Hydrological cycle. Effect of snowfall on water balance

Cykl hydrologiczny

Climate change resulting in an increase in global air temperature is obviously affecting water resources. Based on current IPCC projections, it is estimated that the phenomenon of air temperature increase between 2011 and 2020 is slightly more than 1°C compared to the pre-industrial era (1850-1900) and will continue in the future, and the effects of climate change may be irreversible. The cause-and-effect relationship between climate change and water resources is grounded in the hydrological cycle. As temperatures rise, the phase of precipitation changes.

Snow vs. seasonal water cycle and winter drought

Melting snow determines the water regime of most Polish rivers. The function of accumulated snow is not only to raise their condition. The melting snowpack seeps into the soil, moistening and fertilizing it, and recharging groundwater deposits. Currently, the winter season sees an increase in the frequency of rains with a simultaneous decrease in the frequency of snowfall and a reduction in the duration and thickness of snow cover. There is a decrease in water levels in watercourses due to reduced snowmelt runoff. As a result, rivers, instead of recharging aquifers, drain groundwater resources, which is one of the causes of hydrogeological drought.

This is a phenomenon that highly adversely affects the seasonal water balance. Rain is essential in nature in spring and summer, when plants are developing. After a rainy winter or one with many deep thaws, no “water reserve” is created from snow cover and frozen water in the ground, resulting in shortages during the growing season. These phenomena are expected to increase in the future. Snow is the basis for the renewal of groundwater resources that feed aquatic ecosystems (rivers, reservoirs, lakes) and water-dependent ecosystems (wetlands, marshes, bogs), so its absence can lead to the formation and worsening of water deficits.

This is a very important change in shaping the relationship between the components of the water balance. Snow is “free” retention in winter. This element of the water balance is currently changing dramatically. Increasingly, there is talk of the phenomenon of “winter drought,” which is understood as a lack of water in a given period. Although water is temporarily available in the event of a rainy winter, water shortages and drought are felt in the spring (e.g., by plant root systems) due to rapid runoff into watercourses.

Genesis of snow formation and climate change

Snow, in addition to its underappreciated role in the water cycle, also plays an important function as an aggravating factor in climate change. The absence or short-lived snow cover has a significant effect on accelerating the warming process. The explanation for this phenomenon is the high albedo of the snow, which effectively blocks the flow of energy. So, less snow and days with snow cover translate into more heat absorbed by the earth’s surface.

The condition for the formation of snowfall is the clash of moist air from the Atlantic or Arctic with cold masses from another area. These are complex conditions that result from the spatial relationship between low and high barge systems. One of the reasons for snowless winters in Poland is the disruption of the existing circulation of air masses. Today we observe the occurrence of higher temperatures, which should be associated with probable changes in the advection of air masses.

Over the past few decades, one notices a dramatic change in the nature and course of winter in Poland. According to Prof. Malinowski said in 1961-1990 the average number of days with snow cover in the lowlands ranged from approx. 40 in the western parts of Poland to more than 90 in the northeastern parts (November/December-March/April). Between 2011 and 2020, the number of days decreased significantly, from 18-20 in the west of the country to approx. 60 in the northeastern extremities. The thickness of the snow cover in the 1952-1990 period averaged from 2.5 to 12.9 cm, while already in the 1991-2013 period this value decreased to 1.7-9.7 cm.

The last more heavy snowfall occurred in Poland on December 1-3, 2023. Third-highest level alerts were issued by IMGW in the Małopolska and Podkarpackie provinces, while second-highest level threats were declared in the Silesian, Opole and Lublin provinces. An increase of 30 cm to more than 50 cm of snow cover (depending on altitude) and temperature drops of up to -5°C in the northeast to 0°C in the southeast were observed within 24 hours. It would seem, therefore, that we have had a “real winter” this year, which would contradict global climate change.

Nothing could be further from the truth, as it is not the snowfall itself, but its distribution over time that will be crucial. Global warming in the presence of negative temperatures in the winter semester often takes the form of snowfall, while the duration of snow cover is usually short due to the increase in air temperature. This is the course of the phenomenon we observed in December. After about two weeks of frost, a thaw and melt followed. But this is not the end of winter. We did not have to wait long for the return of snow and frost. In the first weeks of January 2024. The northeastern part of Poland experienced heavy snowfall and severe frosts, which gradually spread throughout the country.

The Institute of Meteorology and Water Management issued first-degree alerts on January 1-2 in the Pomeranian and Warmian-Masurian Voivodeships, reporting forecast snow cover up to about 10 cm thick and temperatures of -7°C. According to synoptics, these phenomena are justified by the pattern of highs and lows in the North Atlantic Ocean. Increasingly warmer Atlantic waters are causing the formation of stronger cyclones in temperate latitudes.

Discussed in a wide forum, global climate change is therefore also reflected in the capricious aura. Its projected and currently observed symptom is the disruption of air circulation over the Arctic by an unusually strong influx of heat, both near the surface and high in the atmosphere, which will shift the raw cold masses over Europe. So, paradoxically, as a result of global warming, we are in for a cold snap.

There is no doubt that climate change has a significant impact on the components of the seasonal water cycle. There is a gradual decrease in snowfall during the winter due to global warming. Currently, snowfall phenomena in December, which should be considered normal in our climate zone, have become weather extremes (anomalies). This is the best evidence of the deepening water crisis also in terms of poor climate memory and worse preparedness to deal with the effects of snowfall.

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