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Groundwater Drought in the Nitra River Basin - Identification and Classification

Groundwater Drought in the Nitra River Basin - Identification and Classification J. Hydrol. Hydromech., 60, 2012, 3, 185­193 DOI: 10.2478/v10098-012-0016-1 MIRIAM FENDEKOVÁ, MARIÁN FENDEK Department of Hydrogeology, Faculty of Natural Sciences, Comenius University in Bratislava, Mlynská dolina, Pav. G, 842 15 Bratislava; Mailto: fendekova@fns.uniba.sk Drought as a natural phenomenon becomes more often the subject of the research. It is because of the increasing frequency of extreme climatic events also in mild climate conditions. Groundwater drought indices could be derived for different groundwater parameters, among them for base flow, groundwater head stage, spring yield, or groundwater recharge. Base flow drought assessment methods were proposed in the paper. Base flow values were separated from the discharge hydrograms using the new HydroOffice 2010 program package, lately developed by Gregor. The base flow drought severity index was applied, calculated as the value of the base flow drought deficit volume divided by the drought duration. After that, the standardized base flow drought severity index was proposed as the ratio of the base flow drought index and the average long-term annual base flow. Proposed methods were applied in the Nitra River basin. Base flow drought occurrence was characterized also from the seasonality point of view. KEY WORDS: Drought, Groundwater Parameters, Cumulative Frequency Curve, Threshold Level Method, Standardized Base Flow Drought Severity Index, Nitra River Basin. Miriam Fendeková, Marián Fendek: SUCHO V PODZEMNEJ VODE V POVODÍ TOKU NITRA ­ IDENTIFIKÁCIA A KLASIFIKÁCIA. J. Hydrol. Hydromech., 60, 2012, 3; 28 lit., 4 obr., 6 tab. Sucho ako jeden z prírodných javov sa coraz castejsie stáva predmetom výskumu. Prícinou je narastajúca frekvencia výskytu extrémnych klimatických situácií aj v miernych zemepisných sírkach. Klasifikacné kritériá výskytu sucha v podzemnej vode mozno odvodi pre rôzne parametre podzemnej vody, napríklad pre hodnoty podzemného odtoku, úrovne hladiny podzemnej vody, výdatnosti pramea alebo dopania zásob. V príspevku sú navrhnuté metódy hodnotenia sucha v podzemnom odtoku. Hodnoty podzemného odtoku boli z hydrogramu prietokov odseparované pomocou nového statistického balíka HydroOffice 2010, ktorý bol v nedávnej minulosti vytvorený Gregorom. Index závaznosti sucha bol vypocítaný ako podiel nedostatkového objemu podzemného odtoku a dzky trvania sucha. Následne bola hodnota indexu závaznosti sucha normalizovaná vydelením dlhodobou priemernou hodnotou podzemného odtoku. Navrhnutá metodika bola pouzitá pre povodie toku Nitra. Výskyt sucha v podzemnej vode bol hodnotený aj z pohadu jeho sezónnosti. KÚCOVÉ SLOVÁ: sucho, parameter podzemnej vody, ciara prekrocenia, metóda hranicnej hodnoty, normalizovaný index závaznosti sucha v podzemnom odtoku, povodie toku Nitra. Introduction Hydrological drought is being broadly studied within last decades in many countries. It is because of increasing frequency of drought periods occurrence also in mild climate conditions. The increasing air temperature, often combined with the lack of precipitation is manifesting in decreasing of available water amounts in river basins, among them also in groundwater. There were many studies done devoted to drought. Processes and estimation methods for surface and groundwater drought were ana- lyzed by Tallaksen and van Lanen Eds. (2004), inter-comparison of circulation patterns and drought occurrence was done by Fleig et al. (2010). Regional characteristics of meteorological and hydrological droughts were proposed and summarized by Hisdal and Tallaksen (2003). Streamflow trends in the Europe were studied by Stahl et al. (2010); Kliment et al. (2011). Hisdal et al. (2001) paid their attention to streamflow drought severity changes in Europe. Drought occurrence in Nordic countries was analyzed by Wilson et al. (2010), the most severe droughts in UK since 1976 were described and 185 analyzed by Marsh et al. (2007). National procedure for low flow estimation in Austria was proposed by Laaha and Bloeschl (2007), low flow regionalization for southwest Germany was done by Schreiber and Demuth (1997). Space-time modeling of catchment scale drought characteristics was published by Tallaksen et al. (2009). Winter drought in Europe was studied by van Loon et al. (2010); its occurrence in the upper Rhine area was analyzed by Pfister et al. (2006). Groundwater drought occurrence in Slovakia was studied by Fendeková and Zenisová, Eds., (2010), Machlica and Stojkovova (2008), Machlica et al. (2010), low flows in Slovakia were studied by Demeterová and Skoda (2009), seasonality of low flows in Slovakia was analyzed by Kohnova et al. (2009). Ecological aspects connected to drought occurrence in the upper part of the Torysa River catchment, Slovakia, were analyzed by Fendekova et al. (2011). Groundwater drought can be expressed using several quantitative parameters of groundwater, such as groundwater runoff, spring yield, groundwater table value, groundwater recharge and others. Groundwater runoff is not measurable directly, except of spring yields measurements. In the hydrogeological practice, is substituted by the base flow values, derived from the discharge time series. After that, the groundwater drought analysis can be done, consisting of identification of drought periods occurrence, their length and severity, followed by the groundwater drought classification. The paper is devoted to groundwater drought; the attention is paid to the base flow drought analysis and assessment in the Nitra River basin. Drought occurrence was estimated and classified, drought propagation downstream the Nitra River basin was analyzed. Differences in drought parameters in a small mountainous sub-basin and larger lowland sub-basins were discussed. Study area description Nitra River Basin is a sub-basin of the Váh River with the total area of 4,063.66 km2 up to Nové Zámky profile. The river flows in approximately north-south direction, the most important right-side tributaries are Nitrica and Bebrava, the most important left-side tributaries are represented by Handlovka and Zitava. All climatic-geographical types of climate occurring in the Slovak territory are present in the basin: warm through moderately warm up to cold, depending on the altitude (Land186 scape atlas of SR, 2002). According to KöppenGeiger classification, the moderately warm, humid continental climate of the Dfb type is typical for the basin. The mean altitude reaches 372 m a.m.s.l. with the lowest value of 108 m a.m.s.l. (closing profile of the basin) and the highest value of 1346 m a.m.s.l. (Mount Vtácnik in Vtácnik Mts.). The average annual areal precipitation amount was estimated on 696 mm for the long-term period of 1961 ­ ­ 2000 (Danácová et al., 2010). The annual runoff has a long-term value of 142 mm, and the runoff coefficient has a value of 0.20 (Danácová et al., 2010). The evapotranspiration reaches 554 mm in average; the mean yearly areal air temperature is 8.4°C (Danácová et al., 2010). Values of all climatic elements are altitude-dependant. Geological structure and hydrogeological settings in the headwater part differ importantly from those in the central and lower part of the basin. The upper part of the basin is bordered by mountain ranges of variegated geological structure and hydrogeological settings. Most of the mountains are built by crystalline core (Strázovské vrchy Mts. in the west, Malá Fatra Mts. in the north, Ziar Mts. in the east, Tribec Mts. in the south-east), on which Mesozoic, mostly carbonatic complexes with the nappe structure are placed. In the eastern upper part of the basin, Vtácnik Mts. is located, the eastern border in the central part of the basin is created by Pohronský Inovec Mts., both being products of the Neogene volcanism. The intra-mountainous depression of the Horná Nitra basin is filled by Palaeogene and Neogene sediments, in the area of Nováky and Handlová brown coal layers of the Neogene age occur. The upper-most layers covering the Neogene filling are built by fluvial sediments of Quaternary age consisting of loam, sand and gravel. The geological structure of the central and lower part of the basin is quite simple, flat, filled by Neogene sandy and clayey sediments, being covered by fluvial sediments. The highest amounts of groundwater are accumulated in Quaternary sediments of the Nitra River alluvial plain and its larger tributaries with the thickness up to 10 meters. Groundwater of alluvial plain is mostly hydraulically interconnected with surface streams. The influence of surface streams on groundwater level can be observed up to the distance of 100 ­ 300 m, it ends up in the distance of 300 ­ 1000 m (Anon, 2000). Well yields reach up to 15 l s-1 (Anon, 2000). Material and methods Data from six discharge gauging profiles regularly observed in the Nitra River Basin were used in the study (Fig. 1, Tab. 1). Fig. 1. Location of utilized gauging profiles. Gauging profile in Kacno represents a small sub-basin in the mountainous upper part of the basin, which is not influenced by human activities. Catchment area up to Nedozery profile is larger; the profile is located in the intra-mountainous depression of the Horná Nitra. One of the longest time series were at the disposal for Nedozery profile, T a b l e 1. Basic parameters of the discharge gauging profiles. Profile Number and Name 6500 Kacno 6540 Nedozery 6710 Nadlice 6730 Nitrianska Streda 6820 Vieska nad Zitavou 6772 Nové Zámky River Nitra Nitra Bebrava Nitra Zitava Nitra having duration of more than 65 years. The profile in Nitrianska Streda represents the central part of the basin, and the Nové Zámky profile is the closing profile of the whole basin located at the place from which the Nitra River is artificially channeled to the Váh River. Both profiles represent the discharge influenced by various kinds of human activities as for instance regulation of discharges at water reservoir Nitrianske Rudno, dewatering of brown coal mines in Handlová and Nováky, water transfer from Nitrica to Nitra River sub-basin, irrigation and others. Two most important tributaries of Nitra River were also assessed. Nadlice profile represents the downstream profile of Bebrava River sub-basin; Vieska nad Zitavou profile represents the Zitava River sub-basin. Basic parameters of profiles are in Tab. 1. Because of inconsistency of discharge data in Nové Zámky profile, only the period of hydrological years of 1991 ­ 2006 was used in the study. The input data on daily discharges in all profiles (Tab. 1) were processed statistically, and then used in the program package HydroOffice 2010 (Gregor, 2008). The package enables: - separation of the base flow from the average daily discharges using BFI+2 sub-routine, - calculation of the base flow duration curve parameters (cumulative frequency curve for the separated base flow) in FDC sub-routine (www.hydrooffice.org), - utilization of base flow duration curve parameters as threshold levels for estimation of drought periods occurrence in TLM sub-routine (www.hydrooffice.org). Altitude [m a.m.s.l.] 471.84 287.00 171.11 158.25 154.14 108.73 Area [km2] 10.50 181.57 598.81 2093.71 295.46 4063.66 Years of observation 1975­2006 1941­2006 1941­2006 1973­2006 1961­2006 1990­2006* *time series shortened due to changes in profile location Sub-routine BFI+2 is based on the original method of base flow (BF) separation from the average daily discharges (BFI model), which was developed in the Institute of Hydrology (1980). N-day consecutive and non-overlapping periods are used for calculation of minimal discharges which represent turning points on the base flow hydrogram. The original program calculated with the fixed Nvalue, equal to 5 days. Application of the original BFI model on Slovak conditions gave much higher base flow values than other methods usually utilized in Slovak hydrogeological practice. The innovation of the original program consists in possibility to choose the length of the N value. The new 187 calculation algorithm also implements the suggestion of Tallaksen and van Lanen Eds. (2004) and some other methods for base flow separation based on recursive filters. The value of N equal to 15 or 30 days gives the best values (depending on hydrogeological settings), as confirmed by Machlica (2010). An example of the results of base flow separation for Nedozery profile is in Fig. 2. Discharges larger than ten cubic meters per second were cut in the figure, in order to make the base flow separation more distinct. Fig. 2. Base flow separation from discharges for Nedozery profile. Separated values of the base flow, representing groundwater runoff from the catchment, were statistically processed. Obtained statistical parameters were compared with those estimated for the original average daily discharge time series. Base flow values were then used for calculation of the average long-term base flow from the basin. The long-term average value was used as a comparative value during the process of the base flow drought extremity assessment. Respective years on the low base flow side were classified as years with: - extremely low base flow in the case when the average yearly base flow was lower than 70 % of the long-term average, - very low base flow in the case when the average yearly base flow ranged in the interval of 70 % to 80 % of the long-term average, - low base flow in the case when the average yearly base flow ranged in the interval of 80 % to 90 % of the long-term average. Years with the base flow value higher than 90 % of the long-term average were classified as normal (90 ­ 110 %). Accordingly, years with the high (110 ­ 120 %), very high (120 ­ 130 %) and extremely high (more than 130 %) base flow were also defined. Drought occurrence was evaluated using the threshold level method (Stahl, 2001; Tallaksen, van Lanen, Eds., 2004). Threshold level values were 188 calculated using the TLM 2.1 sub-routine, which is a part of the HydroOffice 2010 package (Gregor, 2008). Base flow duration curves were constructed for each hydrological year of the whole evaluated period, after that the master base flow duration curve was constructed using the average values of respective percentiles. The base flow value of BF90mc calculated from the master base flow duration curve was used as the base flow drought threshold level value in further analysis of the base flow drought. Value of the BF90mc represents approximately 330-day long-term average base flow. An example of the master base flow duration curve construction for the Nedozery gauging profile is in Fig. 3. According to base flow values in the range of BF90mc to BF97mc, the years 1956 and 1957 belonged to wet years, years 1992, 1994, 2003 and 2004 were dry. The long-term base flow duration curve was also constructed using the long-term time series of the base flow values. The comparison of both ­ master base flow duration curve and long-term base flow duration curve values was done. Base flow drought extremity was assessed using two methods. In the first one, the base flow drought was classified using the BF90mc, BF80mc and BF70mc. Occurrence of dry periods was evaluated also from the point of view of their duration and categorization according to the season of the year. Drought periods shorter than 10 days were not assessed. Pooling of consequent droughts was done for each two droughts which were separated by not more than two days with discharges exceeding the threshold level value. Drought periods longer than 10 days were classified into three categories: with the duration (D) 10 ­ 49 days, 50 ­ 99 days and 100 and more days (Fendeková, Zenisová, Eds., 2010). Seasons were defined for the base flow drought classification as follows: Sp: spring season (March­ May), S: summer season (June­August), A: autumn season (September­November) and W: winter season (December­February). A multiyear drought occurred when the drought lasted for more than 365 days, or from one season of the previous year to the same season of the next year. The classification scheme was as follows: - extreme base flow drought occurred when the average daily discharge did not reach the value of BF90mc, - medium base flow drought occurred when the average daily discharge varied in the range BF90mc ­ BF80mc, - weak base flow drought occurred when the average daily discharge varied in the range BF80mc ­ BF70mc, - discharges with the value higher than BF70mc indicated no base flow drought. Fig. 3. Construction of the master base flow curve for Nedozery profile. Such a classification is in accordance with the classification of year wetness applied on discharges and precipitation (Majercáková et al., 2007). Another possibility to classify the base flow drought gives the Sequent Peak Algorithm method (SPA method), which enables to estimate not only the beginning and ending date of the drought, and its duration, but also the deficit volume during the drought period using the threshold level value. The BF90mc value was used as the threshold level. At present, value of the 80-th percentile (BF80mc) becomes more often to be used as the threshold value. However, in such case, more droughts of shorter duration are estimated, which often do not cover larger areas. Therefore, the threshold value of the 90-th percentile (BF90mc) was used in the study, detecting more severe droughts covering larger areas. Drought severity index was calculated as the ratio of the total deficit volume (in respective drought period) and the drought period duration. In the end, value of the drought severity index was divided by the individual average long-term base flow value in the respective profile, receiving the standardized base flow drought severity index. Calculation of the standardized index enabled to compare the base flow drought severity in different sub-basins. Results and discussion Results of basic statistical evaluation of the average daily discharges and base flow values for the Nedozery gauging profile are given in Tab. 2 as an example of the input data statistical evaluation. T a b l e 2. Basic statistical evaluation of discharges and base flow values (1941 ­ 2006). Value Discharge Base flow N 23741 23741 xaver 2.13 1.32 xmed 1.33 1.08 xmod 0.7 0.7 xmax 62.8 8.9 xmin 0.14 0.14 sx 2.54 0.92 CV 119.1 70.0 CS 5.07 2.32 CSst 303 146 E 50.4 8.23 Est 1585 259 Values of input data count (N), arithmetic mean (xaver), median (xmed), modus (xmod), maximum (xmax), minimum (xmin), standard deviation (sx), coefficient of variation (CV), skewness (CS), standardized skewness (CSst), kurtosis (E) and standardized kurtosis (Est) were calculated. The results show that the frequency distribution of both variables is strongly asymmetric (skew), but the measure of asymmetry represented by the ratio of xaver : xmed : xmod, as well as by values of CS and CSst is lower for the base flow in comparison with the original discharge values. The variability of base flow values ensemble expressed by standard deviation and coefficient of variation also decreased from CV equal to 119 % by discharges to 70 % for the base flow ensemble. Anyway, the ensemble still does not have the normal frequency distribution, which is documented by values of CSst and Est. Comparison of percentiles obtained from the master base flow duration curve (BFmc) and the average base flow duration curve for the long-term base flow time series (BFl-t) showed that the values of BFmc are higher than the values of BFl-t. The differences are the highest at the highest percentiles and decrease towards the median value. Comparison of selected percentiles obtained for base flow values at Nedozery profile for the period 1951 ­ ­ 2005 is in Tab. 3. Therefore, it is necessary to give the information, which procedure was used for base flow duration curve construction. The authors prefer to use the master base flow duration curve, which is not biased by cumulating of low values in the lower tail of the curve (80th ­ 99th percentile), as it is in the case of the average base flow duration curve. Dry years according to base flow value were classified using the ratio of the average yearly base flow (BFY) on the long-term base flow value (BF A). Classification of dry years in Nedozery profile is in Fig. 4. T a b l e 3. Comparison of master and long-term base flow duration curve values. Value Master base flow duration curve Long-term base flow duration curve BF50 0.815 0.767 BF70 0.677 0.600 BF80 0.609 0.520 BF90 0.559 0.409 BF95 0.536 0.362 Fig. 4. Evaluation of dry years in the base flow values for Nedozery profile. It can be seen in Fig. 4 that extremely low base flow occurred in the profile in 1946 and 1947, 1969 and 1970, 1973, 1989 to 1991, 1993, 2000 190 and 2001, as well as 2004 to 2006. Tab. 4 shows number of dry years according to base flow classification, expressed in absolute numbers, and as percentage of the total number of processed years for all evaluated profiles. Tab. 4 also shows that the dry years occur more often in the upper ­ mountainous part of the basin in comparison with the central and downstream part. This is valid not only for the common period of 1976 ­ 2006 in five of six evaluated profiles (except of Nové Zámky), but also for the 1991 ­ ­ 2006 period (16 years), in which all six profiles were assessed. Years with the extremely low base flow prevailed in the upper part of the basin, on the T a b l e 4. Number of base flow periods below normal. other hand, the low and normal to extremely high base flow was more frequent in the central part of the basin (profile Nitrianska Streda). Only in Nové Zámky profile, the number of years with normal to extremely high base flow was higher than number of years with one of the low base flow categories. Occurrence of dry periods was evaluated also from the point of view of their duration (D). The results in Tab. 5 show that longer droughts (100 days and more) prevail in the basin. Base flow Profile Kacno Nedozery Nadlice Vieska n. Zitavou Nitrianska Streda Nové Zámky 1976 ­ 2006 1991 ­ 2006 1942 ­ 2006 1991 ­ 2006 1976 ­ 2006 1942 ­ 2006 1991 ­ 2006 1976 ­ 2006 1962 ­ 2006 1991 ­ 2006 1976 ­ 2006 1976 ­ 2006 1991 ­ 2006 1991 ­ 2006 Extremely low (EL) 10 9 (56 %) 14 9 7 (43.5 %) 12 8 6 (37.5 %) 5 4 2 (4.4 %) 2 2 (12.5 %) 3 (19 %) Very low (VL) 1 1 (6 %) 2 2 2 (12.5 %) 7 5 3 (18.7 %) 5 2 2 (6.4 %) 3 3 (19 %) 2 (12. 5 %) Low (L) 3 3 (19 %) 13 6 4 (25 %) 11 8 5 (31 %) 12 8 5 (31.3) 8 6 (37.5 %) 2 (12.5 %) Total low (EL+VL+L) 14 13 29 17 13 30 21 14 22 14 9 13 11 7 Normal to extremely high 17 (55 %) 3 (19 %) 36 (55 %) 14 (45 %) 3 (19 %) 34 (52 %) 10 (32 %) 2 (12.5 %) 23 (51 %) 17 (55 %) 7 (44 %) 18 (58 %) 5 (31 %) 9 (56 %) T a b l e 5. Number of base flow drought periods of respective duration. Number of periods D: 10 ­ 49 days 2 2 3 3 2 2 2 0 3 1 1 2 1 1 Number of periods D: 50 ­ 100 days 1 1 2 2 1 2 2 2 4 2 2 4 2 1 Number of periods D: 100 days and more 5 5 13 9 8 11 8 5 10 5 3 5 4 3 Total number of periods 8 8 18 14 11 15 12 7 17 8 6 11 7 5 Total duration 1723 1723 2910 2023 1667 3482 2723 1837 2677 1485 1116 2024 1665 924 Profile Kacno Nedozery Nadlice Vieska n. Zitavou Nitrianska Streda Nové Zámky 1976 ­ 2006 1991 ­ 2006 1941 ­ 2006 1991 ­ 2006 1976 ­ 2006 1941 ­ 2006 1991 ­ 2006 1976 ­ 2006 1941 ­ 2006 1991 ­ 2006 1976 ­ 2006 1971 ­ 2006 1991 ­ 2006 1991 ­ 2006 The frequency of drought periods, as well as total duration of droughts is higher in the upper ­ mountainous part of the basin. Long droughts occurred in the whole basin in 1991 ­ 1994. The longest drought in the observed period occurred in the same period in Nadlice profile; lasting for 1062 days. The next base flow drought covering the whole basin was the 2003 ­ 2004 drought, which in the Nitrianska Streda profile lasted till the beginning of 2005. The comparison of the drought severity in respective sub-basins according to standardized base 191 flow drought severity index for the common period of 1991 ­ 2006 was done. Only the most severe droughts in each respective profile are showed in Tab. 6. In this period, the most severe drought according to standardized base flow drought index occurred in Nadlice profile in the years 1991 ­ 1994. The most severe drought in the Nitrianska Streda profile was divided into two sub-periods, the first one occurred in 1990 ­ 1991 (lasting for 182 days) with the index value of 62.8, being followed within a few months with the second one in 1991 ­ 1993 period with the value of 53.1 and lasting for 566 days. However, the most severe drought in the basin during the whole observed period occurred in Vieska nad Zitavou profile in 1962 ­ 1963 with the standardized base flow drought severity index of 159.6 (lasting for 313 days) and in Nedozery profile in 1946 ­ 1948 with the index value of 127.6 (lasting for 564 days). T a b l e 6. The most severe droughts in evaluated profiles in the period 1990 ­ 2006. Kacno 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 106.7 D: 1062 Nedozery Nadlice Vieska n. Zitavou 90.56 D: 222 Nitrianska Streda 62.8 D: 182 53.1 D: 566 Nové Zámky 97.82 D: 239 45.8 D: 150 83.64 D: 651 What the drought seasonality is concerned, in the upper part as well as in the central part of the basin, the summer-autumn and summer-winter droughts have approximately the same occurrence frequency. In the lowest profile at Nové Zámky, the summer autumn drought prevails and in the highest profile at Kacno, the spring-winter drought was the most frequent. When comparing the frequency of drought periods and their total length in the base flow with the discharge drought occurrence in the same profiles, it was documented by Fendeková and Fendek (2011) that discharge drought is more frequent, but lasts shorter ­ droughts with the duration of 10 ­ 49 days prevail. According to the same authors (Fendeková, Fendek, 2011), drought in the groundwater heads occurred two-times more frequently in the upper part of the basin, but it lasted shorter as in the lowland part. Conclusion Drought as one of the more frequently occurring phenomenon can be accessed from different points of view using various characteristics. Such disparateness is followed by more and more drought indices occurring in the literature. Authors of this paper evaluated the drought in a yearly scale. The standardized base flow drought severity index was proposed for base flow drought characterization. It was documented that years with the extremely low base flow occur much often in the mountainous part of the Nitra River Basin than in the lowland part. Similarly, differences in seasonality and base flow drought duration between the upper, central and downstream part of the basin were showed. As it follows from the most severe droughts occurrence analysis, there is no unified pattern of the drought occurrence in the Nitra River basin. It was confirmed that the upper part of the basin is more sensitive on extreme climatic situation occurrence leading to drought in surface and groundwater. Acknowledgment. The authors would like to thank to FP6 Watch project (contract Number 036946), from which financial sources the research was supported. Groundwater drought in the Nitra River basin ­ identification and classification REFERENCES ANON 2000: Hydroekologický plán povodia Nitry v úseku od pramea po zaústenie prelozky Nitry. Cas A. Slovenský vodohospodársky podnik s. p. 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Sci. J., 42, 6, 845­ ­858. STAHL K., 2001: Hydrological drought ­ a study accross Europe. PhD. Thesis Albert-Ludwigs Universitat Freiburg, Freiburger Schriften zur Hydrologie no. 15, Freiburg, Germany. STAHL K., HISDAL H., HANNAFORD J., TALLAKSEN L.M., VAN LANEN H.A.J., SAUQUET E., DEMUTH S., FENDEKOVÁ M., JÓDAR J., 2010: Streamflow trends in Europe: evidence from a dataset of near-natural catchments. Hydrology and Earth System Sciences, 14, 12, 2367­2382. TALLAKSEN L. M., VAN LANEN H. A. J. (Eds.), 2004: Hydrological drought: processes and estimation methods for streamflow and groundwater. Developments in water science; 48, The Netherlands, Amsterdam: Elsevier Science, pp. 579, ISBN 0-444-51688-3. TALLAKSEN L. M., HISDAL H., VAN LANEN H. A. J., 2009: Space-time modeling of catchment scale drought characteristics. J. Hydrol., 375, 363­372. WILSON D., HISDAL H., LAWRENCE D., 2010. Has streamflows changed in the Nordic countries? ­ Recent trends and comparison to hydrological projections. J. Hydrol., 394, 334­346. www.hydrooffice.org (march 2012) Received 23 August 2011 Accepted 13 March 2012 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Hydrology and Hydromechanics de Gruyter

Groundwater Drought in the Nitra River Basin - Identification and Classification

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J. Hydrol. Hydromech., 60, 2012, 3, 185­193 DOI: 10.2478/v10098-012-0016-1 MIRIAM FENDEKOVÁ, MARIÁN FENDEK Department of Hydrogeology, Faculty of Natural Sciences, Comenius University in Bratislava, Mlynská dolina, Pav. G, 842 15 Bratislava; Mailto: fendekova@fns.uniba.sk Drought as a natural phenomenon becomes more often the subject of the research. It is because of the increasing frequency of extreme climatic events also in mild climate conditions. Groundwater drought indices could be derived for different groundwater parameters, among them for base flow, groundwater head stage, spring yield, or groundwater recharge. Base flow drought assessment methods were proposed in the paper. Base flow values were separated from the discharge hydrograms using the new HydroOffice 2010 program package, lately developed by Gregor. The base flow drought severity index was applied, calculated as the value of the base flow drought deficit volume divided by the drought duration. After that, the standardized base flow drought severity index was proposed as the ratio of the base flow drought index and the average long-term annual base flow. Proposed methods were applied in the Nitra River basin. Base flow drought occurrence was characterized also from the seasonality point of view. KEY WORDS: Drought, Groundwater Parameters, Cumulative Frequency Curve, Threshold Level Method, Standardized Base Flow Drought Severity Index, Nitra River Basin. Miriam Fendeková, Marián Fendek: SUCHO V PODZEMNEJ VODE V POVODÍ TOKU NITRA ­ IDENTIFIKÁCIA A KLASIFIKÁCIA. J. Hydrol. Hydromech., 60, 2012, 3; 28 lit., 4 obr., 6 tab. Sucho ako jeden z prírodných javov sa coraz castejsie stáva predmetom výskumu. Prícinou je narastajúca frekvencia výskytu extrémnych klimatických situácií aj v miernych zemepisných sírkach. Klasifikacné kritériá výskytu sucha v podzemnej vode mozno odvodi pre rôzne parametre podzemnej vody, napríklad pre hodnoty podzemného odtoku, úrovne hladiny podzemnej vody, výdatnosti pramea alebo dopania zásob. V príspevku sú navrhnuté metódy hodnotenia sucha v podzemnom odtoku. Hodnoty podzemného odtoku boli z hydrogramu prietokov odseparované pomocou nového statistického balíka HydroOffice 2010, ktorý bol v nedávnej minulosti vytvorený Gregorom. Index závaznosti sucha bol vypocítaný ako podiel nedostatkového objemu podzemného odtoku a dzky trvania sucha. Následne bola hodnota indexu závaznosti sucha normalizovaná vydelením dlhodobou priemernou hodnotou podzemného odtoku. Navrhnutá metodika bola pouzitá pre povodie toku Nitra. Výskyt sucha v podzemnej vode bol hodnotený aj z pohadu jeho sezónnosti. KÚCOVÉ SLOVÁ: sucho, parameter podzemnej vody, ciara prekrocenia, metóda hranicnej hodnoty, normalizovaný index závaznosti sucha v podzemnom odtoku, povodie toku Nitra. Introduction Hydrological drought is being broadly studied within last decades in many countries. It is because of increasing frequency of drought periods occurrence also in mild climate conditions. The increasing air temperature, often combined with the lack of precipitation is manifesting in decreasing of available water amounts in river basins, among them also in groundwater. There were many studies done devoted to drought. Processes and estimation methods for surface and groundwater drought were ana- lyzed by Tallaksen and van Lanen Eds. (2004), inter-comparison of circulation patterns and drought occurrence was done by Fleig et al. (2010). Regional characteristics of meteorological and hydrological droughts were proposed and summarized by Hisdal and Tallaksen (2003). Streamflow trends in the Europe were studied by Stahl et al. (2010); Kliment et al. (2011). Hisdal et al. (2001) paid their attention to streamflow drought severity changes in Europe. Drought occurrence in Nordic countries was analyzed by Wilson et al. (2010), the most severe droughts in UK since 1976 were described and 185 analyzed by Marsh et al. (2007). National procedure for low flow estimation in Austria was proposed by Laaha and Bloeschl (2007), low flow regionalization for southwest Germany was done by Schreiber and Demuth (1997). Space-time modeling of catchment scale drought characteristics was published by Tallaksen et al. (2009). Winter drought in Europe was studied by van Loon et al. (2010); its occurrence in the upper Rhine area was analyzed by Pfister et al. (2006). Groundwater drought occurrence in Slovakia was studied by Fendeková and Zenisová, Eds., (2010), Machlica and Stojkovova (2008), Machlica et al. (2010), low flows in Slovakia were studied by Demeterová and Skoda (2009), seasonality of low flows in Slovakia was analyzed by Kohnova et al. (2009). Ecological aspects connected to drought occurrence in the upper part of the Torysa River catchment, Slovakia, were analyzed by Fendekova et al. (2011). Groundwater drought can be expressed using several quantitative parameters of groundwater, such as groundwater runoff, spring yield, groundwater table value, groundwater recharge and others. Groundwater runoff is not measurable directly, except of spring yields measurements. In the hydrogeological practice, is substituted by the base flow values, derived from the discharge time series. After that, the groundwater drought analysis can be done, consisting of identification of drought periods occurrence, their length and severity, followed by the groundwater drought classification. The paper is devoted to groundwater drought; the attention is paid to the base flow drought analysis and assessment in the Nitra River basin. Drought occurrence was estimated and classified, drought propagation downstream the Nitra River basin was analyzed. Differences in drought parameters in a small mountainous sub-basin and larger lowland sub-basins were discussed. Study area description Nitra River Basin is a sub-basin of the Váh River with the total area of 4,063.66 km2 up to Nové Zámky profile. The river flows in approximately north-south direction, the most important right-side tributaries are Nitrica and Bebrava, the most important left-side tributaries are represented by Handlovka and Zitava. All climatic-geographical types of climate occurring in the Slovak territory are present in the basin: warm through moderately warm up to cold, depending on the altitude (Land186 scape atlas of SR, 2002). According to KöppenGeiger classification, the moderately warm, humid continental climate of the Dfb type is typical for the basin. The mean altitude reaches 372 m a.m.s.l. with the lowest value of 108 m a.m.s.l. (closing profile of the basin) and the highest value of 1346 m a.m.s.l. (Mount Vtácnik in Vtácnik Mts.). The average annual areal precipitation amount was estimated on 696 mm for the long-term period of 1961 ­ ­ 2000 (Danácová et al., 2010). The annual runoff has a long-term value of 142 mm, and the runoff coefficient has a value of 0.20 (Danácová et al., 2010). The evapotranspiration reaches 554 mm in average; the mean yearly areal air temperature is 8.4°C (Danácová et al., 2010). Values of all climatic elements are altitude-dependant. Geological structure and hydrogeological settings in the headwater part differ importantly from those in the central and lower part of the basin. The upper part of the basin is bordered by mountain ranges of variegated geological structure and hydrogeological settings. Most of the mountains are built by crystalline core (Strázovské vrchy Mts. in the west, Malá Fatra Mts. in the north, Ziar Mts. in the east, Tribec Mts. in the south-east), on which Mesozoic, mostly carbonatic complexes with the nappe structure are placed. In the eastern upper part of the basin, Vtácnik Mts. is located, the eastern border in the central part of the basin is created by Pohronský Inovec Mts., both being products of the Neogene volcanism. The intra-mountainous depression of the Horná Nitra basin is filled by Palaeogene and Neogene sediments, in the area of Nováky and Handlová brown coal layers of the Neogene age occur. The upper-most layers covering the Neogene filling are built by fluvial sediments of Quaternary age consisting of loam, sand and gravel. The geological structure of the central and lower part of the basin is quite simple, flat, filled by Neogene sandy and clayey sediments, being covered by fluvial sediments. The highest amounts of groundwater are accumulated in Quaternary sediments of the Nitra River alluvial plain and its larger tributaries with the thickness up to 10 meters. Groundwater of alluvial plain is mostly hydraulically interconnected with surface streams. The influence of surface streams on groundwater level can be observed up to the distance of 100 ­ 300 m, it ends up in the distance of 300 ­ 1000 m (Anon, 2000). Well yields reach up to 15 l s-1 (Anon, 2000). Material and methods Data from six discharge gauging profiles regularly observed in the Nitra River Basin were used in the study (Fig. 1, Tab. 1). Fig. 1. Location of utilized gauging profiles. Gauging profile in Kacno represents a small sub-basin in the mountainous upper part of the basin, which is not influenced by human activities. Catchment area up to Nedozery profile is larger; the profile is located in the intra-mountainous depression of the Horná Nitra. One of the longest time series were at the disposal for Nedozery profile, T a b l e 1. Basic parameters of the discharge gauging profiles. Profile Number and Name 6500 Kacno 6540 Nedozery 6710 Nadlice 6730 Nitrianska Streda 6820 Vieska nad Zitavou 6772 Nové Zámky River Nitra Nitra Bebrava Nitra Zitava Nitra having duration of more than 65 years. The profile in Nitrianska Streda represents the central part of the basin, and the Nové Zámky profile is the closing profile of the whole basin located at the place from which the Nitra River is artificially channeled to the Váh River. Both profiles represent the discharge influenced by various kinds of human activities as for instance regulation of discharges at water reservoir Nitrianske Rudno, dewatering of brown coal mines in Handlová and Nováky, water transfer from Nitrica to Nitra River sub-basin, irrigation and others. Two most important tributaries of Nitra River were also assessed. Nadlice profile represents the downstream profile of Bebrava River sub-basin; Vieska nad Zitavou profile represents the Zitava River sub-basin. Basic parameters of profiles are in Tab. 1. Because of inconsistency of discharge data in Nové Zámky profile, only the period of hydrological years of 1991 ­ 2006 was used in the study. The input data on daily discharges in all profiles (Tab. 1) were processed statistically, and then used in the program package HydroOffice 2010 (Gregor, 2008). The package enables: - separation of the base flow from the average daily discharges using BFI+2 sub-routine, - calculation of the base flow duration curve parameters (cumulative frequency curve for the separated base flow) in FDC sub-routine (www.hydrooffice.org), - utilization of base flow duration curve parameters as threshold levels for estimation of drought periods occurrence in TLM sub-routine (www.hydrooffice.org). Altitude [m a.m.s.l.] 471.84 287.00 171.11 158.25 154.14 108.73 Area [km2] 10.50 181.57 598.81 2093.71 295.46 4063.66 Years of observation 1975­2006 1941­2006 1941­2006 1973­2006 1961­2006 1990­2006* *time series shortened due to changes in profile location Sub-routine BFI+2 is based on the original method of base flow (BF) separation from the average daily discharges (BFI model), which was developed in the Institute of Hydrology (1980). N-day consecutive and non-overlapping periods are used for calculation of minimal discharges which represent turning points on the base flow hydrogram. The original program calculated with the fixed Nvalue, equal to 5 days. Application of the original BFI model on Slovak conditions gave much higher base flow values than other methods usually utilized in Slovak hydrogeological practice. The innovation of the original program consists in possibility to choose the length of the N value. The new 187 calculation algorithm also implements the suggestion of Tallaksen and van Lanen Eds. (2004) and some other methods for base flow separation based on recursive filters. The value of N equal to 15 or 30 days gives the best values (depending on hydrogeological settings), as confirmed by Machlica (2010). An example of the results of base flow separation for Nedozery profile is in Fig. 2. Discharges larger than ten cubic meters per second were cut in the figure, in order to make the base flow separation more distinct. Fig. 2. Base flow separation from discharges for Nedozery profile. Separated values of the base flow, representing groundwater runoff from the catchment, were statistically processed. Obtained statistical parameters were compared with those estimated for the original average daily discharge time series. Base flow values were then used for calculation of the average long-term base flow from the basin. The long-term average value was used as a comparative value during the process of the base flow drought extremity assessment. Respective years on the low base flow side were classified as years with: - extremely low base flow in the case when the average yearly base flow was lower than 70 % of the long-term average, - very low base flow in the case when the average yearly base flow ranged in the interval of 70 % to 80 % of the long-term average, - low base flow in the case when the average yearly base flow ranged in the interval of 80 % to 90 % of the long-term average. Years with the base flow value higher than 90 % of the long-term average were classified as normal (90 ­ 110 %). Accordingly, years with the high (110 ­ 120 %), very high (120 ­ 130 %) and extremely high (more than 130 %) base flow were also defined. Drought occurrence was evaluated using the threshold level method (Stahl, 2001; Tallaksen, van Lanen, Eds., 2004). Threshold level values were 188 calculated using the TLM 2.1 sub-routine, which is a part of the HydroOffice 2010 package (Gregor, 2008). Base flow duration curves were constructed for each hydrological year of the whole evaluated period, after that the master base flow duration curve was constructed using the average values of respective percentiles. The base flow value of BF90mc calculated from the master base flow duration curve was used as the base flow drought threshold level value in further analysis of the base flow drought. Value of the BF90mc represents approximately 330-day long-term average base flow. An example of the master base flow duration curve construction for the Nedozery gauging profile is in Fig. 3. According to base flow values in the range of BF90mc to BF97mc, the years 1956 and 1957 belonged to wet years, years 1992, 1994, 2003 and 2004 were dry. The long-term base flow duration curve was also constructed using the long-term time series of the base flow values. The comparison of both ­ master base flow duration curve and long-term base flow duration curve values was done. Base flow drought extremity was assessed using two methods. In the first one, the base flow drought was classified using the BF90mc, BF80mc and BF70mc. Occurrence of dry periods was evaluated also from the point of view of their duration and categorization according to the season of the year. Drought periods shorter than 10 days were not assessed. Pooling of consequent droughts was done for each two droughts which were separated by not more than two days with discharges exceeding the threshold level value. Drought periods longer than 10 days were classified into three categories: with the duration (D) 10 ­ 49 days, 50 ­ 99 days and 100 and more days (Fendeková, Zenisová, Eds., 2010). Seasons were defined for the base flow drought classification as follows: Sp: spring season (March­ May), S: summer season (June­August), A: autumn season (September­November) and W: winter season (December­February). A multiyear drought occurred when the drought lasted for more than 365 days, or from one season of the previous year to the same season of the next year. The classification scheme was as follows: - extreme base flow drought occurred when the average daily discharge did not reach the value of BF90mc, - medium base flow drought occurred when the average daily discharge varied in the range BF90mc ­ BF80mc, - weak base flow drought occurred when the average daily discharge varied in the range BF80mc ­ BF70mc, - discharges with the value higher than BF70mc indicated no base flow drought. Fig. 3. Construction of the master base flow curve for Nedozery profile. Such a classification is in accordance with the classification of year wetness applied on discharges and precipitation (Majercáková et al., 2007). Another possibility to classify the base flow drought gives the Sequent Peak Algorithm method (SPA method), which enables to estimate not only the beginning and ending date of the drought, and its duration, but also the deficit volume during the drought period using the threshold level value. The BF90mc value was used as the threshold level. At present, value of the 80-th percentile (BF80mc) becomes more often to be used as the threshold value. However, in such case, more droughts of shorter duration are estimated, which often do not cover larger areas. Therefore, the threshold value of the 90-th percentile (BF90mc) was used in the study, detecting more severe droughts covering larger areas. Drought severity index was calculated as the ratio of the total deficit volume (in respective drought period) and the drought period duration. In the end, value of the drought severity index was divided by the individual average long-term base flow value in the respective profile, receiving the standardized base flow drought severity index. Calculation of the standardized index enabled to compare the base flow drought severity in different sub-basins. Results and discussion Results of basic statistical evaluation of the average daily discharges and base flow values for the Nedozery gauging profile are given in Tab. 2 as an example of the input data statistical evaluation. T a b l e 2. Basic statistical evaluation of discharges and base flow values (1941 ­ 2006). Value Discharge Base flow N 23741 23741 xaver 2.13 1.32 xmed 1.33 1.08 xmod 0.7 0.7 xmax 62.8 8.9 xmin 0.14 0.14 sx 2.54 0.92 CV 119.1 70.0 CS 5.07 2.32 CSst 303 146 E 50.4 8.23 Est 1585 259 Values of input data count (N), arithmetic mean (xaver), median (xmed), modus (xmod), maximum (xmax), minimum (xmin), standard deviation (sx), coefficient of variation (CV), skewness (CS), standardized skewness (CSst), kurtosis (E) and standardized kurtosis (Est) were calculated. The results show that the frequency distribution of both variables is strongly asymmetric (skew), but the measure of asymmetry represented by the ratio of xaver : xmed : xmod, as well as by values of CS and CSst is lower for the base flow in comparison with the original discharge values. The variability of base flow values ensemble expressed by standard deviation and coefficient of variation also decreased from CV equal to 119 % by discharges to 70 % for the base flow ensemble. Anyway, the ensemble still does not have the normal frequency distribution, which is documented by values of CSst and Est. Comparison of percentiles obtained from the master base flow duration curve (BFmc) and the average base flow duration curve for the long-term base flow time series (BFl-t) showed that the values of BFmc are higher than the values of BFl-t. The differences are the highest at the highest percentiles and decrease towards the median value. Comparison of selected percentiles obtained for base flow values at Nedozery profile for the period 1951 ­ ­ 2005 is in Tab. 3. Therefore, it is necessary to give the information, which procedure was used for base flow duration curve construction. The authors prefer to use the master base flow duration curve, which is not biased by cumulating of low values in the lower tail of the curve (80th ­ 99th percentile), as it is in the case of the average base flow duration curve. Dry years according to base flow value were classified using the ratio of the average yearly base flow (BFY) on the long-term base flow value (BF A). Classification of dry years in Nedozery profile is in Fig. 4. T a b l e 3. Comparison of master and long-term base flow duration curve values. Value Master base flow duration curve Long-term base flow duration curve BF50 0.815 0.767 BF70 0.677 0.600 BF80 0.609 0.520 BF90 0.559 0.409 BF95 0.536 0.362 Fig. 4. Evaluation of dry years in the base flow values for Nedozery profile. It can be seen in Fig. 4 that extremely low base flow occurred in the profile in 1946 and 1947, 1969 and 1970, 1973, 1989 to 1991, 1993, 2000 190 and 2001, as well as 2004 to 2006. Tab. 4 shows number of dry years according to base flow classification, expressed in absolute numbers, and as percentage of the total number of processed years for all evaluated profiles. Tab. 4 also shows that the dry years occur more often in the upper ­ mountainous part of the basin in comparison with the central and downstream part. This is valid not only for the common period of 1976 ­ 2006 in five of six evaluated profiles (except of Nové Zámky), but also for the 1991 ­ ­ 2006 period (16 years), in which all six profiles were assessed. Years with the extremely low base flow prevailed in the upper part of the basin, on the T a b l e 4. Number of base flow periods below normal. other hand, the low and normal to extremely high base flow was more frequent in the central part of the basin (profile Nitrianska Streda). Only in Nové Zámky profile, the number of years with normal to extremely high base flow was higher than number of years with one of the low base flow categories. Occurrence of dry periods was evaluated also from the point of view of their duration (D). The results in Tab. 5 show that longer droughts (100 days and more) prevail in the basin. Base flow Profile Kacno Nedozery Nadlice Vieska n. Zitavou Nitrianska Streda Nové Zámky 1976 ­ 2006 1991 ­ 2006 1942 ­ 2006 1991 ­ 2006 1976 ­ 2006 1942 ­ 2006 1991 ­ 2006 1976 ­ 2006 1962 ­ 2006 1991 ­ 2006 1976 ­ 2006 1976 ­ 2006 1991 ­ 2006 1991 ­ 2006 Extremely low (EL) 10 9 (56 %) 14 9 7 (43.5 %) 12 8 6 (37.5 %) 5 4 2 (4.4 %) 2 2 (12.5 %) 3 (19 %) Very low (VL) 1 1 (6 %) 2 2 2 (12.5 %) 7 5 3 (18.7 %) 5 2 2 (6.4 %) 3 3 (19 %) 2 (12. 5 %) Low (L) 3 3 (19 %) 13 6 4 (25 %) 11 8 5 (31 %) 12 8 5 (31.3) 8 6 (37.5 %) 2 (12.5 %) Total low (EL+VL+L) 14 13 29 17 13 30 21 14 22 14 9 13 11 7 Normal to extremely high 17 (55 %) 3 (19 %) 36 (55 %) 14 (45 %) 3 (19 %) 34 (52 %) 10 (32 %) 2 (12.5 %) 23 (51 %) 17 (55 %) 7 (44 %) 18 (58 %) 5 (31 %) 9 (56 %) T a b l e 5. Number of base flow drought periods of respective duration. Number of periods D: 10 ­ 49 days 2 2 3 3 2 2 2 0 3 1 1 2 1 1 Number of periods D: 50 ­ 100 days 1 1 2 2 1 2 2 2 4 2 2 4 2 1 Number of periods D: 100 days and more 5 5 13 9 8 11 8 5 10 5 3 5 4 3 Total number of periods 8 8 18 14 11 15 12 7 17 8 6 11 7 5 Total duration 1723 1723 2910 2023 1667 3482 2723 1837 2677 1485 1116 2024 1665 924 Profile Kacno Nedozery Nadlice Vieska n. Zitavou Nitrianska Streda Nové Zámky 1976 ­ 2006 1991 ­ 2006 1941 ­ 2006 1991 ­ 2006 1976 ­ 2006 1941 ­ 2006 1991 ­ 2006 1976 ­ 2006 1941 ­ 2006 1991 ­ 2006 1976 ­ 2006 1971 ­ 2006 1991 ­ 2006 1991 ­ 2006 The frequency of drought periods, as well as total duration of droughts is higher in the upper ­ mountainous part of the basin. Long droughts occurred in the whole basin in 1991 ­ 1994. The longest drought in the observed period occurred in the same period in Nadlice profile; lasting for 1062 days. The next base flow drought covering the whole basin was the 2003 ­ 2004 drought, which in the Nitrianska Streda profile lasted till the beginning of 2005. The comparison of the drought severity in respective sub-basins according to standardized base 191 flow drought severity index for the common period of 1991 ­ 2006 was done. Only the most severe droughts in each respective profile are showed in Tab. 6. In this period, the most severe drought according to standardized base flow drought index occurred in Nadlice profile in the years 1991 ­ 1994. The most severe drought in the Nitrianska Streda profile was divided into two sub-periods, the first one occurred in 1990 ­ 1991 (lasting for 182 days) with the index value of 62.8, being followed within a few months with the second one in 1991 ­ 1993 period with the value of 53.1 and lasting for 566 days. However, the most severe drought in the basin during the whole observed period occurred in Vieska nad Zitavou profile in 1962 ­ 1963 with the standardized base flow drought severity index of 159.6 (lasting for 313 days) and in Nedozery profile in 1946 ­ 1948 with the index value of 127.6 (lasting for 564 days). T a b l e 6. The most severe droughts in evaluated profiles in the period 1990 ­ 2006. Kacno 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 106.7 D: 1062 Nedozery Nadlice Vieska n. Zitavou 90.56 D: 222 Nitrianska Streda 62.8 D: 182 53.1 D: 566 Nové Zámky 97.82 D: 239 45.8 D: 150 83.64 D: 651 What the drought seasonality is concerned, in the upper part as well as in the central part of the basin, the summer-autumn and summer-winter droughts have approximately the same occurrence frequency. In the lowest profile at Nové Zámky, the summer autumn drought prevails and in the highest profile at Kacno, the spring-winter drought was the most frequent. When comparing the frequency of drought periods and their total length in the base flow with the discharge drought occurrence in the same profiles, it was documented by Fendeková and Fendek (2011) that discharge drought is more frequent, but lasts shorter ­ droughts with the duration of 10 ­ 49 days prevail. According to the same authors (Fendeková, Fendek, 2011), drought in the groundwater heads occurred two-times more frequently in the upper part of the basin, but it lasted shorter as in the lowland part. Conclusion Drought as one of the more frequently occurring phenomenon can be accessed from different points of view using various characteristics. Such disparateness is followed by more and more drought indices occurring in the literature. Authors of this paper evaluated the drought in a yearly scale. The standardized base flow drought severity index was proposed for base flow drought characterization. It was documented that years with the extremely low base flow occur much often in the mountainous part of the Nitra River Basin than in the lowland part. Similarly, differences in seasonality and base flow drought duration between the upper, central and downstream part of the basin were showed. As it follows from the most severe droughts occurrence analysis, there is no unified pattern of the drought occurrence in the Nitra River basin. It was confirmed that the upper part of the basin is more sensitive on extreme climatic situation occurrence leading to drought in surface and groundwater. Acknowledgment. The authors would like to thank to FP6 Watch project (contract Number 036946), from which financial sources the research was supported. Groundwater drought in the Nitra River basin ­ identification and classification REFERENCES ANON 2000: Hydroekologický plán povodia Nitry v úseku od pramea po zaústenie prelozky Nitry. Cas A. Slovenský vodohospodársky podnik s. p. 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Has streamflows changed in the Nordic countries? ­ Recent trends and comparison to hydrological projections. J. Hydrol., 394, 334­346. www.hydrooffice.org (march 2012) Received 23 August 2011 Accepted 13 March 2012

Journal

Journal of Hydrology and Hydromechanicsde Gruyter

Published: Sep 1, 2012

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