MAIN HYDROLOGICAL STATISTICAL CHARACTERISTICS OF LOW WATER AND HIGH WATER ON THE TUR/TÚR STREAM (II)

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1 MAIN HYDROLOGICAL STATISTICAL CHARACTERISTICS OF LOW WATER AND HIGH WATER ON THE TUR/TÚR STREAM (II) K. KONECSNY 1, Z. NAGY 2 ABSTRACT. Main hydrological statistical characteristics of low water and high water on the Tur/Túr stream. The Tur/Túr river network (1261 km 2 ) is the secondary left bank tributary of Tisa. The catchment is shared by Romania (96%) and Hungary (4%). Daily streamflow data for the description of low flow conditions are available since, namely 49 years long series for, (RO) and (HU) consequently. Lower reaches of the river were considerable modified during the 19 th and 2 th century river training and flood protection works including the topology and geometry of channel network, while the hydrological regime of the upper hilly and undulating regions remained nearly natural until Discharge series until 1973 are or only slightly influenced, while the impact of reservoir Călineşti-Oaş the temporal distribution of runoff is felt after that period. Key words: minimum and maximum discharge, low flow periods, high flow periods, hydrological statistical analysis, water uses, discharge values probabilities. 3. Some hydrologic statistical characteristics of floods Characteristic of Tur are annual peaks in winter and early spring, when the development of flood waves is of mixed type. On the highland section of the river, at the section the annual maxima of discharges occur most frequently in March (28, %) and April (18,4 %), when the snow melting significantly influences the river development. In the summer semester (IV-IX) the rate of annual maxima is higher (1,%) than in the winter semester (X-III). There did not occur any annual maximum discharges in February, June or September (Figure 11). Considering that snow melts faster in the lowland river section and in winter several snow melt periods can occur, the maxima of streamflows occur the most frequently in March ( 21, %, 22,66 %) and January ( 1 National Inspectorate for Environment, Nature and Water, 116 Budapest, Hungary, konecsnyk@gmail.com 2 Upper Tisza Region Environmental Protection and Water Directorate, 44. Nyíregyháza, Hungary, nagyz@fetikovizig.hu 183

2 21,4 %, 2,8 %). In case of both sections September was the month when none annual maximum discharge occurred in the analysed 49 years. At the Călineşti-Oaş reservoir, situated below but above and, due to the temporary retain of flood waves, the maximum discharges decrease and extend in time, but the duration of the high streamflows due to the pre and post drain significantly increases. On the river, the maximum discharges of the flood wave in March, 21, were the following: 14,2 m 3 s -1, Călineşti-Oaş 16 m 3 s -1, 223 m 3 s -1, 21 m 3 s -1. The flood peak of the reservoir decreases, so the maximum discharge run-off flowing into the reservoir reached 229 m 3 s -1 on the th of March, the maximum discharge let out from the reservoir was only 16 m 3 s -1. So the flood decreasing effect was of 4 % (Konecsny et al. 21). The reservoir decreased the height of peak water levels, but in the same time it increased the run-off time % , Neg reş ti Oaş 18,4 12,2 1,2 1,2 8,2 4,1 4,1 4,1,,, I II III IV V VI VII VIII IX X XI XII % ,4 21, 16,1 8,9 7,1 7,1 7,1 3,6 1,8 1,8 3,6, I II III IV V VI VII VIII IX X XI XII % ,6 2,8 13,2,7 7, 7, 7,,7 3,8, 1,93,8 I II III IV V VI VII VIII IX X XI XII Figure 11. Seasonal frequencies of annual maxima of daily mean flow. In case of the three sections on Tur, the linear trend of the annual maxima of discharges and the annual maxima of daily mean flow is decreasing, so in the analysed period, -29, the annual maxima are getting smaller (Figure 12). Q (m3/s) Qmax Qmax mean day Qmax Qmax mean day Qmax Qmax mean day Figure 12. The development of annual maxima and linear trend of streamflows between

3 This decreasing trend is equally applicable for the sections of the hydrographical stations at, and. As the Negreşti- Oaş hydrometrical station is situated above the Călineşti-Oaş reservoir, in this case the reservoir cannot influence the high streamflow, which leads to the conclusion that the direction of change occurred due to natural processes and it cannot be solely explained by the flood decreasing effect of the reservoir. Compared to the section, the smaller decrease on the section can be explained with the fact that the maximum discharge of the high flood waves observed especially in the first half of the period due to the flood wave flatting and the quantity of water flowed through the breach in dike was significantly lower at than at. Therefore, for example the two highest annual maxima of discharge were 19 m 3 s -1 (197) and 314 m 3 s -1 () at while at it was 274 m 3 s -1 (197), 29 m 3 s -1 (). In the hydrologic statistical investigation of the high water streamflow we established a critical high water discharge (Q max kr ), such a threshold that if it is reached or exceeded by the streamflow, it is considered high water discharge. When we established these values we considered the discharge corresponding to the Is level flood control level (which corresponds to the state of the saturated low water bed). Due to the natural river bed processes and the technical interventions (for example dredging), the connexion between the discharge and the water level is varying in time. Therefore, for example at the hydrographical station Túr in the period and 29 the discharge corresponding to the Ist grade flood control level varied between, m 3 s -1 and 88, m 3 s Figure 13. The development in time of the discharge corresponding to the Ist grade flood control level depending on the Q-H correlations at hydrographical station. 18

4 Table 4. The development of discharges corresponding to the Ist grade flood control level at the three hydrographical stations situated on the Tur River Station Ist grade control in 29 Q max mean mon [m 3 s -1 ] Q max mean mon 2% [m 3 s -1 ] Q max cr [m 3 s -1 ] H [cm] Q [m 3 s -1 ] 17 17, 4,11 6,29, 36 47,6 43,6 6,6, 3 3, 36,7,4, Therefore, to determinate the threshold of critical high discharge (Q max cr ) more hydrological characteristics were considered (Table 4). These are: - the value of discharge corresponding to the Ist grade flood control level indicating the saturation of the low water bed; - the many years mean value calculated based on the 143 values of the monthly maxima of mean discharges (Q max mean mon ) in the period with a natural close fluctuation, - the value with a probability of 2% of the monthly maxima of discharges in the period -1972(Q max mean mon 2% ). Based on the thresholds of high water discharge (Q max cr ) concerning to the analysed stations, the annual number of high flow periods, the annual number of days with high water, the duration (days) and the linear trend of the annual longest high flow periods were determined for the period between -29. At the Tur station, in the analysed period there were totally identified 9 high flow periods, but in 8 years did not occur such a discharge. Yearly occurred an average of 1,9 events, maximum 6 events. The time series indicates a slight decreasing trend. At station, in the analysed period there were totally 161 high flow periods, but in years none critical high water flow occurred. Yearly there occurred an average of 3,3 events, maximum 7 events. The linear trend of the time series is significantly increasing. At there totally occurred 14 high flow periods, but in 6 years there did not occur any critical high water flow. Every year there where an average of 2,9 events, maximum 9 events. The linear trend of the time series does not indicate any significant change. At Tur station, in the analysed period the annual avarage number of days with a flow exceeding the high water threshold (Q max cr ) was 4,3 days, maximum 17 days, at 11,9 days and 31 days, at 11,2 days and 28 days. The time series indicates a slight decreasing trend at, a significant increase at, a slight increase at (Figure 1). If we consider the annual number of days with the longest high flow periods exceeding the threshold (Q max cr ), at the mean value is 2,6 days, the maximum is 11 days, at,9 days and 16 days, at 6 days and 1 days. 186

5 Events number 6 4 Events number 6 4 Events number Figure 14. Annual number of high flow periods exceeding the critical discharge value (Q max cr ) Days 2 1 Days 2 1 Days Figure 1. The annual number of days with discharge exceeding the critical discharge value (Q max cr ). The main data concerning the discharge in high flow periods are collected in Table. Table. Characteristic discharges in extreme high water years at three Hydro metric stationon Tur stream Q max [m 3 s -1 ] Q max day [m 3 s -1 ] Duration of high flow periods below Q max cr [day] Number of low flow events, over Q max cr [-] 12,9 27,4/197 8,79 21,8/197 4,3 17/1962 1,9 6/ / /197 11,9 31/199 3,3 7/,9, / / ,2 28/1979 2,9 9/

6 3. The comparison of low flow and flood extremes The difference between the annual maxima and minima of discharges (Q max -Q min ) and the development of the rate between these two extreme discharge values (Q max /Q min ) indicates the extreme characteristic of water level fluctuation (Figure 16 and Figure 17). At the many years average of difference Q max -Q min was 12,8 m 3 s -1, the minimum difference was 3,9 m 3 s -1 (), the maximum difference was 27,3 m 3 s -1 (197). At the many years average of difference Q max -Q min was 144 m 3 s -1, the minimum 36, m 3 s -1 (), the maximum 19 m 3 s -1 (197). At the many years average of difference Q max - Q min was 122 m 3 s -1 t, the minimum difference was 27,3 m 3 s -1 (), the maximum difference was 296 m 3 s -1 (1974) Figure 16. The annual maxima and minima of differences between discharges (Q max -Q min ). Every year the annual maximum discharge exceeded many times the annual maximum, thus at the average of Q max /Q min rate was 218, but it varied between the extreme values like 37 (199) and 91 (1964). At the average of Q max /Q min rate was 14, but it fluctuated between extreme values such as 66 (198) and 226 (1964). At the average was 222, but it varied between extreme values like 34 (1982) and 964 (1964) Figure 17. The rates of annual extreme discharge value (Q max /Q min ). 188

7 The two index values indicate that of the three hydrometric stations at the station situated below the Călineşti-Oaş reservoir but relatively close to it the water level fluctuation is the most extreme, and at highland the flutuation is the least extreme. 6. CONCLUDING REMARKS In the analysed period between -29 (49 years), at the hydrometric stations from, and, low flow periods below threshold (Q ) could be observed only in 84-96% of years. The average number of days with flows below the discharge threshold typically occurred in 17-2% of days in a year. The time series of the annual minima of discharges and the annual number of low flow days in 49 years does not indicate a definite directional trend. As regards the short-term distribution, it was observed that in April and May it has never occurred any annual minima of discharge, moreover, at not in March. The longest low water periods typically occurred in the period between July-November, but in extreme situation they started even from the end of spring (May), this time the water deficit can be even -1 million m 3 -, the replacing of which being a difficult task even besides the present capacity of the reservoir. Among the main characteristics of the high flows in the investigated period of 49 years, let us highlight that periods with discharges exceeding critical high flow values (Q max cr ) could be observed in % of years. The average number of days with discharges above the critical high flow is 4 on the upper river section, and days on the lower section. The linear trend of the annual maxima of discharges and the annual maxima of daily mean discharge can be equally characterised by a decreasing trend. On the highland river section, the annual maxima of discharge occurred the most frequently in March and April, on the lowland sections in March and January. In case of all three sections, in September did not occur any annual maximum discharge, but on the upper section nor in February or June. The analyse of the extreme discharge value indicates that of the three hydrometric stations, at the station, which is situated below the Călineşti- Oaş reservoir but relatively close to it, the water level fluctuation is the most extreme, and at the highland the fluctuation is the least extreme. The extremes within the year, so the differences between the annual minima and maxima, as well as the rates, all decreased. REFERENCES 1. Hisdal, H., Tallaksen, L.M., Peters, E., Stahl, K., Zaidman, M.. (21), Drought event definition, In: Assessment of the regional Impact of Droughts in Europe, S. 189

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