Oliver Koit, a hydrologist at Tallinn University (TLÜ), noted that with a "classic" Estonian winter, the first snows usually arrive in November, but may not persist at that point. Then, snow cover is typically on the ground by December and mostly stays that way until March. If the accumulated winter snow mass melts all at once, a spring flood period inevitably follows.

However, Koit said it is not uncommon in Estonia either for the snow cover that fell in December to melt before the end of the year, or in early January. In these cases, a more permanent snow cover forms only in January or February, if at all. Consequently, due to continuous runoff, spring floods either do not happen or remain relatively modest, Koit explained.

He said: "In the past, there have also been in hydrological sense warmer and drier periods, followed by times with more snow cover and more substantial spring melts. However, some shifts have been identified. Primarily, winters have been observed to have been shifting to later."

Koit's main area of study has been over the past ten years. "For example, in 2019 and 2020, there were winters where a permanent snow cover never formed," he continued.

"Runoff occurred through the year. The same happened this year — the snow came, melted away, came back, and melted again. Those winters with meter-high snowdrifts, fondly remembered by our grandparents, and the corresponding spring floods, have been rare over the last decade. Of course, they have happened, but there have also been years with little snow," the hydrologist added.

Drought-prone springs shift focus on to winter crops

Ilmar Tamm, head of the plant breeding department at the Center of Estonian Rural Research and Knowledge (METK), said that mild winters can exert both positive and negative effects on winter crops, depending on conditions. However, his belief is that mild winters in general benefit winter crops, meaning their cultivation will likely increase in the future.

What is more damaging is freeze-thaw action, however.

"If the ground is frozen and a major thaw takes place, the water largely remains on the surface. Then if another cold spell follows and the surface water refreezes, this can severely damage winter crops. The plants can no longer get aerated. Another risk is if a sudden cold spell hits bare open ground which has no snow cover; this can also damage winter crops," Tamm went on.

Water abundance manifests cyclically

Veteran hydrologist Arvo Järvet stressed that this is not a one-way shift in hydrological conditions; rather, there is cyclic variability in inland water bodies. This is affirmed by over 150 years of data on the Emajõgi River, plus 120 years of data on Peipsi järv and the Narva River.

Järvet said: "The most clearly observed cycle lasts about 25–28 years. This does not mean that all cycles are equal in length or identical, but there is a definite pattern. There are also cycles lasting six or seven years, but their recurrence is less frequent."

According to Järvet, the last water-rich phase began in 2008 and continues to the present, though he was unable to say exactly how long it will last. "The first five or six years of this period saw very high spring floods. Between 2010 and 2013, there were consecutive water-rich years. After those years, conditions have become slightly drier," he added.

"The extent of the spring flood largely determines the year's overall water abundance," Järvet concluded.

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