Do seasonal flow dynamics depend on snow conditions?

In a recent issue of Water Matters there was a mention of a publication by ten authors, mostly from Tsinghua University in Beijing, describing how streamflow seasonality in a snow-dwindling world is changing. The paper is based on a large amount of measurement data and has been solidly written, as evidenced by its place of publication, the journal Nature. Its conclusion is that these dynamics are complex, ambiguous, and full of nuances, and the authors themselves emphasize that they undermine the previous heuristics of less snow – earlier peak flow.

Interest in river flows on a very large scale

It’s hard to disagree with such conclusions. The natural world is more complicated than it may seem to theory makers, and there is an exception to every rule. This is especially true in the real world, not in physical models governed by exceptionless laws and equations. Nonetheless, such a conclusion, which can be colloquialized by reducing it to saying: it’s a twofer, is ultimately trivial. At the same time, the challenged heuristics did not come out of nowhere, but from previous analyses. It seems that the authors, wanting to embrace the whole reality, specifically the northern hemisphere of the Earth, overstepped.

The work is, of course, of indisputable value, but the Polish reader is likely to be more interested in something from his own garden, rather than in such wide-ranging analyses. It turns out that Polish hydrologists are also looking at seasonal flow dynamics. Such results may have greater value in the domestic knowledge market. Several reports were presented at a recent conference Hydrology and Man. Interactions and Challenges, and some were published in a special issue of Acta Geographica Lodziensia. It is a much less prestigious journal than Nature (according to the list of journals scored by the Ministry of Science as of January 2024, it is 70 and 200 points, respectively), but the reader can judge for himself whether this difference matters for credibility.

Flows – conclusions from the study of Polish rivers

Indeed, one of the publications in Acta Geographica Lodziensia shows that on the Polish scale, as well as the world scale, changes in flow dynamics are complex and difficult to generalize. The analysis focuses on the problem of stability of monthly maximum and minimum flows [1]. Another publication shows that there are no rules without exceptions, but the picture it presents has a clearer shape. Its author is renowned Polish hydrologist Urszula Somorowska, and the title itself summarizes the results of the analysis: Earlier emergence of winter-spring maximum streamflow across Poland, 1981-2020 [2].

The author analyzed flow data from 102 IMGW-PIB water gauge stations on Polish rivers from the Bieszczady section of the San River to the Ina River near Szczecin. This is quite representative of Poland, although there is no position on the Vistula, and the Oder is represented only by Chałupki, the place where this river crosses the Czech-Polish border. Also, the Narew or Warta are measured at sites where they have the character of medium or large rivers, but not yet very large.

They represent 5 main types of river regime according to Dynow typology – from snow-fed to rain-snow with different degrees, which means that the maximum flow occurs in them from March to August. If the maximum average monthly flow in March or April (in high-mountain rivers in May) is almost twice as large (more than 1.8 times) as the annual average, the supply is considered to be strongly snowy, if slightly larger (less than 1.3 times) – weakly snowy.

Somorovskaya’s analysis is in line with the heuristics criticized by the authors of the study in Nature – over the past 40 years, the amount of snow has decreased and the maximum flow has shifted toward the beginning of the year. Out of 102 cases, only 15 points broke out of this rule. In this only one the deadline has moved towards the end of the year. So, yes, there are exceptions, but the change in flow dynamics shows a pretty clear regularity.

The biggest exception is Bialka Tatrzanska (in Lysa Polana), where the date of maximum flow has moved 9 days forward. The lack of change mainly affects the rivers along the eastern border – the upper San and its tributaries, the Chelm section of the Bug and its tributaries, the Narewka, Marycha and Szeszupa. There are exceptions to this rule, too, as a lack of change in seasonal flow dynamics has been found at more central points, such as on the Prosna River, while a point close to the border on the Narew has a maximum shifted back 40 days.

Averaging, it can be said that most rivers have a maximum flow accelerated by about a month. In the case of rivers, where this shift is stronger, you can also see the rule. This applies mainly to the northwest. The record was set on the Łupawa, where this acceleration was as high as 108 days, with the Leba and Wieprza not far behind.

Seasonal flow dynamics in modern Poland

The author doesn’t provide all of the raw data (though there’s probably some way to get it from IMGW-PIB) or the sub-analyses, but from the ones she cites, it appears that the shift in timing was rarely linear. More often there was a jump in the 1990s. last century, and since then the dynamics have been much more volatile than before. It can be noted that in the 1980s. The terms were quite repetitive, and later the dynamics became disjointed.

Seasonal flow dynamics undoubtedly depend on snow conditions. Its pattern has changed due to less snow and shorter snow duration. Nowadays, thaws in the middle of winter are increasingly typical, allowing them to rise during a period with traditionally low flows. Because of this, less snow remains until the spring thaw. At the same time, it can be seen that this effect is weakest in the easternmost regions of Poland. This probably has to do with the continentalism of the climate and the fact that thermal spring comes noticeably later there than in the west.

Meteorological data from the IMGW-PIB show that in recent years, in some places, thermal autumn turns into thermal spring as early as January, without a typical winter phase. Negative temperatures then have the character of frost rather than frost. Even if it snows, it melts so quickly that the final result is not very different from the situation resulting from rain.

So, although every case is different and there are exceptions to every rule, it looks like the less-snow-earlier peak flow heuristics are working quite well in Poland. Regardless of how this differs from the situation in other areas of the world.

In the article, I used, among other things. From the works:

Wrzesinski D. (2024) Changes in selected features of the river regime in Poland under conditions of climate warming. Acta Geographica Lodziensia 115, 175-195, https://doi.org/10.26485/AGL/2024/115/10.

Somorowska U. (2024) Earlier emergence of winter-spring maximum streamflow across Poland, 1981-2020. Acta Geographica Lodziensia 115, 109-124, https://doi.org/10.26485/AGL/2024/115/6.