Hrvatske vode, Vol. 31 No. 125, 2023.
Original scientific paper
Hydrological analysis of the Sava River water level and flow series at the Zagreb hydrological station
Ognjen Bonacci
orcid.org/0000-0002-7123-3421
; Sveučilište u Splitu, Fakultet građevinarstva, arhitekture i geodezije Matice hrvatske 15, Split, Hrvatska
*
Tanja Roje-Bonacci
; Sveučilište u Splitu, Fakultet građevinarstva, arhitekture i geodezije Matice hrvatske 15, Split, Hrvatska
* Corresponding author.
Abstract
The paper analyses the series of water levels (1920-2021) and discharges (1926-2021) observed at the Zagreb hydrological station on the Sava River. Significant variations between the series of minimum and average annual levels were determined. In the 1920-1974 sub-period, no trends were observed. In the 1975-1993 sub-period, a more significant downward trend (about 2 m) was observed. Minimal annual water levels are an indicator of the lowering (erosion) of the riverbed bottom. In 1983, a submerged weir, named Te-To sill, was constructed. It is located about 6400 m downstream of the Zagreb hydrological station. The lowering of the Sava riverbed bottom on the Zagreb profile stopped in 1994. In the 1994-2021 period, the minimum and average annual water levels rose by about 70 cm. A series of maximum annual water levels in the 1920-2021 period showed a statistically insignificant increase. At the Sava Podsused hydrological station, about 11 km away, the process of varying minimum and average annual water levels occurs similarly, but with a slight time shift, as it does at the Sava Zagreb hydrological station. The minimum and average water levels there began to decline simultaneously, while the increase began seven years later (in 2001). At the Sava Čatež hydrological station, which is located in Slovenia, the minimum annual water levels decreased in 1967, eight years earlier than at the Sava Zagreb location.
For the series of minimum annual flows of the Sava near Zagreb, a statistically significant drop in average annual flows was observed in the 1979-2021 sub-period (Q=71.4 m3/s) compared to the previous 1926-1978 sub-period (Q=88.3 m3/s). A statistically significant drop in average annual flows was observed in the 1981-2021 sub-period (Q=287 m3/s) for a series of average annual flows of the Sava near Zagreb compared to the previous 1926-1980sub-period (Q=326 m3/s). For a series of maximum flows in the recent 2004-2021 sub-period (Q=2033 m3/s), a statistically significant increase was observed compared to the previous 1926-2003 sub-period (Q=1772 m3/s). For a series of average annual flows in the 1926-2021 period, the value of the Hurst exponent was 0.691. A series of average daily flows of the Sava near Zagreb was analysed using the day-to-day flow variability (DTDQ) method.
Keywords
water level, flow; Sava Zagreb; day-to-day flow variability; Hurst exponent
Hrčak ID:
316420
URI
Publication date:
28.9.2023.
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