IDENTIFICATION OF ATMOSPHERIC DROUGHT PERIODS IN NORTH-WEST POLAND OVER 1965-2004

Eliza Kalbarczyk, Robert Kalbarczyk
Department of Meteorology and Climatology, Agricultural University of Szczecin, Poland

ABSTRACT

The aim of the study was to estimate the frequency of occurrence, a degree of intensity and time and space variability of atmospheric drought periods during vegetation season of plants determined on the basis of relative precipitation index (RPI) and hydrothermal index (K) in north-west Poland. The material was mean-monthly values of air temperature and monthly total precipitation for the vegetation season (from April to September) over 1965-2004 from 12 IMGW stations situated in Pomerania. Occurrence of drought in Pomerania is a phenomenon which is remarkably space-variable and it applies to all Pomerania. However, the area located in the south-west can be considered as the most threatened with drought, and the area situated in the north of Pomerania as the least threatened. In the centre and east part of Pomerania the month with the most frequent occurrence of drought of different severity according to RPI was April, and in the western part – it was July. In almost all Pomerania drought occurred the least frequently in August, only along the Baltic coast – in September. In 1965-2004 no statistically significant tendency of RPI index variability was recorded. The values of the K hydrothermal index showed a downward tendency in subsequent years in April and May, which was significant at the level α <0.1 in April – in the south and north of Pomerania, and only in the south – in May.

Key words: atmospheric drought, relative precipitation index, hydrothermal index, Pomerania.

INTRODUCTION

During the period from April to September the north-west part of Poland (Pomerania) receives about 55-65% of total annual precipitation in this area. According to Rojek [17] the largest potential precipitation deficiency in this area occurs in May (climatic water balance ranges from below –30 to over –40 mm), and next in April and in August (climatic water balance from below –10 to more than –30 mm). In June and July in the north part of Pomerania there is a small, up to 10 mm, advantage of precipitation over evaporation, which in September already covers about a half of Pomerania and amounts to over 20 mm. In the period from April to September precipitation deficiency in relation to evaporation is altogether from about 20 mm in the north to over 160 mm in the west, south and east ends of the region. The north part of the research area is also characterized by the highest rainfall variability, particularly from April to July (from 55 to 70%) [5]. The longest, over sixty-day precipitation-free period in the years 1971-2000 was also recorded in this area in 1992 [11]. Although Pomerania belongs to the areas receiving more than average total precipitation in Poland, months with precipitation deficiency occur more frequently here than months with its excess [12].

Preventing the negative effects of drought in agriculture requires an evaluation the risk of drought occurrence in a given area and an assessment of a degree of their severity. The phenomenon of drought is not explicitly defined in the literature [1,2,4,15], that is why there is a need for an index which is specific to a given region, working in local circumstances.

The aim of the study was to estimate a frequency of occurrence, a degree of severity and time and space variability of atmospheric drought periods during the plant vegetation season, determined on the basis of RPI and K indexes over 1965-2004, in north-west Poland.

MATERIAL AND METHODS

The meteorological data containing mean-monthly values of air temperature and monthly total precipitation from the vegetation season (from April to September) over 1965-2004 came from 12 IMGW (National Hydrological and Meteorological Services) stations situated in north-west Poland (Bydgoszcz, Chojnice, Gdańsk, Gorzów Wlkp., Koszalin, Lębork, Piła, Resko, Szczecin, Szczecinek, Swinoujscie, Ustka) published in Monthly Agri-Meteorological Review [14] and Bulletin of National Hydrological and Meteorological Service [3] as well as those made available by IMGW in Warsaw. The data for the station Bydgoszcz from July 1982 till the end of 2004 was read off from maps of isotherms and isohyets using the graphical interpolation method.

Drought periods were determined by two methods – according to the value of the relative precipitation index RPI (in %) and on the basis of hydrothermal index K.

On the basis of the RPI index [2], calculated according to the formula:

RPI = (P / ) 100%       (1)

where:

P – total precipitation in a given period, mm,
– mean multi-year precipitation value in a given period, mm,

drought periods were determined using a criterion by Kaczorowska [8] in which a total precipitation less than 25% of a multi-year total (a norm) is considered as an extremely dry month, if a total precipitation ranges from 25 to 49% of the norm – as a very dry month, and if a total precipitation ranges from 50 to 75% of the norm – as a dry month. The criteria for longer periods are: an extremely dry period – precipitation below 50% of the norm, very dry – 50-74% and dry – 75-89% mean multi-year total precipitation, respectively.

Sielianinov K hydrothermal index:

K = P / 0.1 Σ t       (2)

where:
P – total precipitation, mm
Σ t – total air temperature > 0°C

allowed three periods with different drought intensity to be distinguished: an extremely dry period – K ≤ 0.4, a very dry period – 0.4 < K ≤ 0.7, and a dry period – 0.7 < K ≤ 1.0. The K index is applicable to periods where the mean daily temperature is at least 8°C. For that reason drought periods determined in April with lower temperature are less accurate than in the other months.

RESULTS AND DISCUSSION

In the years 1965-2004 an average total precipitation from April to September in Pomerania was 350 mm, from below 320 mm in the west to above 400 mm in the north part of the research area (Fig. 1). Total precipitation variability throughout the vegetation season amounted to about 25%, and was the smallest in the west part of the coast, and the largest in the north-east, in the Kaszubskie Lake District.

During forty years, a vegetation season with a total precipitation below the average occurred 18 times altogether, including 10 times when precipitation did not exceed 90% of the norm, and 6 times – when it did not exceed 75% of the norm (Fig. 2). Vegetation seasons with total precipitation deficiency for two subsequent years occurred twice – in 1975-1976 and 1982-1983, hence a probability that a year with the dry vegetation season repeats itself in the next year amounted to 5%. The lowest total precipitation altogether for the research area occurred in 1982 (207 mm), and next in 1992 (230 mm), although the absolute minimum values at particular IMGW stations were recorded in different years: 1969 – in Gorzów Wielkopolski, 1975 – in Koszalin and Lębork, 1989 – in Chojnice, Gdańsk and Swinoujscie, 2003 – in Szczecinek. In 1992 the absolute minimum of total precipitation during the vegetation season in forty years was recorded in Piła – that is 122 mm (35% of the norm). The years 1982 and 1992 belonged to the driest in the second half of the 20th century, also in other regions of Poland [16]. The highest total precipitation occurred in 1980 (484 mm), and in the same year the absolute maximum was recorded – 652 mm (186% of the norm) in Bydgoszcz, and at the other stations also in the following years: 1972, 1987, 1991, 1993, 1995, 1996 and 1998. The lowest precipitation during the vegetation season was usually recorded in April (approximately 38 mm, from 32 mm in Gdańsk to 47 mm in Resko, which made 11% of total precipitation in the vegetation season), and then in May (on average 52 mm, from 46 mm in Ustka to 58 mm in Koszalin and Lębork, which made 15% of total precipitation). The highest total precipitation at most stations occurred in July, on average 74 mm, from 57 mm in Swinoujscie to 89 mm in Koszalin, which was 21% of total precipitation during the vegetation season; only in Ustka the largest total precipitation fell in September. The advantage of total precipitation in July over the other months in Pomerania was also found by Czarnecka et al. [6]. April, June and September were the months of the most variable precipitation, the coefficient of variation amounting to about 60%. The precipitation in May was characterized by the least variability – the coefficient of variation was 45%. The absolute minimum of monthly total precipitation – 0 mm, was recorded in July (in Swinoujscie), the minimum monthly total precipitation was 1 mm in April and in June, and 3 mm in September. The maximum monthly total precipitation was recorded in Bydgoszcz in June and it was 292 mm.

Over 1965-2004 in the period from April to September from 54 to 90 months altogether occurred assessed as dry, very dry or extremely dry according to the RPI index (Table 1). Months with various drought severities were observed the least frequently, that is below 60, in the north of Pomerania, and the number of drought cases increased to over 80 in the west, south and east of the area (Fig. 3). About a half of all the months with drought were periods with less drought intensity, and from 5 to 17% of cases were extremely dry months. The number of months assessed as dry increased from below 30 in the north-west to over 39 in the south and south-east.

A number of very dry months increased from below 22 in the north to more than 31 towards the west, south and east ends of the area examined, whereas the largest number of extremely dry months that is over 9 occurred along the coast and in the south-west (Fig. 4). At the majority of stations situated in central and eastern parts of the research area the month of most frequent (more than once every 2 years) occurrence of drought with different severity was April, and at the stations located in the west – July (once every 2-2.5 years). The most frequent occurrence of drought in July in the vicinity of Gorzów and Szczecin confirmed the previous study [9,10]. Drought of different severity occurred the least frequently in August (once every 4-7 years) in almost all Pomerania, only along the Baltic coast – in September (once every 7-10 years). A month which was most frequently assessed as dry or very dry at the majority of stations in Pomerania was April, whereas the occurrence of extreme drought, which was most frequently recorded at numerous stations in July, but also in June or April was considerably more variable (Table 1). In the years 1951-1990 atmospheric drought in Poland occurred most frequently in March and during the period June – September [7].

A development of atmospheric drought in two chosen years is presented in Fig. 5. In 1992 and 2003 a drought of different degree of severity occurred in all months of the vegetation season. In April 1992 precipitation deficiency was recorded in the south, east and north-east of Pomerania, and locally – in the vicinity of Piła and Gdańsk – precipitation did not exceed 50% of the norm (very dry), while in 2003 April was very dry in over half of the area of Pomerania and only the areas situated in the north remained drought-free. In May 1992 the drought range increased (only in the north-west precipitation remained at the standard level), while in 2003 – it decreased to the south and south-west fragments of Pomerania. June was the month with the most drought severity in 1992 – all Pomerania was covered with drought, its severity increased as well – an extreme drought occurred in the south-west and along the east part of the Baltic coast. June 2003 brings a further increase in drought range and drought occurrence moving to the north of Pomerania. In two subsequent months in 1992 the range and severity of drought decreased, and then in September there was dry again in the south and west of the research area, and even very dry in the vicinity of Gorzów. In 2003 drought almost subsided in July, and then it occurred again in a substantial part of Pomerania in two subsequent months – in August it was dry and very dry in over three-fourths of the area, except the Gorzowska Basin and the area along the Coast, and in September – in the east and center of Pomerania. Extreme drought did not occur in any months of the vegetation period in 2003.

According to the K index, from below 60 (on the middle coast) to over 100 (in the south-west) dry, very dry or extremely dry months were established altogether in the period from April to September (Fig. 6). About 45% cases (from 33% in Piła to 52% in Koszalin) of months with different drought severity were the months assessed as dry, of which the least, below 32, occurred in the north and south of the research area, and the most, over 50 – in the south-west (Fig. 7). The number of very dry months increased from below 22 in the north to over 31 in the south of the research area. Extremely dry months made from 14 to 24% of all cases of drought and they occurred the least frequently, in less than 13 cases, in the center of Pomerania and on the central coast, and the most frequently, in more than 22 cases, in the south-west. At the majority of stations August was the month with the most frequent occurrence of drought with different intensity (approximately from once every 1.5 years in the south-west to once every 3 years in the north). Drought occurred less frequently in April (once every 3 years in the south and south-west, and once every 4-6 years – in the other area). The month assessed as dry and very dry according to K in most Pomerania was mainly August, while extremely dry months were August and July (Table 2).

Development of drought determined with the hydrothermal index over 1992 and 2003 is presented in Fig. 8. In the years 1992 and 2003 drought occurred in all the months of the vegetation season, just as that determined on the grounds of RPI. In April 1992 drought of weak intensity occurred along the valleys of the Warta, Noteć and Vistula rivers, and so it was in 2003, but its range and severity was remarkably higher. In May 1992 the drought range shifted considerably to the north, and a severe drought developed along the Warta and Noteć valleys, while in 2003 there was a shift of drought occurrence from the Vistula valley towards the central heights of Pomerania. In 1992 drought (including extreme drought) reached its highest severity in June, when it extended over all Pomerania – the north, west and south. Also in June 2003 drought spread over almost all Pomerania, its severity, however, was not so high. In July drought began to subside, in 1992 mainly around the Gdańska Bay, and in 2003 – nearly in all Pomerania. August was characterized by a different course of drought. In the years in question – in 1992 drought kept subsiding, especially in the west and north; whereas in August 2003 there was another development of drought, which spread through nearly all the area of Pomerania. In September in both years the range of drought decreased; in 1992 drought of a weak intensity occurred mainly in the central parts of Pomerania, and in 2003 drought subsided in the north-west. In April, May, August and September 2003 a part of the area was covered with extreme drought. By contrast, in 1992 a drought of extreme severity was recorded only in June and July. In June 1992 drought covered all Pomerania, in 2003 there was no such a month, and the largest range of drought occurred in August.

During the forty studied years a decrease in the value of the K index was observed; at all stations in Pomerania – in April, at most stations – in May (Table 3). In April a statistically significant negative trend of the hydrothermal index occurred at the stations situated in the north (Swinoujscie, Koszalin, Ustka) and south (Piła, Bydgoszcz) of Pomerania, while in May a statistically significant trend occurred only at three stations situated in the south (Gorzów, Piła, Bydgoszcz). No significant precipitation trend in any month of the period examined was recorded, just as in the study of time variability of total precipitation in Pomerania in the years 1951-2000 [5,6] and in the Szczecińska Lowland in the years 1961-2000 [13]. In the study by Briff et al. [4] a statistically insignificant increase in humidity resources in Europe and south-west Asia in the years 1882-1991 was reported. For a comparable multi-year period, a downward rainfall trend, particularly in winter, in the south of Europe was described by Tran et al. [18]. Also the research by Balling [1] for the United States showed a slight upward trend of drought.

CONCLUSIONS

1. Occurrence of drought in Pomerania is a phenomenon which is considerably space-variable. This applies to all Pomerania, yet the area located in the south-west may be considered as the most threatened with drought, and the area situated in the north of Pomerania as the least threatened.

2. A larger number of drought months was determined with the hydrothermal index than using the RPI index. A number of months assessed as extremely dry was also larger.

3. In the central and eastern part of Pomerania the month of most frequent occurrence of drought of different severity according to RPI was April, and in the western part it was July. In nearly all Pomerania drought occurred the least frequently in August, only along the Baltic coast – in September.

4. An estimation of hydrothermal conditions indicates the most frequent drought occurrence in August, and the least frequent – in April.

5. In the years 1965-2004 a statistically significant tendency of the RPI index variability was not recorded. The values of K hydrothermal index showed a significant downward tendency in subsequent years in April and May, in April – in the south and north of Pomerania, and in May – only in the south.

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Accepted for print: 3.11.2006

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