HEAVY rain, which turned Zagreb into a swimming pool in the evening of July 24, happens very rarely, on average once in several centuries, but climatologists warn that there are clear indications that global warming has already resulted in more frequent and worse extremes, for both rain and drought.

The record in the hourly precipitation intensity was broken

The precipitation that fell in Zagreb at midnight on July 24-25 did not break the record for the daily total precipitation. However, the record for the hourly precipitation intensity was broken, and the problem is that the rain was falling most intensely in the city center.

According to DHMZ (Croatian Meteorological and Hydrological Service), the majority of the measured rainfall of 58.9 mm fell in a very short period of time between 8 pm and 9 pm at the Zagreb-Gric area.

"This is a record precipitation intensity that exceeded the previous hourly precipitation record of 52 mm from 1976. Such an hourly precipitation intensity is an extremely rare event that happens at that location only once in about 100 years on average! In the following hour, another 18.1 mm of precipitation fell, thus amounting to a two-hour precipitation intensity of 77 mm/2 h, an event so rare that it happens once in more than 400 years on average," our experts explain.

Due to climate change, there will be more extremes

Climatologists have recently been warning that scientific analyses show that events such as rain, drought, and strong winds might get more extreme in the future due to warmer atmosphere storing more moisture and more energy.

Rising air and ocean temperatures, melting sea ice, and rising mean sea levels are clear indicators of climate change. These are not just natural variances.

Changes in hydrology are somewhat slower

Changes in the hydrological system occur somewhat more slowly, and the natural variance is larger, making it more difficult to obtain an equally clear sign in that part of the system.

Climatologist Ivan Güttler, Ph.D. from DHMZ says that theory and measurements indicate that as the temperature rises, so does the amount of water vapor in the atmosphere. This additional amount of water vapor will be added to the existing water changes in the climate system, which can increase precipitation in terms of intensity, duration, and coverage area.

"However, precipitation is also affected by the dynamics of airflow. Climate projections for Europe indicate a shift of most cyclonic disturbances to the north. This leads to an increase in precipitation in northern Europe and a decrease in precipitation in the south and the Mediterranean. By the end of the 21st century, we expect an increase in precipitation in winter and a decrease in summer. In the next 30 years, we expect an increase in the winter and a decrease in the summer amounting to 10-15% in Croatia, depending on the region and the season, with a tendency of growth for the rest of the century depending on the greenhouse gas emissions. It is important to emphasize that the observed and modeled air temperatures in Croatia already show properties of statistical significance in all seasons. When it comes to precipitation amounts, both mean and extreme, the changes will become clearer in the coming decades," explains Güttler.

Ksenija Cindric Kalin, a climatologist from DHMZ, explains that the analysis of the duration of droughts in Croatia indicates a significant trend of their prolongation in the period from 1961 to 2015 in the northern Adriatic, especially in summer.

"However, this trend does not exclude the possibility of heavy rainy seasons in the dry summer climate in the Mediterranean," says Cindric Kalin.

How to prepare for the future?

After heavy rains in Zagreb, some media and people on social networks criticized DHMZ's reaction.

"One of the most common climatological analyses that DHMZ does is the analysis of maximum short-term precipitation intensities, which is used in planning construction projects and sizing of drainage systems in cities. We use statistical models to estimate the probability of exceeding certain extreme precipitation amounts and calculate the reliability of these estimates," says Percec Tadic.

"That is where our expertise ends. DHMZ is obliged to announce and issue warnings of dangerous weather phenomena timely. We cooperate with other sectors, from agriculture, through energy to construction, which use meteorological, climatological, and hydrological data, or data on air quality, so they can plan and strategize their work and development in accordance with the components of the climate system," says our climatologist.

Zagreb's drainage system was constructed in the 19th century

DHMZ hydrologists point out that one of the key problems of the city of Zagreb in situations of heavy rainfall is the mixed sewage system from 1892, which means that both sanitary and rainwater are drained into it.

At that time, Zagreb had less than 83,000 inhabitants.

"As the city continues to grow, there are more asphalted and watertight areas, which causes a significant increase in the surface runoff that drains into the drainage system, along with large amounts of earth, gravel, and organic material (leaves and branches). The drainage system is under capacitated in such situations, and pressurized flow occurs, causing the sewage water in the lower parts to come to the surface and mix with rainwater, which further complicates the situation," hydrologists point out.

The problem is that sewage water and rainwater are mixing

Prominent Zagreb architect prof. Drazen Juracic states that the Medvescak stream was canalized at the end of the 19th and the beginning of the 20th century when a network that ended at the railway line was built.

"According to the standards of the time, rainwater and sewage water were not separated," Juracic explains.

"That system has been abandoned in most larger and smaller cities because it has one basic drawback – it prevents the treatment of fecal water treatment since such mixed water has varying dilutions and quantities. This system remained unchanged when parts of Zagreb south of the Sava river were built, which turned out to be a grave mistake. New systems should have been built south of Sava in order to separate the rainwater from the fecal water. There was an attempt to purify that water using an expensive purifier, but due to the variation in dilution and the quantity of water, it was unsuccessful. When only fecal water is collected, it flows in a constant quantity. However, if you add rainwater to the fecal water, the variations in quantity are huge because of the periods of drought and periods of heavy precipitation," explains Juracic.

The system is not maintained or cleaned

An outdated system is only part of Zagreb's problem. The second part is that the spills on the roads are not regularly cleaned. Therefore, even the network that could be used for drainage on the tops of the hills is not functioning, and everything flows down the roads into the main canal.

"The floods that occur consist of the sewage water. It is a great sanitary risk that is not talked about enough. It is completely unacceptable for fecal matter to flood basements, business premises, and even grocery stores," states our expert.

There are various possible solutions

Juracic states that it is difficult to say what the best solution would be.

"We could mitigate some damage by properly maintaining the existing drainage system. Also, non-return sewer valves could be used in order to prevent the fecal water from returning to homes. If we install these valves, fecal water would be drained into the streets, but it would not return to the buildings. Lastly, we could separate the fecal water from rainwater, which was done in practically all the coastal towns," explains Juracic.

Zagreb has the most precipitation in summer and autumn

Precipitation characteristics differ in continental, mountain, and coastal Croatia.

If we look at the data on the average monthly precipitation of the Zagreb-Maksimir Observatory from the period from 1951 to 2018, Croatia has the most precipitation in June and September, when above 90 mm of rain falls per month on average. More than 80 mm of precipitation on average falls in July, August, and November.

Average daily amounts also have their seasonal course, and these amounts are also higher in summer and autumn in the Zagreb-Maksimir station. In August and September, average daily precipitation values ​​amount to around 7.5 mm on rainy days.

"However, the greatest difficulties are caused by extremes, especially if we are not prepared for them. The maximum daily precipitation can be many times higher than the average daily amount," notes climatologist Percec Tadic.

How common are heavy rains for Zagreb?

Precipitation amounts greater than 50 mm, which is the threshold commonly used in the assessment of high precipitation amounts, are not unusual in Zagreb. In 68 years, there were 31 days with precipitation of more than 50 mm in Zagreb, mostly in summer and autumn.

But Percec Tadic emphasizes that people's impression of how frequent episodes with heavy rainfall are mostly depends on the consequences they have on an area.

"The damage is not only a consequence of extreme weather conditions but also of measures that were adopted or not adopted during the planning and construction phases, as well as during further maintenance of the system," said Percec Tadic.

Coastal areas hold the highest records in Croatia

The highest recorded daily precipitation since the beginning of measuring at the Zagreb-Gric station amounted to 118.8 mm in 1926, while at the Zagreb-Maksimir station, the highest daily precipitation was also measured in 1926, and amounted to 139.0 mm.

However, the historical extremes in Croatia are much higher. According to the data from the Climate Atlas of Croatia, the highest maximum daily precipitation was measured along the Adriatic coast (352.2 mm in Zadar on September 11, 1986, 315.0 mm in Crikvenica on September 25, 1987, 201.5 mm in Cavtat August 19, 1979), on the Kvarner islands (250.8 mm on September 5, 1993, and 231.7 mm in Cres on September 25, 1951), on the slopes of the mountain hinterland (Gorski kotar: 319.5 mm in Marcelji on September 27, 1956, 254.0 mm in Gerovo on November 14, 1987), on the windward side of the Snijeznica mountain along the Konavle field in Pridvorje (225.9 mm on October 17, 1976), and on the pass between Mosor and Biokovo in Zadvarje (225.6 mm on July 7, 1992).