

The group’s discussion of how can we deal with Black Swans in our theories/models mirrored many of the suggestions already made by others in this blog. These included:
• Looking at other places (comparative hydrology): what is a Black Swan here could have happened already somewhere else. Retrieving information from the “ungauged” past: e.g., water issues caused the decline of ancient civilisations in India and America, do we know what happened? What can we learn from it?
• Don’t exclude outliers – embrace them. Apparently anomalous behavior that falls far from the mean may be representative of rare events that have a large impact. These events need to be studied.
• Creative, out-of-the-box thinking is critical as a way to circumvent blinkered expectations about what knowledge is useful. We need to imagine things that might be important in the future, even if they have never been in the past. Structured brainstorming on events is necessary
• In systems where a small number of events (in time) or points (in space) play a dominant role in the behavior of the system, it is necessary to collect many many observations. Oversampling a system relative to its mean behaviour is needed to characterize the tails of the distribution of behaviours.
• Interdisciplinarity: human, ecological, geomorphic, seismic etc. processes may drive hydrologic Black Swans more than the rainfall-runoff itself.
• A systematic, axiomatic approach to hydrologic modelling may help us identify blind-spots that hide Black Swans. This approach involves clearly listing the assumptions that go into a prediction, and critically assessing those assumptions.
• Last but not least, have humility: Black Swans reveal our limited ability to predict and control nature. And if prediction is not feasible, we should focus on resilience, preparedness, robustness, which is indeed the conclusion in the book of Taleb.

It was clear by the end of the discussion that one day was not enough to design a framework to deal with Black Swans in hydrology. As humans continue to interact with the hydrologic cycle and long term natural and anthropogenic change influences catchment evolution, we hope that the discussion of Black Swans in hydrology can considered in the next hydrologic decade.

Stacey Archfield, Ciaran Harman, Alberto Viglione, Ross Woods and all participants to the 2012 Vienna Catchment Science Symposium.

ref: Taleb, Nassim Nicholas: The Black Swan – the Impact of the Highly Improbable, 366 pages, Random House, New York, 2007.

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