12 Dynamics, Range, and Severity of Hydrological …
249
delay in the Oder. Frequency coefficients of CI in the Sudetes are more variable than
in the Carpathians and range from 13 to 45%. The Sudeten rivers also show slightly
higher spatial diversity of the hydrological drought seasonality index (30–60%) than
the Carpathian ones. Low SI is typical for cross-sections located on the rivers with
abundant base flow from capacious aquifers (the Oława, the Bystrzyca) or placed
downstream of the dam reservoirs (the Nysa Kłodzka).
In most upland and lowland rivers, seasonal concentration date of hydrological
drought occurs in August (Fig. 12.11). CI dates from the first half of this month
are typical for rivers the water resources of which are slightly smaller, e.g., for
right tributaries of the middle Oder, left tributaries of the upper and middle Warta,
the Bzura system, the Vistula and the Bug inter-fluve area, and some rivers of the
lake districts. Rapid depletion of small water resources results in a relatively early
seasonal concentration date of hydrological drought in these rivers. In lake systems
with high lake density and a large number of flow-through and outflow lakes, the
drought concentration date occurs slightly later (the Pisa, the Drw˛ eca, the Gwda, the
Drawa, the upper Note´ c) [32]. In the coastal rivers, CI indicates a relatively early
seasonal concentration date of hydrological drought, particularly in the eastern part
of the region. This is directly linked to an early seasonal concentration date of total
flow [33].
Seasonal concentration date of hydrological drought in lowland rivers is highly
stable for a multi-year period. The reason for this is the dominance of summer
half-year low flows. At the same time, seasonality of hydrological drought is high
or very high due to the high similarity of factors determining the formation and
severity of the low flows. In many lake and upland river systems characterized by
a large contribution of groundwater flow, the seasonal concentration date is also
stable for a multi-year period, as permanent base flow effectively buffers the impact
of random precipitation events. This is the reason why upland catchments with the
large and stable contribution of groundwater flow (e.g., the Wieprz) feature very low
seasonality indices of hydrological drought.
The situation may differ in some large transit rivers, in which low flow trends
depend on characteristic features of their tributaries (Fig. 12.11). Mean multiannual
seasonal concentration date of hydrological drought for the Vistula downstream of
the Goczałkowice Reservoir depends on the dynamics of the Carpathian tributaries
and typically falls on 16 October. Downstream of the San entrance, CI occurs 10 days
earlier, and from the mouth of the Bug and the Narew until the mouth of the Vistula,
the date is shifted to 22 September. The difference in mean seasonal concentration
date between the lower Vistula and its autochthonous tributaries may even exceed
two months. Slightly smaller differences are observed for the Oder and the Bug,
where the gradual impact of subsequent tributaries is visible, but the distinctiveness
of low-flow regime is maintained until the estuaries. Although the middle and lower
Note´ c “inherits” low flows from the upper part of the river system and tributaries
from catchments of high lake density, its seasonal concentration date of hydrological
drought falls relatively early. In the transit rivers, the frequency coefficient C along
the river course is lowered when low flows in the tributaries are not synchronized
249
delay in the Oder. Frequency coefficients of CI in the Sudetes are more variable than
in the Carpathians and range from 13 to 45%. The Sudeten rivers also show slightly
higher spatial diversity of the hydrological drought seasonality index (30–60%) than
the Carpathian ones. Low SI is typical for cross-sections located on the rivers with
abundant base flow from capacious aquifers (the Oława, the Bystrzyca) or placed
downstream of the dam reservoirs (the Nysa Kłodzka).
In most upland and lowland rivers, seasonal concentration date of hydrological
drought occurs in August (Fig. 12.11). CI dates from the first half of this month
are typical for rivers the water resources of which are slightly smaller, e.g., for
right tributaries of the middle Oder, left tributaries of the upper and middle Warta,
the Bzura system, the Vistula and the Bug inter-fluve area, and some rivers of the
lake districts. Rapid depletion of small water resources results in a relatively early
seasonal concentration date of hydrological drought in these rivers. In lake systems
with high lake density and a large number of flow-through and outflow lakes, the
drought concentration date occurs slightly later (the Pisa, the Drw˛ eca, the Gwda, the
Drawa, the upper Note´ c) [32]. In the coastal rivers, CI indicates a relatively early
seasonal concentration date of hydrological drought, particularly in the eastern part
of the region. This is directly linked to an early seasonal concentration date of total
flow [33].
Seasonal concentration date of hydrological drought in lowland rivers is highly
stable for a multi-year period. The reason for this is the dominance of summer
half-year low flows. At the same time, seasonality of hydrological drought is high
or very high due to the high similarity of factors determining the formation and
severity of the low flows. In many lake and upland river systems characterized by
a large contribution of groundwater flow, the seasonal concentration date is also
stable for a multi-year period, as permanent base flow effectively buffers the impact
of random precipitation events. This is the reason why upland catchments with the
large and stable contribution of groundwater flow (e.g., the Wieprz) feature very low
seasonality indices of hydrological drought.
The situation may differ in some large transit rivers, in which low flow trends
depend on characteristic features of their tributaries (Fig. 12.11). Mean multiannual
seasonal concentration date of hydrological drought for the Vistula downstream of
the Goczałkowice Reservoir depends on the dynamics of the Carpathian tributaries
and typically falls on 16 October. Downstream of the San entrance, CI occurs 10 days
earlier, and from the mouth of the Bug and the Narew until the mouth of the Vistula,
the date is shifted to 22 September. The difference in mean seasonal concentration
date between the lower Vistula and its autochthonous tributaries may even exceed
two months. Slightly smaller differences are observed for the Oder and the Bug,
where the gradual impact of subsequent tributaries is visible, but the distinctiveness
of low-flow regime is maintained until the estuaries. Although the middle and lower
Note´ c “inherits” low flows from the upper part of the river system and tributaries
from catchments of high lake density, its seasonal concentration date of hydrological
drought falls relatively early. In the transit rivers, the frequency coefficient C along
the river course is lowered when low flows in the tributaries are not synchronized