250
E. Tomaszewski and M. Kozek
with the low flows of the main river [31]. Such situations happen for the Vistula
downstream of the San entrance and for the Oder after it is joined by the Bóbr and
the Nysa Łu˙ zycka and then the Warta.
12.6 Summary and Conclusions
The study demonstrates that employing low flows as indicators of hydrological
drought development seems promising. The adopted identification and separation
criteria allowed for unambiguous isolation of mild and severe hydrological droughts
in Poland in the years 1985–2014. Transformation of the parameters describing the
duration and the relative deficit of streamflow and the share of the catchment in the
entire investigated area enabled the assessment of hydrological droughts in terms of
their duration, severity, and range. It also allowed for pointing out periods differing in
the intensity of the assessed parameters. The evaluation of seasonal and multiannual
variability, as well as analysis of genetic relationships between selected estimators
of hydrological drought, provided new and valuable cognitive insights and made it
possible to identify a group of factors the determine drought development. Practical
conclusions, especially those concerning regional barriers for development and regularities of the duration and range of hydrological drought can significantly expand
water management activities aimed at mitigation of drought effects at the national
and regional scale.
The analysis proves that both the seasonal concentration date and intensity of
hydrological drought in Poland show significant and multidirectional variability.
In the mountain rivers, low-flow regimes are significantly affected by genetically
different summer and winter low flows (CI – September-December), while the rest
of the country is dominated by summer half-year deficits (CI – July–September).
Apart from hydrometeorological conditions, seasonal distribution of hydrological
drought is also determined by local factors associated with water resources and the
rate of their exchange in the hydrologically active zone and with some aspects of
water management. Large transit rivers gradually change the features of their lowflow regime along with the entrance of successive tributaries but retain their regime
specificity up to the estuary. Allochthonous features of low-flow regime inherited
from the upper part of the basin, and larger tributaries are present in the regimes of
the Vistula, the Oder, the Bug, the Note´ c, the Nysa Kłodzka and the Dunajec. The
high degree of hydrological drought seasonality occurs when its average seasonal
concentration date falls in the summer months (Fig. 12.10). When the CI is shifted
towards autumn months, the seasonality index decreases as droughts of the winter
half-year become more and more important.
E. Tomaszewski and M. Kozek
with the low flows of the main river [31]. Such situations happen for the Vistula
downstream of the San entrance and for the Oder after it is joined by the Bóbr and
the Nysa Łu˙ zycka and then the Warta.
12.6 Summary and Conclusions
The study demonstrates that employing low flows as indicators of hydrological
drought development seems promising. The adopted identification and separation
criteria allowed for unambiguous isolation of mild and severe hydrological droughts
in Poland in the years 1985–2014. Transformation of the parameters describing the
duration and the relative deficit of streamflow and the share of the catchment in the
entire investigated area enabled the assessment of hydrological droughts in terms of
their duration, severity, and range. It also allowed for pointing out periods differing in
the intensity of the assessed parameters. The evaluation of seasonal and multiannual
variability, as well as analysis of genetic relationships between selected estimators
of hydrological drought, provided new and valuable cognitive insights and made it
possible to identify a group of factors the determine drought development. Practical
conclusions, especially those concerning regional barriers for development and regularities of the duration and range of hydrological drought can significantly expand
water management activities aimed at mitigation of drought effects at the national
and regional scale.
The analysis proves that both the seasonal concentration date and intensity of
hydrological drought in Poland show significant and multidirectional variability.
In the mountain rivers, low-flow regimes are significantly affected by genetically
different summer and winter low flows (CI – September-December), while the rest
of the country is dominated by summer half-year deficits (CI – July–September).
Apart from hydrometeorological conditions, seasonal distribution of hydrological
drought is also determined by local factors associated with water resources and the
rate of their exchange in the hydrologically active zone and with some aspects of
water management. Large transit rivers gradually change the features of their lowflow regime along with the entrance of successive tributaries but retain their regime
specificity up to the estuary. Allochthonous features of low-flow regime inherited
from the upper part of the basin, and larger tributaries are present in the regimes of
the Vistula, the Oder, the Bug, the Note´ c, the Nysa Kłodzka and the Dunajec. The
high degree of hydrological drought seasonality occurs when its average seasonal
concentration date falls in the summer months (Fig. 12.10). When the CI is shifted
towards autumn months, the seasonality index decreases as droughts of the winter
half-year become more and more important.