23 (+1) unsolved questions in hydrology

Contributed by Bettina Schaefli

Have you heard about the “23 unsolved problems” initiative of IAHS, in reference to the 23 unsolved problems of Hilbert? After the launch of this initiative in November 2017, the web-based discussion culminated in a round of brainstorming at EGU 2018 and in a voting process at the Vienna Catchment Science Symposium.

Personally, I did not contribute any question at the discussion stage. I simply could not think of a question that relates to observed phenomena, is universal and specific (the three original requirements to formulate the problems). Are all questions that I work on not related to a particular climate region or to modeling rather than observed phenomena?

The proposed questions and the brainstorming session clearly showed that most colleagues did not self-censor their ideas and simply proposed anything what hydrologists currently work on.

At this stage, I really asked myself how we could possibly bring the hundreds of questions and problems (around 260) down to a reasonable number. And what would the added value of such a process be? Asking this question around me during the brainstorming session, I got an interesting answer: at the very least, we can learn something about what our research community is concerned about.

Of course! Why did I not consider this aspect before? With renewed enthusiasm and with a complete perspective change, I went to the Vienna Catchment Science Symposium following EGU; not to decisively influence the final choice of “the unsolved problems” but to observe a scientific experiment of a new kind: put 40 scientists in a room, together with a moderator. Give them a list of 60+ questions and roughly 1.5 h to reformulate, rank or delete them. Of course the process starts slowly. People do not know each other, some hesitate to openly say what they think about questions that were obviously formulated by some of the most famous hydrologists. Should I really vote to delete the favorite question of the moderator?

The great thing about a direct-democratic process, with hand-voting, is that it creates its own dynamic. Everyone can see what you vote or that you don’t vote; and there is no time for deep thinking. Over the course of the exercise, it becomes more and more fun: the moderator announces a problem number, the audience yells “delete” or votes for gold, silver or bronze. Hands go up and down, and even the most intriguing problems are voted within a few seconds.

Incredibly enough, after three such rounds in three parallel rooms, the list was brought down to 16 gold questions and 29 silver questions. The final outcome is now in the hands of a paper drafting team and will be published in a paper with a giant author list in the Hydrological Sciences Journal. Once you read the questions/problems, you will no longer be able to decipher what complex processes have led to these specific questions. But every reader might find a few unexpected questions that trigger new thinking; and together, the selected problems nicely reflect what the hot topics are in hydrology at the moment. With some gaps however. The question of how to make hydrology more open and replicable is not reflected in the retained questions, for example.

And: the single most important problem has been completely forgotten: why is hydrology not more gender-balanced?