Research by Estonian scientists shows "short-duration rainfall" in Estonia has intensified by 4 percent per decade. Co-author Ottar Tamm, an assistant professor of hydroinformatics at the Estonian University of Life Sciences, said that several years ago, water engineers began to express concern that the existing calculations for rainfall amounts might be too conservative. More rain was falling than forecast.

"There was a suspicion that stormwater pipe systems in Estonia might be underdimensioned and that this should be investigated. So we gathered all the short-term precipitation measurement data. We went through the archives, reviewed everything, digitized the data, and conducted a statistical analysis based on measurements taken in Estonia," Tamm told ERR.

"We can now quite confidently state that extreme precipitation in Estonia has become more intense, with statistical analysis showing an increase of 4 percent per decade. We have also received confirmation of the trend magnitude from climate model projections," he added. "Some models show a bit less, some a bit more, but the scale remains around four percent."

Tamm said it is "certainly reasonable to assume" the trend will continue in the near future. The further into the future we look, the more the reliability of the forecast decreases, but it can be trusted for the next 20 years.

There is a simple physical explanation for the intensification of precipitation: the warmer the air, the more water it can hold. Since Estonia has gradually become warmer, the air contains more moisture. "If the air temperature is higher during a downpour, then the chance of more water falling is greater. Clouds are like containers, and with warmth, those containers become larger," the researcher explained.

Downpours are heavier, but droughts more frequent

Droughts are also becoming more common in Estonia and farmers are unlikely to be pleased. For farmers, rain no longer falls at the right time, in the right place, or in the right amount, Tamm said.

"The variability of precipitation — its frequency and duration — has changed. For example, it increasingly happens that it does not rain at all for almost a month, then suddenly a large amount of rain falls, and after that it is dry again for a long time," the hydrologist explained.

"A farmer would ideally want it to rain a little every three to four days, but not like this, where, figuratively speaking, everything scorches for a month and then a heavy downpour comes, dropping 50 millimeters in a day. The soil cannot absorb that water; it all runs off across the surface into ditches, and a week later there is too little water again," Tamm continued.

Heavy downpour can also cause other problems. For example, Tamm pointed to stormwater culverts running under roads. Last year, in the Tartu area, roads flooded as the amount of water exceeded the drainage capacity.

When talking about flooding, the assistant professor noted that knowing how much it rains in a 24-hour period does not help drainage planners. At least in cities, the runoff time for stormwater is much faster. "When rain falls in a city, it moves quite quickly over hard surfaces. Here we are talking more about 10 to 80 minutes, depending on the size of the catchment area where the water collects."

Where can rainwater that falls in the city go?

Ottar Tamm said that for both Tallinn and Tartu, the main concern tends to be where to direct rainwater once it has fallen. New knowledge shows that the problem must be addressed more urgently. This is already accounted for in the standards updated in 2021, which define how stormwater is calculated. Going forward, the 4 percent trend will be taken into account in stormwater system design. Tamm helped write the new rules.

"In the past, we did not have this knowledge. That means that the currently built stormwater systems cannot accommodate any more water. If we cannot direct the water somewhere, it will always flow to the lowest point. In such places, there are usually roads and sometimes someone's home. If we do not regulate stormwater, then the risk of flooding will increasingly become part of the urban landscape," he told ERR.

Assessment is the first step to taking action. For that, various calculation models are used. For example, water company Tallinna Vesi prepares detailed stormwater models for different city districts, based on which problematic areas and opportunities to reduce flood risk can be identified.

"Essentially, we can do two things: the first option is to make the drainage pipes very large. But this is not a financially reasonable solution. First, it would require a completely disproportionately large pipe. Second, the kinds of rains that cause major flooding happen so rarely that the pipe would almost never operate at full capacity. It would just be an enormous financial cost," Tamm explained.

The second approach is gaining increasing support both in the scientific community and in Estonia. "There are nature-based solutions like rain gardens and green areas where water can be temporarily directed and stored. Essentially, they function as reservoirs where stormwater is temporarily diverted and from which the water later continues into the stormwater system, for example, into a pipe," the researcher explained. Green roofs can be used in a similar way.

As it is not possible to remove all hard surfaces in a city such as Tartu, Tamm suggests that plastic reservoirs regulating stormwater drainage could be installed under the asphalt. As pipes have limited capacity, additional loads can be accommodated beneath the surface. He said this has already started happening in both Tallinn and Tartu.

"I like that stormwater is no longer treated as a problem like in the past, where it had to be removed as quickly as possible. Stormwater is a resource that can be used to empower green solutions in the urban landscape. I also prefer walking in green areas rather than on hot asphalt," Tamm said, summing up the issue.

Cities' water regimes should become more nature-based

Professor of urban water systems at Tallinn University of Technology Ivar Annus stated that pipe-based systems are designed both in land drainage and in cities, taking into account the intensity and duration of rainfall, which is usually derived from historical data.

"In the University of Life Sciences study, historical data was reexamined, supplemented with data from recent decades, and it was found that the intensity of rainfall in Estonia has indeed changed. When it rains, it often comes down in large amounts at once," said Annus.

He said the current water systems were mostly built 30–40 years ago, in some cases 20 years ago. "This means they tend to flood more frequently, simply because they were not designed for the kind of rainfall we are seeing today," noted the professor. When water no longer fits in the pipes in certain places, it begins to push back up through storm drains, which are supposed to direct it underground, and spills back onto the street.

Similarly to Tamm, Annus found that the long-prevailing philosophy of removing stormwater as quickly as possible has started to change. "In the case of heavy downpours, this may no longer work, because we would have to build excessively large pipes. In normal conditions, those would simply fill up with silt and sand. The water volumes are usually just not large enough to keep them clean," said the scientist.

Annus said the urban water systems should seek to imitate nature. This means creating places where excess water can be temporarily stored.

"These can be rain gardens, where water is directed not immediately into a storm drain, but first onto a green area where, for example, a small temporary body of water can be formed. Only then is the water directed into a storm drain, or infiltration is encouraged, if possible. Another option is to create larger park areas that can be deliberately flooded during heavy rainfall," explained the professor.

In other words, areas are created in the urban environment for controlled flooding, where no additional damage occurs. However, according to Annus, there are too many hard-surfaced areas in Estonian cities at the moment, especially in Tallinn, to manage stormwater problems in this way.

"When our research group was involved in compiling Tallinn's most recent public water supply and sewerage development plan, a very general analysis was conducted. To cope with heavy downpours, approximately 11 percent of the surface area of Tallinn's streets would need to be converted into green areas. It is not fully feasible to implement that, but that is the order of magnitude we are talking about," he noted.

Annus said it would be more sensible to design stormwater systems with controlled flooding capacity. "We're changing the water regime a bit more like how nature behaves. If we need to, we will buffer it in the meantime and only then direct the water forward," said Ivar Annus.

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