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CLIMATOLOGICAL MONITORING OF TERRITORY AFFECTED BY CONSTRUCTION OF THE DANUBE HYDROELECTRIC POWER PROJECT AND EVALUATION OF INITIAL IMPACT

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Milan LAPIN

Slovak Hydrometeorological Institute, Jeseniova 17, 83315 Bratislava, SLOVAKIA

CONCLUSIONS Climatological study revealed significance of regional time trends of critical climatic elements in the period 1901-1994. From the point of view of climate development, all changes of climatic elements acted negatively and unidirectional in the sense of increasing requirements for soil moisture, decreasing water content in the upper layers of soil, decreasing ground water levels and discharges in rivers. Climatology has to disposition results of long series of measurements, while another scientific disciplines are lacking such precise data. That is why some changes of regional character might be inappropriately ascribed to local anthropogenic interventions of natural environment.

From the results presented in the study it follows that regional trends of air temperature and potential evapotranspiration are unambiguously increasing. Within 90 years (1901-1990), air temperature rose by about 0.8 °C, while potential evapotranspiration increased by about 14%. In warm half years (IV-IX), air temperature increased by 0.5 °C and potential evapotranspiration by 11%. Trend of precipitation totals, sunshine duration and relative air humidity was decreasing, precipitation dropped by 15% (in warm half years by 20%), sunshine duration was shorter by 2% (in warm half years by 3%) and relative humidity was lower by 5% (the same drop was recorded in warm half years). These trend characteristics suggest that in the course of the last 90 years, regional climate was subjected to changes that have significantly affected several factors of environment. Rise in air temperature and at the same time drop in precipitation and relative humidity correspond well to increase of potential evapotranspiration. Due to these facts, requirements for soil miosture increased. As a consequence, soil humidity, level of ground water and river discharges decreased in a larger area. The last decade was substantially poorer for precipitation, especially in the summer half years.

Entire 1991-1993 period was above normal as to the temperature, especially in summer. There was no month during these three years below the normal temperature. From the analysis of relative air humidity U it follows that in each year of the period 1991-1993 U value was lower than long-term means of the period 1951-1980, as well as those of dry period 1981-1990. Potential evaporation had reached 791 mm in decade of 1961-1970, 784 mm in 1971-1980 and 821 mm in 1981-1990, with no significant differences found within the area.

Observed devastation of flora in this territory is partially associated with air pollution emitted from the sources in Bratislava. In the rest of territory, level of air pollution corresponded to regional middle European background. Hydroelectric Power Structures will not influence directly level of air pollution, however, production of net energy associated with savings of fossil fuels will contribute to decrease of Slovak emissions of SO2, NOx, and ash by 5-7%.

#### INTRODUCTION

Project of climatological monitoring of the territory affected by construction of Hydroelectric Power Structures (HPS) was reviewed and accepted in June 1991. The project has been aimed at:

- collection and analysis of meteorological data and data on chemistry of atmosphere;
- elaboration of the first part of complex climatological study directed to analysis of pre-building conditions in the period 1951-1980 and trends of climatological characteristics in the period 1901-1990;
- calculation of selected characteristics of evapotranspiration based on meteorological and hydrological data collected in the period 1961-1990.

Final goal of this climatological study has been to provide basic data for the analysis of environmental consequences of HPS construction that would help in objective differentiation between natural changes of environment and changes caused by anthropogenic activity in the monitored area.

#### PREPARATION OF BASIC DATA

In order to perform monitoring of the territory touched by HPS construction, collections of daily data registered in the period 1961-1990 in 4 meteorological stations and 2 precipitation-recording stations as well as monthly data of the 1951-1990 period from additional 4 meteorological and 6 precipitation-recording stations were completed and checked in the Slovak Hydrometeorological Institute.

To analyse changes in the chemistry of atmosphere in the area of interest, data obtained in the period 1985-1990 from background station EMEP in Topo3/4niky and from air pollution monitoring stations in Bratislava and near surroundings were used.

#### REVIEW OF METHODS USED IN THE STUDY

After initial processing, primary data were stored in archives and databases of Slovak Hydrometeorological Institute (SHMU(. Climatological characteristics were evaluated according to international methodical instructions of World Meteorological Organization (WMO) and in agreement with methodical instructions valid in Slovak Republic. Climatological trend analysis was applied to series of data from meteorological measurements that fulfil requirements of time homogeneity. Linear and quadratic trends and running means were used to obtain characteristic time curves of individual elements. Evapotranspiration fluxes were calculated on the basis of algorithms postulated in DAIR, DINUND and GLOBAL models accepted in the Slovak Republic for active surfaces of lawn, floodplain forest and fields covering the monitored area. Data for analysis of changes in chemistry of atmosphere were routinely processed in SHMU and obtained results were specially interpreted following selection of those characteristics that significantly influence pedosphere and biosphere of the area. Methods of measurements, chemical analyses and processing of data are in accord with the manual of WMO and with methodical instructions of SHMU.

#### EVALUATION OF CLIMATIC CONDITIONS AND THEIR DEVELOPMENT

In the frame of climatological study entitled "Climatological monitoring of the territory affected by construction of HPS", three groups of problems were subjected to analysis, each of them having certain impact on characterisation of conditions before (until 1980) and in the course of HPS construction (1981-90).

#### Time trends of important climatic characteristics

The first part of study under the title "Regional trend of selected climatic elements" is directed to analysis of time trends of changes in air temperature, precipitation, sunshine, relative humidity and potential evapotranspiration in the period from 1901 to 1990. For identification of trend characteristics, localities unaffected by alterations of local conditions (e.g. Hurbanovo) were selected to illustrate regional climatic trend of general validity. From the results presented in the study it follows that regional trends of air temperature and potential evapotranspiration are unambiguously increasing. Within 90 years, air temperature rose by about 0.8 °C, while potential evapotranspiration increased by about 14%. In warm half years (IV-IX), air temperature increased by 0.5 °C and potential evapotranspiration by 11%. Trend of precipitation totals, sunshine duration and relative air humidity was decreasing, precipitates dropped by 15% (in warm half years by 20%), sunshine duration was shorter by 2% (in warm half years by 3%) and relative humidity was lower by 5% (the same drop was recorded in warm half years). These trend characteristics suggest that in the course of the last 90 years, regional climate was subjected to changes that have significantly affected several factors of environment. Rise in air temperature and at the same time drop in precipitation and relative humidity correspond well to increase of potential evapotranspiration. Due to these facts, requirements for soil moistening increased. As a consequence, soil moisture, level of ground water and river discharges decreased in a larger area. Because of a higher variability in sunshine duration, changes of this parameter could not substantially affect natural environment. Trend of sunshine duration was partially influenced by increased dimming of the lower troposphere due to air pollution.

From the analysis of individual characteristics, as well as from the time course of their annual values, it is evident that the conditions of critical period from 1971 to 1990 (e.g. just before and immediately after the beginning of water engineering works construction) roughly correspond to their overall trends and variability during the period after the year 1940. Thus, it could be concluded that the analysed trend characteristics are valid both for period of 1951-1980 as well as for that of 1981-1990.

Initial trend analyses of main climatic parameters have shown that the changes of air temperature, relative air humidity, precipitation, eloudiness, potential and actual areal evaporation, are of the highest significance with respect to identification of possible consequences of HPS construction and operation. Figs. 1-3 illustrate linear trends of important climatic characteristics recorded at meteorological station in Hurbanovo during the years 1901-1994. Values of the period 1871-1894 serve as an information supplementing the results of above mentioned studies [1,2].

Meteorological observatory at Hurbanovo is localised beyond reach of HPS climatic influence and also additional factors in surroundings of Hurbanovo play no important role. From this reason, Hurbanovo was chosen as a monitoring place for identification of regional climatic changes. It is worth to mention, that meteorological observations made at Hurbanovo belong to the most precise and reliable not only in the frame of Slovakia, but also in whole Central Europe.

Fig. 1 shows a typical curve of annual means of air temperature and precipitation totals during the warm half years (IV-IX) of the period from 1871 to 1994. Despite a high variability, time course of air temperature T (interpreted as a smoothed curve of mean annual values) has unambiguously increasing tendency, rising by about 1.3 °C within 124 years, mean value of 1991-1994 period being above the total trend. Especially the years 1992 and 1994 belong to the warmest years in the history of instrumental meteorological observations. On the other hand, trend of precipitation totals R in the warm half years is decreasing by about 48 mm, with even higher variability. Fluctuation of R parameter is expressed as a smoothed curve of mean annual values. Noteworthy, precipitation values in the warm half years substantially determine water balance of lowlands with vegetation not supplied by ground water or moistening.

Fig. 2 presents linear trend of relative air humidity U in the spring months (II-V) during the period 1901-1994. Graph reveals drop in U value of about 9% within 94 years as well as cumulating of low mean U values during the last 14 years. Relative air humidity is to some extent determinant of potential evaporation and the higher is air temperature, the more negative are effects of low U values. Summer and autumn U values have similar, although less decreasing tendency of about 5% within 94 years than the spring ones. Low U values at Hurbanovo indicate regional drop in U caused by decrease of precipitation totals and increase of air temperature in the larger area [3]. Additional potential effects of changes in atmosphere circulation in the middle Europe as well as influence of regional changes of water balance (especially decrease of water content in zone of aeration and changes in level of ground water) were not subjected to satisfactory analyses.

Fig. 3 shows trends of main elements of water balance, e.g. potential evaporation Eo, actual evaporation E and precipitation R at Hurbanovo during the period 1901-94. R is expressed both in the form of smoothed curve and individual points of annual totals. These data represent changes and fluctuation of precipitation of regional character. The most important fact related to water balance in the years 1991-94 appears to be high potential evaporation associated with low precipitation totals, which could be also considered for regional. Decreasing difference between precipitation and actual evaporation E is direct consequence of this fact. Taking into account total water balance in Danube lowland, during the years 1990-1994 were reached extreme values of this century.

Evaluation of the trends of important climatological parameters of this century revealed their extraordinary tendency culminating in the period 1990-94 in extreme values. Comparison with other lines of observation in the Central Europe supported the view, that these trends represent regional changes of climatic conditions which are certainly unrelated to HPS construction. It is likely that also another components of environment in the surroundings of HPS which are affected by climatic parameters have similar tendency. Due to the lack of long term observations of these additional components of environment, this unfavourable tendency in the years 1990-1994 could be inappropriately associated with construction and operation of HPS. By the end of the year 1992, HPS has been partially put into operation by filling Hrusov dam. However, this could not influence climatic characteristics in Hurbanovo. High inter-annual fluctuation of climatic parameters made impossible to express the contribution of Hrusov dam filling to trends of climatic characteristics even in immediate surroundings of HPS. Such analysis could be accomplished only after several years (at least 10) of climatological observation.

#### Comparison of climatic characteristics with normal values

The second part of study entitled "Climatic analysis of the periods 1951-1980 and 1981-1990" has been aimed at evaluation of climatic conditions by comparison of means and deviation characteristics of individual elements in the course of certain time periods. Mean values of precipitation totals recorded at 10 meteorological stations in the vicinity of HPS in the periods 1951 -1980 and 1981-1990 indicate that the last decade was substantially poorer for precipitation, especially in the warm half years. From additional data it follows that annual precipitation totals were in the range of 333 to 874 mm, once a four years raising to more than 600-650 mm and dropping to less than 400-500 mm. During each winter season, snow cover was present at least during one day and mean annual maximum of snow cover reached depth from 17 to 21 cm. By comparison to period 1931 -1960 the period 1951-1980 appears to be generally poorer for precipitation (in Bratislava by 4%, in Hurbanovo by 5%, in Samorin by 3%), in the warm half years being the same or slightly higher in 1951-1980 period than in 1931-1960. Annual means of air temperature were almost the same in the period 1951-1980 compared to 1931-1961, however, in summer mean value was lower by 0.2-0.3 OC and in winter it was higher by 0.2-0.4 OC. It is worth to mention that at Gabcikovo observatory at the border of floodplain forest air temperature was in the warm half years by 0.2-0.3 OC lower than that in regions outside the forest. This unambiguously lower air temperature is due to evapotranspiration with high energy consumption from sun radiation balance. Global warming in the last decade was uniformly manifested over the larger area and was not affected by HPS construction. According to the additional sources, temperature of the period 1981-1990 was more variable compared to 1961-1980 taking into account all temperature characteristics. Also in this case no influence of HPS was noted.

Characteristics of sunshine duration and cloudiness are more than other elements dependent on general circulation of atmosphere and position of frontal zone in global or at least regional extent. That is why variability and changes of these elements do not correlate so tightly with air temperature and fluctuation of precipitation. Periods of 1951-1980 and 1981-1990 were characterised by a larger cloudiness and shorter sunshine duration than the period 1931-1960 by about 10%, however drop in sunshine duration was stopped during 1981-1990 and begun to rise moderately. On the other hand, it is worth to stress that sun radiation balance serves as a source of energy not only for warming of lower layer of atmosphere by Earth surface, but also for evapotranspiration.

Relative air humidity results from interaction among rate of circulation's, conditions of evapotranspiration, precipitation, temperature and wind. Its trend and conditions represent well also regional climatic conditions. From the evaluation of this parameter over different time periods it is apparent that in localities which are well supplied by ground water (e.g. Gabcikovo and Ziharec) mean U values are not changed in the course of long time periods, while in localities fully dependent on precipitation (e.g. Hurbanovo and Bratislava) U value dropped during 1981-1990 by 2-3%. This fact is well reflected by differences of U values among localities, U at Gabcikovo being 3-6% higher than at Hurbanovo and 2-5% higher than in Bratislava. The difference is more striking within individual months, especially in vegetation season being about 3-8% higher. Also in this parameter analysis, no influence of HPS on meso(climatic changes of relative air moisture was found.

#### Remarks to the development in the years 1991-1993

With respect to the precipitation, period 1991-1993 was below the normal levels, especially during the spring, winter and VIIIth month. No significant differences were found to be related to long term means between localities close to HPS (e.g. Hrusov) and those that are more distant. It is worth to mention that regarding higher than normal air temperatures, this precipitation deficit was more pronounced due to higher humidity requirements of vegetation.

Entire 1991-1993 period was above normal as to the temperature, especially in summer. There was no month during these three years below the normal temperature. Evaluation of additional temperature characteristics (occurrence of days and periods with certain temperature values, fluctuation of air temperature etc.) is of minimum significance within this short period. Regional evaluation of air temperature revealed no effect of HPS on the changes of air temperature.

From the analysis of relative air humidity U it follows that in each year of period 1991-1993 U value was lower than long-term means of period 1951-1980, as well as those of dry period 1981-1990. As a rule, U used to be below the normal value at above-normal air temperature or at low precipitation. These suppositions were fulfilled quite often at climatic evaluation of individual months during 1991-1993 period. Any of the months of entire period 1991-1993 can not be considered for above-normal from the point of view of relative air humidity. In comparison with long-term means, U value was substantially lower in following months: VI.,VIII.,IX. of 1991; III.,IV.,V.,VII.,VIII.,IX. of 1992; IV.,V.,VI.,VII.,VIII. 1993, it means in the majority of months of vegetation season. This fact was doubtlessly the cause of increased requirements for soil moisture, since it is known that drop in U is accompanied by decrease of soil moisture and rise of potential evapotranspiration.

Numbers of days with snow cover (SC) and totals of daily snow depths (DSD) serve as data contributing to illustration of great variability of snow conditions in the analysed region. According to SC and DSH values, the years 1991 and 1993 are within the range of normal years and the year 1992 is below. Great inter-annual and inter-monthly differences in SC and DSD values are typical for this area. In this regard, months I. 1991 and XI. 1993 can be considered for extraordinary, because of zero and normal January snow cover, respectively.

In duration of sunshine (SS), no significant fluctuation was registered in the period 1991-1993. In evaluation of SS values in individual months, it is worth to note relatively high total SS values in months II. and XII. 1991; I.-IX. 1992 (especially VIII. 1992) and I.-VIII. 1993 (especially V. 1993). VIIIth month of 1992 and Vth month of 1993 were most sunny within the last four decades. With respect to this climatic parameter, any influence of neither HPS construction, nor Hrusov dam filling on SS values was detected.

#### Brief evaluation of evapotranspiration conditions

Evapotranspiration at four distinct localities with different active surfaces represents the last parameter that entered into consideration of the periods 1961-1980 and 1981-1990. Calculations were performed in Water Research Institute (VUVH( and in Institute of Hydrology and Hydraulics (UHH( using models DAIR, DINUND and GLOBAL. Potential evaporation had reached 791 mm in decade of 1961-1970, 784 mm in 1971-1980 and 821 mm in 1981-1990, with no significant differences found within the area. Differences in actual evaporation are considerable due to its dependence on precipitation, level of ground water and vegetation cover. Absolute values and time courses of actual evaporation were quite similar in localities Samorin and Dunajska Streda. However, in Gabcikovo having significant capillary supply from ground water, values of actual evapotranspiration were higher, with substantially different time courses during individual years (especially from April to October). Absolute values of evapotranspiration associated with areas covered with maize and barley were affected by vegetative phase of plant products, however, differences among localities were similar to those related to grass. According to results obtained by DINUND method, evapotranspiration of forest surfaces in Gabcikovo was by 16.5% higher than potential evaporation of grass surface. This was caused by draw of energy for evaporation from floodplain forest surroundings accompanied by drop of temperature in immediate vicinity of floodplain forest. Calculations using DINUND model revealed that the period 1981-1990 was characterised by significant decrease in capillary supply of soil layer of 0-0.8 m associated with decrease of soil humidity in this layer. Most convincingly, it is a consequence of regional drop in level of ground water and soil Moisture in major part of Slovak territory. This fact appears to be associated with natural regional changes of climate and possible influences of global warming. Drop of ground water level in Danube surroundings due to decrease of Danube water level and erosion of river bed represent in this respect additional potential factors.

#### EVALUATION OF INITIAL CONDITIONS OF ATMOSPHERIC CHEMISTRY

With exception of Bratislava, there are no large industrial sources of air pollution's in the territory close to HPS construction. According to the level of air pollution, this area can be divided into three zones, e.g. administrative territory of capital city, upper part of Zitny ostrov island about 10 km south-east from Slovnaft refinery and the rest of territory. Total values of selected characteristics of air pollution and chemical composition of precipitation in the period 1985-1990 are summarised in study accomplished in 1992. During the following period, decrease of air pollution by compounds containing sulphur and heavy metals was registered. Concentration of nitrogen compounds and dust did not change, while the acidity of precipitation was higher all over entire period (with mean pH<5). There were some cases of overstepping emission limits in the city intravilan. In the area of upper Zitny ostrov island, several ecological limits were overstepped during monitored period (e.g. critical level of SO2=10 (g.m-3, wet fall of sulphur compounds = 7 kg S.ha-1.year-1). Observed devastation of flora in this territory is partially associated with air pollution emitted from the sources in Bratislava. In the rest of territory, level of air pollution corresponded to regional middle European background. In accordance with middle European trend, decreased concentrations of compounds containing sulphur, nitrogen and heavy metals were recorded (with exception of Pb and Cd in the year 1990). Since 1988, mean concentration of SO2 and wet deposits of sulphides have not exceeded recommended ecological limit. In dry and windy Danube lowland, acidity of precipitation is substantially decreased due to mineral soil dust. Mean pH value of precipitation during the entire monitored period was close to the values of 5.5-5.6, being by 1-2 units higher than real pH of precipitation unaffected by secondary dust. Air pollution in the area, especially in the upper part of Zitny ostrov island, contributes to ecological stress. HPS will not influence directly level of air pollution, however, production of net energy associated with savings of fossil fuels will contribute to decrease of Slovak emissions of SO2, NOx, and ash by 5-7%.

#### SUMMARY

Climatological study revealed significance of regional time trends of critical climatic elements in the period 1901-1994. From the point of view of climate development, all changes of climatic elements acted negatively and unidirectional in the sense of increasing requirements for soil moisture, decreasing water content in the upper layers of soil, decreasing ground water levels and discharges in rivers. Climatology has to disposition results of long series of measurements, while another scientific disciplines are lacking such precise data. That is why some changes of regional character might be inappropriately ascribed to local anthropogenic interventions of natural environment. To differentiate natural regional climatic changes from local influences of anthropogenic activity, additional modelling calculations are needed, especially in the field of evapotranspiration flows and energetic balance of active surfaces. Doubtless, accumulation of local and regional influences of the same direction might induce mesoclimatic alterations that could change natural environment, mainly hydrosphere and biosphere.

As follows from above described brief evaluation of climatic conditions, the period 1991-1994 and also additional years of the period 1981-1990 have significantly abnormal values of almost all climatic elements, mainly of precipitation, air temperature and relative air humidity. Deviations from normal climatic conditions are of regional character and are related to territory larger than whole Danube lowland in Slovakia, or larger that Pannonian lowland (with regard to air temperature). The accomplished study unambiguously confirmed that up till now there it is no direct association between deviation of climatic conditions and construction of Hydroelectric Power Structures (HPS( and putting of Gabcikovo(Hrusov part of the project into operation. Preliminary evaluation suggests favourable effect on local climatic conditions.

Completion of climatologic study accomplished in 1992 by evaluation of the years 1991-1994 confirmed no detectable influence of neither HPS construction, nor Hrusov reservoir on negative changes of climatic conditions of the area near the structures. On the other hand, it became evident that the significant deviation of almost all climatic parameters continuing in the years 1991-1994 has broader regional character. Inappropriate interpretation of this significant deviation of climatic conditions, that persisted since 70th, might be the cause of false association of dry and warm weather in the particular area with the period of construction of HPS.

#### REFERENCES:

  [1] Lapin, M., Mitosinkova, M., Rak, J., Petrovic, P.,1992: Climatological monitoring of area affected by construction of Danube water engineering scheme. Final report of climatologic study performed in 1991-April 1992. Slovak Hydrometeorological Institute, Ministry of Environment of Slovak Republic, pp. 60 (in Slovak)

  [2] Lapin, M. et al., 1994: Climatological monitoring of area affected by construction of Danube water engineering scheme. Final report of climatologic study performed in 1992-April 1994. Slovak Hydrometeorological Institute, Ministry of Environment of Slovak Republic, pp. 26 (in Slovak)

  [3] Lapin, M., Fasko, P., Zeman V., 1994: Contribution to analysis of possible consequences of global warming of atmosphere by 1-2 °C on changes of climatic conditions in Slovakia. NKP SR, Vol. 2, Slovak Hydrometeorological Institute, Ministry of Environment of Slovak Republic, 1994, p. 35-77 (in Slovak)