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P. Jokiel and P. Tomalski
– Average seasonality indices IS changed with the course of both rivers in a very
similar way from 32% to 22% and were comparable to those obtained for large
Carpathian, upland and lowland rivers [4, 12] but clearly larger (by about 10%)
than the values calculated for large lake rivers [8]. Similar changes were observed
for flow seasonality coefficient GMO as it describes basically the same aspect of
flow seasonality. GMO declined from about 15 to 9.5. These results are comparable
to those obtained for the Carpathian rivers [12, 21].
– A clear correlation was found between the concentration date (PK) and halfflow date (TPO) for both rivers. These parameters demonstrate a linear trend
and PK grows in line with TPO. However, their regression lines have different
inclinations. The rise is slower for the Vistula, but this difference ceases as TPO
and PK decrease with a growing area of both basins.
– Covariation between average seasonality indices IS and flow seasonality coefficient GMO is less pronounced in each of the rivers than in the case of PK and
TPO. However, when the water gauges from both river basins are considered,
the relationship between average IS, and GMO is clear and its curvilinear model
indicates IS growth along with GMO increase. Thus, the increase in the catchment
area significantly reduces flow seasonality decline.
The river flow shows long-term oscillations due to changes in the water balance
components. Genetic diversity of multiannual fluctuations in flow time series has
been advocated by many authors [2, 7, 24]. This analysis also shows about 40 years
long cycle of changes in average flow (SQ) for the gauges closing the upper, middle
and lower sections of the Vistula and the Oder. A similar, though slightly longer cycle
was identified for an aggregated flow from both those basins [16]. These fluctuations
are often associated with North Atlantic Oscillation NAO [20, 22, 23], or even North
Atlantic Thermahaline Circulation NA THC [16]. In most cases, a negative correlation
of various NAO indices with the flows of rivers from southern and even central Poland
can be confirmed. This analysis also showed a significant (α = 1%, Student’s t-test)
correlation between the gauges closing the catchments of the upper and middle
sections of the Vistula and the Oder (R: −0.55 and −0.43 for the Vistula and −0.46
and −0.42 for the Oder) with NAO index (Hurrell). Despite the confirmed influence
of the North Atlantic Oscillation NAO on flow size, flow seasonality measures show
only insignificant correlation coefficients with these indices at α = 1% (Student’s
t-test). This may suggest that NAO related circulation changes over Poland affect
the amount of water discharged with the rivers but not seasonality of the flow. Our
analysis enabled drawing some basic conclusions on changes in flow seasonality
over time:
– Multiannual changes in the concentration date (PK) for the Vistula and the Oder
irregularly oscillate around average values, and average PK values for the upper,
middle and lower parts of the catchments are nearly identical (Fig. 10.7).
– The similarity of PK indices in individual years for the Vistula and the Oder in the
analyzed multi-year period occurred in the lower and middle course of the rivers.
For the upper sections, the differences between rivers are larger, and for the last
30 years, there is a growing tendency for earlier concentration date for the Oder.
P. Jokiel and P. Tomalski
– Average seasonality indices IS changed with the course of both rivers in a very
similar way from 32% to 22% and were comparable to those obtained for large
Carpathian, upland and lowland rivers [4, 12] but clearly larger (by about 10%)
than the values calculated for large lake rivers [8]. Similar changes were observed
for flow seasonality coefficient GMO as it describes basically the same aspect of
flow seasonality. GMO declined from about 15 to 9.5. These results are comparable
to those obtained for the Carpathian rivers [12, 21].
– A clear correlation was found between the concentration date (PK) and halfflow date (TPO) for both rivers. These parameters demonstrate a linear trend
and PK grows in line with TPO. However, their regression lines have different
inclinations. The rise is slower for the Vistula, but this difference ceases as TPO
and PK decrease with a growing area of both basins.
– Covariation between average seasonality indices IS and flow seasonality coefficient GMO is less pronounced in each of the rivers than in the case of PK and
TPO. However, when the water gauges from both river basins are considered,
the relationship between average IS, and GMO is clear and its curvilinear model
indicates IS growth along with GMO increase. Thus, the increase in the catchment
area significantly reduces flow seasonality decline.
The river flow shows long-term oscillations due to changes in the water balance
components. Genetic diversity of multiannual fluctuations in flow time series has
been advocated by many authors [2, 7, 24]. This analysis also shows about 40 years
long cycle of changes in average flow (SQ) for the gauges closing the upper, middle
and lower sections of the Vistula and the Oder. A similar, though slightly longer cycle
was identified for an aggregated flow from both those basins [16]. These fluctuations
are often associated with North Atlantic Oscillation NAO [20, 22, 23], or even North
Atlantic Thermahaline Circulation NA THC [16]. In most cases, a negative correlation
of various NAO indices with the flows of rivers from southern and even central Poland
can be confirmed. This analysis also showed a significant (α = 1%, Student’s t-test)
correlation between the gauges closing the catchments of the upper and middle
sections of the Vistula and the Oder (R: −0.55 and −0.43 for the Vistula and −0.46
and −0.42 for the Oder) with NAO index (Hurrell). Despite the confirmed influence
of the North Atlantic Oscillation NAO on flow size, flow seasonality measures show
only insignificant correlation coefficients with these indices at α = 1% (Student’s
t-test). This may suggest that NAO related circulation changes over Poland affect
the amount of water discharged with the rivers but not seasonality of the flow. Our
analysis enabled drawing some basic conclusions on changes in flow seasonality
over time:
– Multiannual changes in the concentration date (PK) for the Vistula and the Oder
irregularly oscillate around average values, and average PK values for the upper,
middle and lower parts of the catchments are nearly identical (Fig. 10.7).
– The similarity of PK indices in individual years for the Vistula and the Oder in the
analyzed multi-year period occurred in the lower and middle course of the rivers.
For the upper sections, the differences between rivers are larger, and for the last
30 years, there is a growing tendency for earlier concentration date for the Oder.