The water temperature of the River Sava in Zagreb has risen significantly in the past 70 years, and its increase in the last 25 years is worrying, reads a paper published recently in the international scientific journal Water.

In the 1948-2020 period, the minimum water temperature of the Sava in Zagreb was 2.29 degrees Celsius, the mean temperature was 12.11 degrees and the maximum 24.2 degrees Celsius.

The increase, however, becomes conspicuous when the period of observation is divided into two periods – from 1948 to 1987 and from 1988 to 2022, warns the paper, authored by hydrologists Ognjen Bonacci and Tanja Roje-Bonacci of Split University and Ana Žaknić-Ćatović of the University of Toronto Scarborough.

The minimum water temperature for the first period was 1.79 degrees Celsius, while in the second period it was 3.18 degrees. The mean temperature for the first period was 11.26 degrees Celsius and in the second 13.63 degrees Celsius. The biggest difference was reported for the maximum average temperature – over the past 30 years it has grown from 22.9 degrees Celsius to 26.7 degrees, almost four degrees Celsius more than in the 1948-1987 period.

The paper also points to a worryingly strong trend of growth in the number of days in the year with the mean daily water temperature of over 20 degrees Celsius. That higher water temperature appears increasingly early in the year, lasts longer and ends later, often extending to September.

Whereas until 1989 the average number of days with higher water temperatures was around 25 days a year, in the 2004-2020 period it was 80 days a year. In as many as three years (2007, 2011 and 2017) the mean daily water temperature of the Sava exceeded 20 degrees Celsius for more than 100 days.

The authors of the paper point to links between the increase in the water temperature and the increase in the air temperature caused by global climate change but note that this only partially explains the phenomenon.

Sava – Zagreb’s key resource, still insufficiently recognised

Admittedly, the primary cause of the higher water temperature of the Sava in Zagreb is the higher air temperature but that increase is augmented by a decrease in the water flow during the warm season, from June to September.

The researchers also point to a drop in the minimum water flow in the Sava, noting that the river is a crucial ecological, social and economic resource for Zagreb, which, unfortunately, is still not adequately recognised and treated.

They support this assertion with the fact that their paper is the first research of the kind and that the problem of strong changes in the temperature environment of the river has not been sufficiently researched.

Upstream from Zagreb, the River Sava has been under significant pressure in the past decades, both due to human interventions and the effect of climate change.

The Sava is formed from the Sava Dolinka and Sava Bohinjka headwaters in northwest Slovenia.

So far, eight small hydropower plants have been built on the Sava in Slovenia, and one more is planned, at Mokrice. That hydropower plant will be the closest to Zagreb.

The authors of the paper note that they did not find any studies on the impact of those hydropower plants on the river’s hydrological regime, notably its temperature environment in Croatia.

There is no sufficient understanding of the very complex and urgent problem of the water temperature of the Sava in Zagreb and the paper points to the need for an interdisciplinary approach to its research. This is particularly accentuated by a recent mass die-off of fish in the Sava near Zagreb. Such incidents are increasingly frequent and it is speculated that they are due to sudden increases in the river water temperature, the researchers say.

Analyses point to a likely further increase in the temperature of the water in the Sava River, notably during the summer months, as well as to a reduced water flow. This calls for urgent action to analyse the possible consequences of such phenomena on ecological processes in the river, the underground waters in the associated aquifier system that supplies Zagreb with water, and the urban processes in Zagreb or some of its parts, notably those located by the river, the paper says.