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Analysis of changes in the occurrence of ice phenomena in upland and mountain rivers of Poland [1]

['Krzysztof Kochanek', 'Faculty Of Building Services', 'Hydro', 'Environmental Engineering', 'Warsaw University Of Technology', 'Warsaw', 'Agnieszka Rutkowska', 'Faculty Of Environmental Engineering', 'Land Surveying', 'Department Of Applied Mathematics']

Date: 2024-08

Abstract The ice phenomena are an inherent component of rivers in temperate, continental, and polar climate zones. Evident progress in global warming leads to a decrease in snow cover on land and ice phenomena in water bodies, disrupting the stability of the hydrological cycle and aquatic ecosystems. Although common observations indicate the disappearance of ice phenomena in rivers over recent decades, detailed quantitative research is lacking in many regions, especially in the temperate zone. In this paper, ice phenomena were analyzed on the rivers of southern Poland, located in the upland and mountain areas of the country, as no such studies have been conducted so far. The temporal changes in the annual number of days with ice (NDI) phenomena were studied in locations where ice phenomena were observed every year for at least 30 years between 1951 and 2021. Using straightforward but commonly accepted procedures, such as the Mann-Kendall test, statistically significant decreasing trends in the annual NDI were revealed for the majority of gauging stations. The Theil-Sen (TS) slope mean values were -1.66 (ranging from -3.72 to -0.56), -1.41 (from -3.22 to -0.29), and -1.33 (from -2.85 to -0.29) for the datasets representing the periods 1992–2020, 1987–2020, and 1982–2020, respectively. The results for the annual NDI were additionally presented within the context of meteorological characteristics such as annual and winter (Nov-Apr) air temperature, precipitation, and water temperature. Correlation and regression analyses revealed that the main factor triggering the decrease in NDI is the increase in the average winter air temperature. An increase in temperature by 1°C results in a decrease in NDI by up to twenty days. If these negative trends continue, ice phenomena may disappear completely from southern Polish rivers within few decades.

Citation: Kochanek K, Rutkowska A, Baran-Gurgul K, Kuptel-Markiewicz I, Mirosław-Świątek D, Grygoruk M (2024) Analysis of changes in the occurrence of ice phenomena in upland and mountain rivers of Poland. PLoS ONE 19(7): e0307842. https://doi.org/10.1371/journal.pone.0307842 Editor: Salim Heddam, University 20 Aout 1955 skikda, Algeria, ALGERIA Received: February 15, 2024; Accepted: July 12, 2024; Published: July 26, 2024 Copyright: © 2024 Kochanek et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: All relevant data are within the manuscript and its Supporting information files. Funding: The author(s) received no specific funding for this work. Competing interests: The authors have declared that no competing interests exist.

Introduction Global climate change poses severe threats to the environment and economies [1]. Beyond the intuitive and widely discussed consequences of global warming, such as temperature rise, changes in precipitation patterns, and related flow regime alterations, the ice phenomena (namely: grease ice, floe, shore ice, ice cover, and ice jams) occurring in surface water bodies within temperate-to-arctic climatic zones are also severely impacted by the changing climate [2–5]. The changes in ice phenomena regarding their duration, occurrence, disappearance, and types correlate with an increase in air temperature and other climatic indices in the region. Most studies revealed climate-induced changes in the duration and spatiotemporal patterns of ice phenomena on rivers, lakes, and coastal waters [6–10]. For example, a study on Russian rivers across Eurasia by Smith [11] found a decrease in ice cover in the 1950s, an increase in the 1980s, and again a decrease in the 1990s, with melt onset shifting negatively by approximately 1 to 3 weeks, consistent with Siberian temperature trends. He also documented interannual and regional variability, such as earlier autumn freezing in Karelian and Kola-region rivers, leading to increased winter ice cover in these areas. Grześ and Ćmielewski [5] confirmed the reduction of days with ice in Arctics (Russian Siberia, Canada, and the USA)–the rivers got frozen later and melt melted earlier in the 1980s when compared to the 1950s, so the number of days with ice was shortened by at least 20 days per 100 years. Contrastingly, Hallerbäck et al. [12] reported a significant reduction in river ice duration across Sweden from the early 20th century to 2021, with the mean number of ice days decreasing by 11 days in the north and 28 days in the south over the last three decades compared to the previous three decades. Similarly, extreme events in recent decades, such as late freezing, early ice melt, and shorter ice cover periods (or no ice cover at all), contribute to the general ice loss in Northern Hemisphere lakes [13]. Klavins et al. [14] noted a reduction in ice cover duration in 17 rivers in the Baltic countries and Belarus based on over 200 years of ice phenomena, hydrological and air temperature observations; in the last several decades the number of days with ice shrunk by 2.8–6.3 days per 10 years, depending on the river. In Poland, observed and modeled increases in seasonal air temperatures [15] during winter (December to February) underscore the shrinking of winter ice phenomena on rivers observed since the early 19th century. Paczowska [16] reported rises in the percentage of ice-free years: an increase from 10% for the River Warta, 4% in the River Vistula, and 0% in the River Neman from 1822–1877 to 40%, 40% and 30% in the period 1926–1935 in Rivers Warta, Vistula, and Neman, respectively. Simultaneously, the duration of time when ice phenomena were observed during winters shortened significantly. For instance, on the River Vistula, ice cover lasting 81–100 days was observed 26 times from 1878 to 1900 and 35 times from 1901 to 1935, whereas 121–160 days of ice cover was observed 22 times from 1878 to 1900 but only 15 times from 1901 to 1935. This reduction in ice cover duration was echoed across other Polish rivers between 1822 and 1935. Similarly, Pawłowski [17,18] analyzed the Vistula River in Toruń in 1814–2003, noting a decline in ice cover duration from 60–120 days in the 19th century to 30–80 days in the second half of the 20th century, and the duration of any type of ice phenomena decreasing from 88 to 53 days (1882–2011) and ice cover from 40 to 7 days. Pawłowski suggested that these reductions might be partially attributed to river regulation and the construction of the Włocławek reservoir upstream of Toruń, which actually goes along with earlier monumental research by Grześ [19]. Fukś [20] reviewed the impact of dam reservoirs on river ice characteristics in the Northern Hemisphere, identifying climate change as the main cause of later freezing and earlier ice melt, and noting the effects of artificial reservoirs on local river ice phenomena. Recent studies, such as by Somorowska [21], reported that increased air temperatures (by 2.7°C over the last 70 years) in central Poland have led to changes in snowfall, rainfall, and baseflow metrics, influencing river flow regime changes in Liwiec River. Additionally, an analysis of ice phenomena duration in several coastal rivers at the southern Baltic from 1956 to 2015 indicated a shortening of ice phenomena occurrence by up to 7 days per decade [8]. For the same region, Łukaszewicz and Graf [22] showed that increasing air temperatures have led to higher river water temperatures, resulting in a shortened duration of ice phenomena. Bączyk and Suchożebrski [23] analyzed the duration of ice cover in a sequence of gauging stations along the River Bug (east Poland) and found that it shortened from approximately 100 days (1903–1960) to about 60 days (2001–2012) due to higher winter temperatures. The results listed above were the main research impact behind the studies on the temporal changes of the NDI in Polish rivers in this paper. Ice phenomena and changes in their spatial and temporal patterns affect a wide range of processes and activities. Economically, icing on lakes and rivers challenges winter transportation and navigation. However, in some regions, icing can also enhance transport across rivers and lakes (e.g., in Siberia). Some authors suggest that ice phenomena could be part of cognitive, environmental, specialist, hiking, winter, seasonal, or occasional tourism [24]. However, inland navigation (technical ice breaking) and low water quality hinder comprehensive documentation of changes in ice phenomena over time [25]. Similarly, while studies of rivers impounded by large reservoirs provide important documentation of anthropogenic influence, they often do not reveal the possible influence of climate-related changes in air and water temperatures on ice phenomena [18]. Hydropower operations can extend ice cover duration upstream of hydroelectric facilities, causing severe ice jams and floods, while significantly shortening ice phenomena downstream [26]. Environmentally, changes in ice phenomena duration and patterns affect water temperature inversion and oxygen circulation in aquatic ecosystems [27]. Knoll et al. [28] discussed cultural ecosystem services and benefits related to ice, while Lindenschmidt et al. [29] indicated that prolonged periods without ice can deteriorate water quality. McBean et al. [30] highlighted the depletion of dissolved oxygen due to continued respiration, potentially reducing primary productivity and reaeration. Under-ice processes impact the annual cycling of energy and carbon through aquatic food webs [31]. Berilsson et al. [32] described how under-ice conditions alter lake physics, affecting auto- and heterotrophic micro-organism distribution and metabolism. Extended periods without ice negatively influence aquatic fauna and flora, extending the growing season for warm-water species and expanding pelagic habitats [33,34]. This extended growing season stresses micro-organisms, favoring competitive species, disrupting life cycles, and altering the reproductive periods of fish and crustaceans [35,36]. Documenting cross-sectoral consequences of changing ice phenomena types, durations, and occurrences is critical. However, extensive, systematic studies covering broad, country-wide data have not yet been conducted in Poland. Despite common perceptions of disappearing ice phenomena on Polish rivers in recent decades, detailed research in the temperate climatic zone is still lacking. The main objective of this paper is to study changes in the occurrence of ice phenomena in the scarcely documented upland and mountain rivers of Poland, in relation to meteorological conditions. Being aware of the newest methodological approaches for detecting trends in environmental and hydrological variables, such as the innovative trend pivot analysis method and trend polygon star concept method [37], machine learning, and artificial intelligence, here the emphasis was put on the temporal changes in the annual number of days with ice (NDI) using commonly accepted statistical methods, i.e., the Mann-Kendall test with Bonferroni correction. Note, that this analysis considers all types of ice that may be formed in rivers. Our results remain an input into the riverine ice investigations and aim to contribute to the recognition of ice phenomena alterations using the widest dataset and geographic extent available.

Discussion The analysis indicated a significant reduction in the annual number of days with ice phenomena in southern Polish rivers since the commencement of national hydrological monitoring. The most substantial change in Polish rivers was noted in the recent periods analyzed, highlighting a marked response of this phenomenon to climate change. While one might expect more pronounced differences in the annual ice days in datasets spanning longer periods, particularly during periods with significant disparities in air temperature indicators, our findings reveal that the last 30 years witnessed the most dynamic and abrupt increases in air temperatures, leading to a decrease in icing periods in rivers. Assuming that the average annual sum of days with ice in Polish rivers during the analyzed periods is 55 days and extrapolate the results with the assumption that historical trends will continue at the same magnitudes in the future (ranging from -3.72 to -0.56 days/year in the 30-year analysis, -3.22 to -0.29 days/year in the 35-year analysis, and -2.85 to -0.29 days/year in the 40-year analysis), we can speculate that under the most conservative scenario, ice phenomena in Polish rivers may vanish in a few decades. This estimation considers a steady decline in ice days toward their likely disappearance by the mid-21st century based on the constant Theil-Sen slope. When examining the potential impact of dam reservoirs on downstream sections with regards to ice formation [20,50], it was found that the locations of gauging cross-sections downstream of dam reservoirs are considerably distant from the dams. For instance, Klęczany at the River Ropa is approximately 26 km from the Klimkówka Dam, Topoliny at the River Ropa is around 52 km from the Klimkówka Dam, and Przemyśl at the River San is over 146 km from the Myczkowce Dam. The operational effect of dams on ice duration is deemed insignificant, attributing the decreasing trends in ice days primarily to climate change. The Klimkówka dam, put into operation in 1994, is located 54.4 km upstream from the river confluence. The distances between the two cross sections analyzed in this paper, namely Klęczany and Topoliny, and the river confluence are 26.9 km and 3 km, respectively. The impact of the Klimówka dam on ice occurrence in the River Ropa in the Klęczany and Topoliny cross-sections, was analysed by means of the MK test for the number of days with ice before and after the dam was constructed in 1994, i.e. 1960–1993, 1994–2021 for Klęczany and 1960–1993, 1994–2019 for Topoliny. The TS slope values in the two periods were -0.25 and -1.11 (Klęczany) and -0.38 and -2.91 (Topoliny), respectively. The hypothesis about higher decrease in number of days with ice (i.e. lower TS slope) in the second period then in the first one was verified with bootstrap method. The results for Klęczany showed no trend in 1961–1993 and a decreasing trend in 1994–2021, however, the difference between the TS slope values is not significant suggesting non-substantial impact of the dam construction on the number of days with ice. Similarly, in Topoliny no trend was detected in 1961–2021 and a decreasing one in 1994–2019. This time, however, the difference between the TS slope values is significant being stronger after the construction of the dam. A deeper insight into the sample values showed the decreasing trend in the second period stemming from several anomalously low values in the years 2010–2019 (see Fig 7 for the Ropa/Topoliny cross-section that direct down the regression line) which also cause a strong decrease in the TS slope. Future climate change impacts projected to affect water resources in Polish catchments, such as decreased snow accumulation and a decline in baseflow [15,51], do not appear to align with the observed general decrease in ice days in southern Polish rivers. This decline occurs uniformly across the analyzed region with consistent, though varying, intensities, indicating that the increase in winter temperatures is anticipated to have a more pronounced and unilateral effect compared to other seasons. Consequently, the warming of winters is expected to drive a widespread reduction in icing durations, likely evolving into a nationwide phenomenon independent of catchment physiography or human intervention levels. From a water management perspective, the outcomes of our study offer insights into planning strategies to mitigate ice-related issues in rivers and address technological challenges associated with managing icing occurrences, such as protecting bridges and dam structures. The diminishing ice phenomena is seen as economically favorable in reducing the risk of water structure damage caused by ice flow. However, it is essential to recognize that adapting to the transition as ice events become less frequent yet still possible in colder winters is crucial to prevent potential damages incurred by river icing. In terms of aquatic ecosystem protection, the declining trends in ice phenomena are anticipated to have adverse long-term implications, particularly for ecosystems dependent on ice processes. The alteration in ice patterns may disrupt crucial ecological processes, such as the formation of anastomosing riverbeds and the cleaning of river bottoms, potentially leading to habitat changes and impacting the breeding success of fish species, e.g. in the River Narew [52]. As the disappearing ice alters habitats and species dynamics, there may be cascading effects on aquatic plant species, life cycles of aquatic organisms, and biogeochemical processes in watercourses [53–58]. Consequently, the absence of ice phenomena could influence nutrient dynamics, potentially accelerating nutrient runoff and eutrophication in downstream water bodies. 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Conclusions The analysis conducted in this study has documented a noteworthy reduction in the duration of ice events in southern Poland. The observed shortening of the icing period in the examined rivers, contingent upon the length of datasets analyzed, spanned from -3.79 days/year to -0.29 days/year. Both mountain catchments (characterized by dynamic flow conditions) and lowland catchments (exhibiting slower flow conditions) are similarly affected by these diminishing icing periods. By examining air temperatures and exploring correlations between trends in air and water temperatures, we concluded that the decline in ice events on rivers in southern Poland is primarily a result of ongoing climate warming and not directly linked to human activities. Notably, a nonlinear trend in this process has been identified, with a more rapid decrease in ice duration observed in recent years (1992–2021) compared to longer time frames. A 1°C rise in mean winter temperature led to a reduction of 12–20 days with ice phenomena in the study area during the period 1992–2021. Climate variables such as precipitation did not display a significant impact on the occurrence of ice events. Should the observed trends persist, it is hypothesized that rivers in southern Poland will likely cease to exhibit ice phenomena around 2040–2070. The gradual disappearance of ice in rivers undoubtedly disrupts the regularity of the hydrological cycle, causing changes in water accumulation and altering conditions for aquatic flora and fauna development in our climatic region. Given the escalated disappearance of ice events, particularly in the last 30 years, in Poland and potentially Central Europe, the comprehensive impacts remain largely unknown. Recognizing the complexity of these consequences, it is vital to concentrate on analyzing how individual components of aquatic ecosystems respond to the absence of ice phenomena. The intricate implications of this disappearance on rivers in Poland and Central Europe as a whole have yet to be fully understood. Looking ahead, conducting studies on the onset and cessation of ice events could provide valuable insights into changes in ice occurrence duration. Furthermore, exploring novel statistical methods for trend detection could be beneficial, although their efficacy should be rigorously evaluated before implementation.

Acknowledgments The data available in the IMGW-PIB repositories enable further and deeper analysis of ice phenomena in Poland. Dr Barbara Nowicka is acknowledged for her help in data validation and acquisition. This work was supported by a subsidy from the Polish Ministry of Science and Higher Education for the Warsaw University of Technology, University of Agriculture in Cracow, Cracow University of Technology, Institute of Geophysics, Polish Academy of Sciences and University of Life Sciences in Warsaw (SGGW).

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[1] Url: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0307842

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