FRIEND '97 — Regional Hydrology: Concepts and Models for Sustainable Water Resource Management <-n (Proceedings of the Postojna, Slovenia, Conference, September-October 1997). IAHS Publ. no. 246, 1997. J?

The variability of hydrological series due to extreme climate conditions and the possible change of the hydrological characteristics with respect to potential climate change

OLGA MAJERCAKOVA Slovak Hydrometeorological Institute, Jeséniova 17, 833 15 Bratislava, Slovak Republic MIRIAM FENDEKOVÂ Department of Hydrogeology, PRIF UK, Mlynska Dolina, 842 15 Bratislava, Slovak Republic DANICA LESKOVÂ Slovak Hydrometeorological Institute, Jeséniova 17, 833 15 Bratislava, Slovak Republic

Abstract In recent years Slovak hydrology has been more oriented towards evaluation of water resources, their variability and possible changes and also towards evaluation of the territory from the point of view of water resources. The results documented significant decrease in the runoff from Slovak streams after 1980. The decrease is most significant in November, negligible decrease was identified in May and in the spring months. The absolute minimum annual discharge for the whole observed period occurred in 1993 in 19% of all water gauging stations considered. The starting time of decreasing spring yields in was in 1987-1988. The decrease in yield will reach up to 60% by 2010.

NATURAL CONDITIONS

The Slovak Republic (SR) is an inland country in central Europe, with an area of 49 036 km2. Slovakia lies on the roof of Europe and occupies territory between the River and the Tatra Mountains. One of the main European watershed divides passes through northern Slovakia. From an orographic point of view, the territory of SR is very heterogeneous, the altitude varying between 94 m a.s.l. (southeastern Slovakia) and 2655 m a.s.l. (Gerlach mountain in the High Tatras). Slovakia is a mostly mountainous country, 60% of the territory being higher than 300 m a.s.l. The country lies in the mild climate zone, where the effects of the ocean and continent meet. The territory is characterized by a regular rotation of four seasons and variable weather throughout the whole year. The mean annual temperature ranges from 10.4°C (south of the Danube Lowland) to -3.7°C (Lomnicky sift in the High Tatras). The mean annual precipitation is about 750 mm. The runoff of surface and subsurface water is relatively quick; the surface and groundwater resources are filled largely by the precipitation. The next source of water is the River Danube, which flows through southwestern Slovakia and saturates the groundwater in near surroundings. The density of the river network varies from 0 to 2000 m km"2 while on average it is 920 m km"2. Approximately 2% of the territory is covered by water. For water balance and optimal management of rivers, the territory of Slovakia is divided into 10 main basins (Fig. 1). 60 Olga Majercâkovâ et al.

MORA VA VI. SLANA II. DUNAJ VII. BODVA I. VÂH VIII. HORNÂD llla. NITRA IX. BODROG IV. HRON X. POPRAD V. IPEL — water gauging stations * springs —r watershed divides Fig. 1 Slovakia: main river basins. Water gauging stations and springs selected for NCP and FRIEND.

The natural conditions determine the relatively dense network of hydrological and climatological observations. The mean daily discharges are measured at about 450 gauging stations. The spring yields and the groundwater levels are measured once a week at some 700 and 1200 points, respectively.

DESCRIPTION OF THE PROBLEM

The long-term water balance of Slovak territory has been derived for the period 1931-1980: 753 mm (Precipitation) = 261 mm (Runoff) + 492 mm (Evaporation) At the Slovak hydrological service, 1931-1980 is considered as a reference period and is the basis for the main long-term hydrological characterization. After 1980 came the series of climatically unfavourable years from the point of view of water resources filling. This unfavourable situation peaked in 1993 and ended in 1994 (year 1994 was the warmest one in the history of measurements). The period 1981-1993 can be characterized by a deficit of precipitation and runoff. The Variability of hydrological series due to extreme climate conditions §\

Table 1 Runoff in 1993 for the main river basins in Slovakia (in % of long-term mean annual runoff).

No. Basin i?1993 in % of Rmg I __ II Dunaj 90% main stream, 50% tributaries III Vâh 55-70% Ilia Nitra 40-80% IV Hron 20-40% V Ipel 10-45% VI Slanâ 20-50% VII Bodva <30% VIII Hornâd 40-80% IX Bodrog 60-90% X Poprad 55-70% Note: Basins 1-X (96% of territory) belong to the ; basin X (4% of territory) belongs to the . differences compared with the long-term averages were -6% for precipitation and -20% for runoff. These values varied during the period 1981-1993 as well as over the territory. For instance, in 1993 in 34% of the territory (southern and southeastern Slovakia) runoff was less than 50% of the long-term average (Table 1). Most Slovak river basins regularly have the low flow season at the end of the summer and during the autumn. During the last 15 relatively dry years this fact was particularly remarkable. It is out of the hydrologists' domain to judge the causes of the climate variability. However, the advent of potential climate change shifts hydrology from evaluation of the past and short term future to prognoses of possible development of the hydrological cycle and water balance. These prognoses are based on the recommended climate scenarios. The hydrological situations of the last 15 years or so create the need for re- evaluation of long-term hydrological characteristics. Among possible tasks for this forthcoming need are: - the analysis of variability of hydrological series and the judgement of the series stationarity; - the analysis of the change of low flow characteristics; - the possible impact of potential climate change on the hydrological characteristics which quantify the water resources. The solution of these presented problems was possible due to the support of national and international projects, mainly: the FRIEND project, the National Climate Programme and the Country Study Programme.

DATA The basic data which were used in the solution of the above-mentioned tasks were: - Sixty-four of the discharge series—daily, mean monthly and mean annual discharge series observed at least from 1931. Thirty-six of these series were included in the National Climate Programme (NCP), 20 in the FRIEND project and 45 were used to evaluate the low flow characteristics. 62 Olga Majercâkovâ et al.

- Sixty-four series of spring yields (mean monthly yield series), the most important of them included in Fig. 1. - In parallel the series of precipitation, air temperatures and meteorological data were evaluated (by the climatological service of SHMI). - Climate scenarios: - the results according to GCMs (scenarios CCCM, GISS and GFD3), - the results of NCP experts (scenarios SD and WP) (Lapin et al., 1995). The selection of hydrological series was determined by the following criteria: - the measurement of discharges at least from 1931; the observation of spring yields over as long a period as possible (starting in the 1970s); - the series representing the watersheds of various sizes, various physio- geographical and hydrogeological regions (from all 10 main basins); - the perspective series (with high probability of further observation); - the series are relatively reliable (the reliability of series means that the series are of good quality and are representative). The verification of reliability was based on the history of measurements, on known activities in watersheds as well as on homogeneity testing (Majercâkovâ & Sedïk, 1994a).

MAIN RESULTS

Analysis of discharge series variability and their stationarity

The results of homogeneity testing, of low frequencies filters (moving averages) and trend analysis of series until 1990 identify the remarkable decrease of the runoff in Slovak streams after 1980. The decrease is most significant in November and following autumn-winter months. Negligible decrease was identified in May and some following spring months, respectively. An example is shown in Fig. 2, where the course of mean annual, May and November discharges is supplemented by linear trends on the Litava watershed at the Plâstovce gauging station. The runoff decrease in SR territory is different. The most influenced basins are: Ipel, Slanâ and Bodva (south and southeastern Slovakia), where the runoff after 1980 was decreased by 30-40%. Behind them follow the basins of Hron, Nitra, Poprad and upper Vâh, where the decrease did not exceed 15-20%. The lowest decrease (to 5-10%) was recorded in northwestern, western and northeastern Slovakia. From the statistical analysis of series it is evident that many of them are non- stationary (in the observed interval). For instance, the proof of the non-stationarity can be significant non-homogeneity, which is not evident throughout the entire year but only in some months. In the future this fact will have to change the methods (or the philosophy) of the series evaluation as well as the derivation of long-term characteristics (Majercâkovâ & Sedîk, 1994b).

Analysis of the spring yield series variability

The results of homogeneity testing and trend analysis of spring yields time series up to 1994 identify the same remarkable decrease of mean, minimum and maximum Variability of hydrological series due to extreme climate conditions 63

annual discharges

19B0 1990

May discharges

November discharges

Fig. 2 Linear trend course of mean annual, May and November discharges for the Litava basin in Plâstovce. 64 Olga Mqjercâkovâ et al. monthly spring yields in almost all cases. The main difference is in the commencement of decreasing trends. In some cases it started in 1982 (two years later than in the case of identified runoff decrease), but the most frequent starting point of spring yield decrease were years 1987-1988 (Fendekovâ et al., 1995; Kullman, 1995; Fendekovâ, 1996). The influence of the documented last warm climate period 1988-1994 on Slovak territory was very distinct for the springs located in structures of sedimentary flysch rocks (alternation of sandstones and clays) and in karst limestone areas, but not so strong in the areas of neo-volcanic rocks. The spatial distribution of decreasing yield is very variable. The most remarkable decreasing trends were documented in the southern part of Slovak territory by springs from karst environment and also in northeastern and northern Slovakia by springs from flysch rocks. It is interesting that decreasing trends were documented as being more distinct in the case of mean and maximum yields. Minimum yields are quite stable.

Analysis of low flow characteristics

The climatological situation after 1980 has evoked an interest in re-evaluating the low flow characteristics, (e.g. Majercâkovâ et al, 1995). We can say that the low flow characteristics have responded to this situation with some delay. For example, the absolute minima occurred only since 1984. Two examples illustrate the change of these characteristics: 1. The minimum annual discharges: up to 1986 the minimum annual discharges were only slightly under the long-term average in approximately 40% of territory. From 1987 a rising number of cases occurred when <2minann. < <2minavg. In 1993 the minimum annual discharges were half of their long-term averages at almost 30% of water gauging stations and 70% of stations had the minimum annual discharge less than the average. The mean minimum discharge for the period 1931-1993 was less than the mean for the period 1931-1980 by about 5- 10% at 40% of stations. 2. Occurrence of the absolute minimum discharges: during the period 1981-1993 the absolute minima occurred rarely, examples occurring in 1984 (at 11% of stations), in 1992 (at 17% of stations) and this phenomenon peaked in 1993 (at 19% of all considered gauging stations). According to the presented and other low flow characteristics, the year 1993 can be considered as the second driest year in our country (Leskovâ, 1996).

Possible impact of potential climate change on the characteristics which quantify the water resources

Runoff The statistical methods for the analysis of runoff series variability were used only exceptionally for the prediction. The hydrological scenarios were based mostly on the application of already developed models (WatBal, DAIR etc.) and on application of the author's methods. For the prediction of the mean monthly and annual runoff change due to potential climate change a statistical linear regression Variability of hydrological series due to extreme climate conditions 65

model was used of the type R = f(P.,, P, T, r, RA), where R is the runoff, P = preci­ pitation, T = air temperature, r = relative air humidity; indexes with the negative sign indicate the mean monthly values of elements in previous months (Majercâkovâ & Sedïk, 1995). This model was applied on 12 watersheds in central SR, (Majercâkovâ et al., 1996). The results can be summarized in the following way: - A significant runoff increase over the whole territory during the winter (from December to March). In the northern regions a smaller increase is expected (about 20%) of longer duration (from November to April—due to the altitude). In central and southern regions there is a more intensive increase in winter runoff (to 40%) but of shorter duration (only January and February). - During the spring and summer until September a very significant decrease can occur; with only one exception which results from the GFD3 scenario. In northern regions the decrease in runoff can reach from 20 to 25%, while in southern regions it can be from 30 to 40% and exceptionally 60%. - According to GCM scenarios we can expect a slight increase in the runoff from October into the winter, the opposite situation to that indicated by the NCP scenarios—a slight or moderate decrease. - The intensity of the runoff change will grow as the year 2075 approaches and will be higher in the southern and southeastern parts of Slovakia. - Although the annual sums of the runoff change can be relatively small, the significant seasonal and monthly changes can influence to a very large extent water management and the other sectors dependent on water.

Spring yield The prediction of spring yields was made by statistical methods of trend analysis. The decrease of spring yields varies from 10 to 65% of mean yield for the whole period of observation. The prediction to the year 2000 was made for springs flowing from karstic structures. The values obtained varied from 0 to 63% decrease. According to Kullman (1996) the decrease of spring yield in accordance with parameters of climatic scenarios GISS, GFD3, CCCM and others, and the decrease of mean yearly yield in crystalline range mountains will reach, in the year 2010, values in the range 10-60%. Only in a few cases was an increase of spring yields documented.

CONCLUSIONS

In recent years Slovak hydrology has been increasingly oriented towards evaluation of water resources, their variability and possible changes, and also on evaluation of the territory from the point of view of water resources. The interest in hydrological processes was less than in the 1960s and 1970s. This orientation was also supported by some of the international projects mentioned previously. The relatively dry and warm years were not favourable for the water resources saturation or for water management (furthermore at a time of economic and social transformation). In the history of hydrological measurements such a long dry period has not before been identified. This is the main reason for describing the present situation, presenting some results from recent years and making some predictions about the future. 66 Olga Majercâkovâ et al.

REFERENCES

Fendekovâ, M. (1996) Spring regimes of selected hydrogeological units of Slovakia (in Slovak with English summary). Podzemnâ Voda 2/1996, SAH, 23-31. Fendekovâ, M., Gavurm'k, J., Kullman, E., Jr & Sadlofiovâ, K. (1995) Influence of global climate change on spring yields on Slovak territory (in Slovak). In: AQUA 95 (Trencin), 8-18. Kullman, E. (1995) Decrease in the yield of groundwater resources in Slovakia and an appraisal of its causes (in Slovak with English summary). Podzemnâ Voda 1/1995, SAH, 58-74. Kullman, E. (1996) Interpretation of possible impacts of climatic and hydrologie condition changes on estimation on the disposable groundwater supply changes. Slovak Republic's Country Study, element 2iii. SHMÛ, Bratislava. Lapin, M., Nieplovâ, E. & Fasko, P. (1995) Climate change scenarios for air temperature and precipitation change in Slovakia (in Slovak with extended English summary). In: NKP 3/95 (National Climate Programme SR), 19-57. Leskovâ, D. (1996) Selected statistical and probable characteristics of low flow. Report for FRIEND Project 1996. SHMU, Bratislava. Majercâkovâ, O. & Sedfk, P. (1994a) The selection of hydrological monthly series and the analysis of their variability and reliability (in Slovak). Final Report for NCP SR. SHMÛ, Bratislava. Majercâkovâ, O. & Sedîk, P. (1994b) The runoff change of the Slovak rivers (in Slovak with extended English summary). In: NKP 2/94 (National Climate Programme SR), 107-137. Majercâkovâ, O. & Sedîk, P. (1995) The estimation of the possible runoff change during the year on the basis of the statistical model (in Slovak). Report for NCP SR. SHMU, Bratislava. Majercâkovâ, O., Leskovâ, D. & Sedik, P. (1995) Selected characteristics of low flows of Slovak rivers. /. Hydrol. Hydromech. 43(4-5), 331-353. Majercâkovâ, O., Minârik, B. & Sedik, P. (1996) The possible runoff change during the year on the Slovak streams due to potential climate change. Slovak Republic's Country Study, element 2iii. SHMÛ, Bratislava.