11 Low-Flows in Polish Rivers
225
Analysis of the relationship between maximum duration of observed and predicted
(99%) low-flow event allows assessing the potential risk of very extremely long
drought period occurrence. This parameter was estimated as a quotation of these
two characteristics (T max / T p.99 ). The highest potential risk if the further low-flow
episode would be much longer than observed, appears in rivers with a low value
of index: lower course of the Vistula and Oder river and some lowland catchments
with restricted water resources (Fig. 11.13). Relationships between the low risk
of crucial low-flow lengthening and available water resources are characterized by
upland (significant groundwater alimentation) and mountainous (high and dynamic
precipitation alimentation) catchments.
Severe low-flow episodes exceeding maximum credible duration appeared very
often (Fig. 11.6). In 10 cases, the limit of 99% probability was exceeded as well.
Moreover, the probability range 95–99% in many catchments involved the interquartile deviation reach. Such image of distribution indicates that hydrogeological limitations, as well as physiographical features of the catchment, are more important than macroscale hydrometeorological conditions. In rivers where maximum
observed severe low-flow duration exceeded level of 99% probability, there is a very
small chance for significantly longer event appearance. Seasonal assessment of this
phenomenon might be important for prediction of water ecosystem condition and
improvement of water management effectiveness. Therefore, for every catchment,
the month with the highest number of low-flow episodes exceeding 95% probability
of its duration was identified (Fig. 11.13). Selected months, which are estimators of
extremely long severe low-flows appearance time, occurred in summer and autumn
season. It proves that precipitation shortage and evapotranspiration process are the
main determinants of severe hydrological drought in the scale of the whole country.
The only excepts are high mountainous areas (Dunajec) where winter low-flows are
basic low-flow regime determinant.
11.6 Conclusions and Recommendations
Poland belongs to territories which are put at risk of occurrence of hydrological
droughts at serious degree of severity. It results in periodic water shortage, a decrease
of groundwater table, and occurrence of low-flows in rivers. Multidirectional analysis of low-flows allowed to assess low-flow development dynamics as well as identify multiannual and seasonal features of drought streamflow deficits regime. Good
knowledge of these processes and phenomena may effectively support the tools and
strategies of optimal reduction of drought results, its prevention, and prediction.
It is worth noticing that knowledge of low-flow regime is very important for water
balance structure which may be crucial for the identification of quantity and dynamics
of available water resources.
Dynamics of low-flow episodes progression is characterized by high seasonal and
multiannual variability. Its basis is determined by strong differentiation of hydrometeorological conditions which generate particular episodes as well as modifying the
225
Analysis of the relationship between maximum duration of observed and predicted
(99%) low-flow event allows assessing the potential risk of very extremely long
drought period occurrence. This parameter was estimated as a quotation of these
two characteristics (T max / T p.99 ). The highest potential risk if the further low-flow
episode would be much longer than observed, appears in rivers with a low value
of index: lower course of the Vistula and Oder river and some lowland catchments
with restricted water resources (Fig. 11.13). Relationships between the low risk
of crucial low-flow lengthening and available water resources are characterized by
upland (significant groundwater alimentation) and mountainous (high and dynamic
precipitation alimentation) catchments.
Severe low-flow episodes exceeding maximum credible duration appeared very
often (Fig. 11.6). In 10 cases, the limit of 99% probability was exceeded as well.
Moreover, the probability range 95–99% in many catchments involved the interquartile deviation reach. Such image of distribution indicates that hydrogeological limitations, as well as physiographical features of the catchment, are more important than macroscale hydrometeorological conditions. In rivers where maximum
observed severe low-flow duration exceeded level of 99% probability, there is a very
small chance for significantly longer event appearance. Seasonal assessment of this
phenomenon might be important for prediction of water ecosystem condition and
improvement of water management effectiveness. Therefore, for every catchment,
the month with the highest number of low-flow episodes exceeding 95% probability
of its duration was identified (Fig. 11.13). Selected months, which are estimators of
extremely long severe low-flows appearance time, occurred in summer and autumn
season. It proves that precipitation shortage and evapotranspiration process are the
main determinants of severe hydrological drought in the scale of the whole country.
The only excepts are high mountainous areas (Dunajec) where winter low-flows are
basic low-flow regime determinant.
11.6 Conclusions and Recommendations
Poland belongs to territories which are put at risk of occurrence of hydrological
droughts at serious degree of severity. It results in periodic water shortage, a decrease
of groundwater table, and occurrence of low-flows in rivers. Multidirectional analysis of low-flows allowed to assess low-flow development dynamics as well as identify multiannual and seasonal features of drought streamflow deficits regime. Good
knowledge of these processes and phenomena may effectively support the tools and
strategies of optimal reduction of drought results, its prevention, and prediction.
It is worth noticing that knowledge of low-flow regime is very important for water
balance structure which may be crucial for the identification of quantity and dynamics
of available water resources.
Dynamics of low-flow episodes progression is characterized by high seasonal and
multiannual variability. Its basis is determined by strong differentiation of hydrometeorological conditions which generate particular episodes as well as modifying the