Snow Cover in Bulgarian Mountainous Regions: Observed Variations and Future Activities

Nadezhda Petkova, Vesselin Alexandrov National Institute of Meteorology and Hydrology - BAS, , [email protected]

Abstract An investigation on observed variations and tendencies in annual snow cover duration (SCD) and maximum snow depth (MSD) in the mountainous regions of Bulgaria over the period 1931-2005 was carried out. Drought anomaly index and standardized anomalies of winter precipitation as well as Ped drought index for winter precipitation and air temperature were applied in effort to explain winter droughts. Annual snow cover temperature sensitivity and rend estimations were made. Twenty one meteorological stations located in Bulgarian mountainous regions were selected for this study. For assessment of climate variations and tendencies various statistical techniques were applied. Snow cover exhibited evidence of significant decadal variability over the investigated period but no proven statistical significant long-term trends. During the last three decades of the 20th century most of the winters have experienced atmospheric drought; winter precipitation was below normal in 63% of the winters and 47% of winters have been dry and warm. Projections for future activities are launched considering the obtained results and consequences that sustained changes in climate affect water resources, agricultural plants and biodiversity, development of mountain ski resorts and tourism and economy in general.

Keywords: snow cover, mountainous regions

Introduction Snow cover in temperate mountain regions of our planet is an important resource for water supply, irrigation, hydro-electricity production, agriculture, winter sports and tourism. In terms of ecosystems, snow cover protects the dormant plants from frost during the cold part of the year. Precipitation during the winter period plays a main role in stockpiling soil moisture for spring droughts. Low winter precipitation, less than 20 mm per month affects significantly the plants if the autumn or next spring is dry. The consequences of insufficient precipitation during the tillering are most sensitive after severe winters or under droughts continue to earning (Alexandrov, 2005). The economic consequences of the negative effects under drought can be perceptive. Long droughts associated with scarce snow cover during the winter causing damages on many crops and lead to significant reduction of flow, for example the period 1982-1994 for Bulgaria. The events during this drought period show many negative impacts of unfavorable climate in Bulgaria, with environmental, economic and social dimensions. These impacts suggest expected consequences of climate change, with future warming and drying over Bulgaria (e.g., Knight et al., (eds.), 2004). In temperate mountainous regions winter temperatures is often close to freezing which means the accumulated snow cover is quite sensitive to climate variations and changes. At the same time th according IV Assessment Report of IPCC temperature will continue to rise over the 21st century even if the concentrations of all GHGs and aerosols had been kept constant at year 2000 levels a further warming of about 0.1°C per decade would be expected (Allali et al., 2007). Warmer winter temperatures will lead to more of the precipitation falling as rain than as snow and earlier snowmelt (Nolin and Dally, 2006). For every degree Celsius increase in temperature the

BALWOIS 2012 - Ohrid, Republic of Macedonia - 28 May, 2 June 2012 1 snowline rise by about 150 m therefore as a result less snow will accumulate at low elevations than today. The hydrological cycle will be enhanced under warmer climatic conditions and the consequences for river runoff are likely to affect not only the watersheds within the mountains themselves, but also in the lowland regions that are heavily dependent on this mountain resource (Beniston, 2003). Shifts in snow pack, duration and amount as a consequence of sustained changes in climate will affect not only water resources, soil, agricultural plants, vegetation and biodiversity, as well as the development of mountain ski resorts and tourism but also on mountain farming, water supply, energy, the economy in general, people's health and behavior (Benisston at al., 2003). This work is an investigation on variations and tendencies in snow cover (depth and duration) in accordance with tendencies in winter precipitation and temperature in Bulgarian mountainous regions for the period 1931-2005. Winter droughts for the autumn-winter (October – March) season are investigated by applying drought anomaly index, standardized anomalies of winter precipitation and Ped drought index. Annual snow cover temperature sensitivity and rend estimations were made

Data and Methods In Bulgaria snow cover is mainly confined to the mountains, which occupy almost one third of the country with typical elevations in the 800-1000÷2925 m range (Petkova et al., 2004). A meteorological and snow cover dataset extending back to the early 1930s for 21meteorological stations in the Bulgarian mountains has assembled and the locations of the stations are shown in Figure 1. Information about the climate observing programs and data quality control are provided in Petkova et al., (2004). SCD was defined as a day with 1 cm or more of snow on the ground. Annual maximum snow depth also has used for it greater relevance to water resources. Drought anomaly index (Koleva, 1988) and standardized anomalies of winter precipitation as well as Ped drought index (Ped, 1975) were additionally used to describe more detailed drought (wet), warm (could) winters and to investigate winter droughts during 1931-2005 period. The drought indexes were calculated for the autumn-winter (October – March) season, substantial for soil water supply (Hershkovitch, 1968). Most of the statistical techniques are described in (Petkova et al., 2004 and Petkova et al., 2010). A 95% level of significance is accepted to define significant trends and the current climate 1961-1990 period is used in computation of snow cover, winter precipitation and air temperature statistics and anomalies.

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D a n u b e P l a i n SER BIA

S t a r a P l a n i n a

Vitosha T r a c i a n L o w l a n d R i l a

MA CE R h o d o p s DO T U R K E Y NIA

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Figure 1: Locations of the mountain meteorological stations used in the study

BALWOIS 2012 - Ohrid, Republic of Macedonia - 28 May, 2 June 2012 2 Results and Discussion 1. Snow cover in Bulgarian mountains Snow cover shows significant spatial and temporal changes, ranging from ephemeral in nature below 800 m but persists for an average of 200 days above 2000 m a.s.l. in the central Mountains (Mt. Cherny vrah - 2286 m a.s.l.) and 190 days in Stara Planina Mountains (Mt. Botev- 2376 m a.s.l.). At higher elevations, such as Mt. Cherny vrah, the mean maximum snow accumulation is 180 cm. Both mean maximum snow depth and snow cover duration are strong functions of the elevation due to the combined influence of the lapse rate effect on air temperature and orography influences on precipitation (Brown and Petkova, 2007)). Maximum snow accumulation and snow cover duration exhibit a strong dependence on elevation, increasing linearly with correlation coefficients (R²) of 0.82 and 0.89 respectively. The observed linear dependence of snow cover and winter temperature on elevation permitted us to estimate mean winter snow line temperature sensitivity of −191 ± 20 m/°C as well as annual snow cover duration and maximum snow depth temperature sensitivities, which are respectively: −17 ± 2days/°C and −16 ± 3 cm/°C respectively. In mountainous regions of the country at the altitudes above 1100-1200 m due to the low winter temperatures suitable conditions for continuous snow cover formation exist (Stanev et al. (Ed.), 1991). The period of continuous snow cover typically extends from December to March (Brown and Petkova). In the Rhodopes at an altitude of 1000/1100 m, continuous snow cover persists approximately in 50% of the winters. In the Balkan, and Vitosha Mountains at this altitude snow is stable approximately in 75-80% of the winters, and above 1,300 m in 100% of the winters.

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D a n u b e P l a i n SER 31 BIA 28 S t a r a 31 31 P l a n i n a 24 31 BLACK Vitosha SEA 24 28 T r a c i a n L o w l a n d 28 31 31 R i l a 26 25 MA 20 29 25 CE R h o d o p s DO Pirin T U R K E Y NIA 25 27

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Figure 2: Average snow cover duration in January in Bulgarian mountains January is a typical winter month with the lowest monthly average temperature and most days with snow cover in Bulgaria. The duration of snow cover in January increases linearly with elevation with correlation coefficient R² = 0.86. At an altitude above 1400 m a.s.l. all days in January have snow cover (Figure 2). The average temperature in January at this altitude is ~ - 4.7°C which provides suitable continuous for snow cover persisting. Staneva B., (1955) was found this “border altitude” is 1200 - 1300 m. A lifting with an average of 150 m during 1961-1990 period by comparison with the 1921-1950 is observed. 2. Snow cover variability and trends The applied trend analysis for annual MSD and annual (SCD) in the mountainous regions of Bulgaria (Fig. 4a and Fig.4b) revealed a decreasing trend which is statistically significant in the western (Petrohan), the central (Mt. Botev) and the Vitosha Mountains (Mt. Cherni Vrah) for MSD. A statistically significant decrease in SCD was observed in the mountain areas such as: the

BALWOIS 2012 - Ohrid, Republic of Macedonia - 28 May, 2 June 2012 3 Vitosha Mountains (Mt. Cherni Vrah) and the western Rhodope Mountains (Yundola). Snow cover duration shows an increasing trend statistically significant in the Rhodope Mountains (Boikovo and Chepelare). At the same time winter precipitation exhibits evidence of reductions, which are statistically significant in many areas such as: the central Balkan Mountains, the Vitosha Mountains, the Rila Mountains and in the Pirin Mountans (Bansko). Only in the western Balkan Mountains (Petrohan) significant increase in winter precipitation is observed. Winter temperature shows an increase tendency which is statistically significant in the highest part of the central Stara Planina, the Ossogovska Mountains, the south eastern slopes of Pirin Mountains and the western Rhodope Mountains.

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D a n u b e P l a i n SER BIA

S t a r a P l a n i n a

Vitosha BLACK SEA

T r a c i a n L o w l a n d R i l a MA R h o d o p s CE DO Pirin T U R K E Y NIA

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Figure 3a

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D a n u b e P l a i n

SER BIA

S t a r a P l a n i n a

Vitosha BLACK SEA

R i l a T r a c i a n L o w l a n d

MA R h o d o p s CE DO Pirin T U R K E Y NIA

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Figure 3b

Decreasing trend Increasing trend Significant decreasing trend Significant increasing trend Not changing trend Tendencies in annual maximum snow depth (Figure 3a) and annual snow cover duration (Figure 3b)

BALWOIS 2012 - Ohrid, Republic of Macedonia - 28 May, 2 June 2012 4 The trend estimations corresponding to the decrease in SCD found an average decrease of 1-2 days during 1931-2005 period and it have reached to15 days in the highest part of the mountains. MSD decreases with an average of 10 cm and decreasing get to 35 cm for the mountain peaks. Winter precipitation has experienced an average reduction of 8-10 mm in mountainous regions for 75 year period. The reduction is 30-35 mm for the highest part of the mountains. At the same time mean winter temperature has increased with 0.8ºC in the whole investigated period.

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-3 Standardized Anomalies Standardized Snow Cover Duration -4 5 per. Mov. Avg. (Snow 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 Cover Duration)

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Figure 4: Variability in maximum snow depth, annual snow cover duration and 5-year running average (bold lines) The variable nature of winter snow conditions is shown in figure 4 representing the regionally averaged annual series of standardized anomalies (computed with respect to a 1961 – 1990 reference period) for maximum snow depth and duration in snow cover for the 1931-2005 period. In the mountainous regions of Bulgaria annual MSD and SCD anomalies exhibit one period of above- average snow cover during the late 1930s and 1940s and two periods of sustained low snow cover: during the late-1950s and early 1960s, and the late 1980s to mid -1990s. Snow cover conditions retrieved close to the normal during the later 1990s but stayed below normal till the end of the investigated period. The above-average snow cover during the late 1930s and 1940s is consequence of the wet winter conditions with above-normal precipitation and low winter temperatures. The early 1960s snow deficit is associated with an extended period of above-average temperature in the fall and early winter that delays the start of the snow seasons while the later 1980s and the early 1990s deficit is associated with a well-documented period of below-average precipitation (Brown and Petkova, 2007). Variability in regionally averaged standardized anomalies of winter as well as Ped’s drought index that reflects atmospheric conditions related to the X-III winter season are presented in figure 5. Winter precipitation was below normal in 63% of the winters during 1971-2000 period and in 77% during 1982-1994 period. According Ped index 47% of winters during the period 1976-2005 and 70% of the winters during the last decade of the 20th century have been dry and warm. This fact could be explained by the influence of atmospheric circulation and frequency of Mediterranean cyclones substantial influencing the character of the winters in Bulgaria. During the last 20 years of the 20th century especially during 1990-1999 a well pronounced tendency of decreasing number of Mediterranean cyclones is observed which resulted in decreasing winter precipitation over Bulgaria (Marinova et all, 2005, cited in Brown and Petkova, 2007).

BALWOIS 2012 - Ohrid, Republic of Macedonia - 28 May, 2 June 2012 5 2.5 2 1.5 1 0.5 0 -0.5 -1 -1.5

Standardized Anomalies Standardized Winter (X-III) Precipitation -2 1931 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 19855 per. Mov.1990 Avg.1995 (Winter2000 (X-III) 2005 Precipitation)

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-2.5 Ped Index -3.5 1931 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 5 per. Mov. Year Avg. (Ped Index) Figure 5: Variability in SPI and Ped drought index (X- III) and 5-year running average (bold lines)

Conclusions 1. The winter climate in the Bulgarian mountainous regions is characterized by warming and reduction in winter precipitation leading to an overall decrease in snow cover duration and maximum snow depth during the period 1931-2005. 2. Snow cover exhibited evidence of significant decadal-scale variability over the 1931-2005 period but no proven statistical significant long-term trends. 3. During the last three decades of the 20th century most of the winters were experienced atmospheric drought; winter precipitation was below normal in 63% of the winters and 47% of winters having been dry and warm. 4. The “border altitude” above which snow cover is continuous in the all January has lifted with an average of 150 m during the 1961-1990 period by comparison with 1921-1950.

Future activities 1) Development of climate scenarios in conjunction with solving the task of assessing the sensitivity of snow cover, to possible future climatic changes that will enable the provision of adaptation measures in areas such as tourism, high - mountain agriculture, water resources. 2) Study of extreme events such as snowstorms, avalanches, forest fires and etc. as well as risk assessment of them to launch the construction of early warning systems.

Acknowledgments This study is partially supported by the international project of Drought Management Centre for Southeastern Europe. The project is funded by the SEE Transnational Cooperation Programmed.

BALWOIS 2012 - Ohrid, Republic of Macedonia - 28 May, 2 June 2012 6 References Allali A., R. Bojariu, S. Diaz, I. Elgizouli, D.Griggs, D. Hawkins, O. Hohmeyer, B.Jallow, L. Kajfez- Bogataj, N. Leary, H. Lee, D. Wratt, (Eds.), Climate Change 2007: Synthesis Report to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Alexandrov V., 2005: Soil drought in Bulgaria (in Bulgarian), www.meteo.bg/meteorology/. Beniston M., 2003: Climatic Change in Mountain Regions: A Review of Possible Impacts, Climatic Change 59: 5–3, Kluwer Academic Publishers. Printed in the Netherlands. Beniston M, F. Keller, B. Koffi and S. Goiette, 2003: Estimates of snow accumulations and volume in the Swiss Alps under the changing climatic conditions, Theoretical and Applied Climatology, 76, 125- 140.Benitson M, 2005: Climatic Change in Mountain Regions: A Review of Possible Impacts, v.59 (1- 2), 5-31, DOI: 10.1023/A: 1024458411589. Brown R., Petkova N., 2007: Snow Cover Variability in Bulgarian Mountainous Regions, 1931-2000, International Journal of Climatology, 27, 1215-1229 (2007). Hershkovitch E., 1968: Repeatability and scope of the different seasonal and damaging to agricultural drought in Bulgaria. In proceedings: Nature of drought and irrigation regime for agricultural crops in Bulgaria, 111-15 Knight G., I. Raev, M. Staneva, (eds.), 2004: Drought in Bulgaria - a contemporary analogue for climate change. ASHGATE, England, ISBN 0-7546-4215-1, 336 pp. Koleva E., 1988: Futures in precipitation distribution in Bulgarian plane part, Problems of Meteorology and Hydrology, v.2.41-48 (in Bulgarian). Nolin A. & Daly C., 2006, Mapping “At risk” snow in the Pacific Northwest, Journal of Hydrometeorology, 7(5), 1164-1171. Ред Д. А., 1975: Index of drought and waterlogged, Proceedings of the Hydrometeorological Center, 156,19-38, (in Russian). Petkova, N., E. Koleva, V. Alexandrov: 2004, Snow cover variability and change in mountainous regions of Bulgaria, 1931-2000. Meteorologische Zeitschrift, vol 13, N 1, 19-23. Petkova N., V. Alexandrov, E. Koleva, 2010: Snow Cover Variability in Bulgaria, 1931-2005. In Proceedings of the International conference: „Geography and regional development”, 14-16 October, 2010. Stanev, S., M. Kjuchukova, S. Lingova (Eds.), 1991: The Climate of Bulgaria, Publishing house of Bulgarian Academy of Sciences, Sofia, 499 p, (in Bulgarian). Staneva B., 1955: On snow cover regime in Bulgaria, Publications of Hydro-meteorological service of Bulgaria, vol. IV, 213-269 (in Bulgarian).

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