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E. Tomaszewski and K. Kubiak-Wójcicka
11.1 Introduction
Hydrological extremes are of great significance for river flow structure. They define
the range of river regime dynamics as well as indicate directions of water management
operations concerning mitigation of their negative effects. Low-flows, in contrast
with floods, develop very slowly. On the one hand, it is relatively simple to estimate
drought streamflow deficit volume because of high process inertia, on the other, it
is difficult to predict when the low-flow episodes terminate because of high inertia
as well. Therefore the occurrence of low-flow periods is determined not only by
stochastic factors connected with restricted alimentation but also depends on the
spatial pattern of water balance structure which allows identifying areas with different
risk levels of drought appearance. Many projections and scenarios of climate changes
in Poland prove that further rise in air temperature is expected. It will result in
higher evapotranspiration and adverse structure of water balance. Moreover, there is
a predicted higher density and intensity of hydrometeorological extremes, especially
in interior zones [1–3]. Wide and multidirectional analyse of the low-flow regime,
in the context of presented facts, seems to be very important for research and practical purposes. Results should improve low-flow prediction methods and support the
implementation of water shortage effect mitigation strategies, especially in the field
of hydropower, water supply, agriculture, inland water transport, etc.
The river low-flow is commonly defined as a period of low flows (water levels)
in a river or flows during prolonged dry weather [4, 5]. This process is usually initiated by rainfall shortage—meteorological drought (see chapter Dynamics, Range,
and Severity of Hydrological Drought in Poland). Prolonged lack of precipitation
combined with intense evapotranspiration results in gradual loss of soil moisture
within the vadose zone—agricultural drought. It may lead to significant depletion
of groundwater resources in the hydrologically active zone (groundwater low-flow)
and in hydraulically connected river beds (surface water low-flow)—hydrological
drought. Thus, the streamflow drought that occurs at the end of the chain of events
(adverse in terms of water management) can be considered as a reliable indicator of
hydrological drought development [5, 6]. Winter low-flows in rivers follow a different
course. Limited runoff is then due to temporary water retention in the snow cover
often combined with riverbed freezing during severe frosts that stop all forms of
drainage. It is worth noticing that the low-flow episode has a highly seasonal nature
in this case because all of the water entrapped in snow cover will break the drought
period during spring snowmelt alimentation. Low-flow appearance may temporary disorder river and valley ecosystems existence. Low-flow episodes can seriously hinder realization of water management tasks and during severe hydrological
drought may lead to serious consequences of social, economic, and environmental
matter—socioeconomic drought [7].
Observations of river low-flows have been conducted from the down of history.
The oldest notes about dramatically low water stages on Polish lands concern
years: 988, 1121, 1332, 1473 [8]. However, the contemporary methodology of
this phenomenon assessment not until 50. of XX century appeared. Its origin was
E. Tomaszewski and K. Kubiak-Wójcicka
11.1 Introduction
Hydrological extremes are of great significance for river flow structure. They define
the range of river regime dynamics as well as indicate directions of water management
operations concerning mitigation of their negative effects. Low-flows, in contrast
with floods, develop very slowly. On the one hand, it is relatively simple to estimate
drought streamflow deficit volume because of high process inertia, on the other, it
is difficult to predict when the low-flow episodes terminate because of high inertia
as well. Therefore the occurrence of low-flow periods is determined not only by
stochastic factors connected with restricted alimentation but also depends on the
spatial pattern of water balance structure which allows identifying areas with different
risk levels of drought appearance. Many projections and scenarios of climate changes
in Poland prove that further rise in air temperature is expected. It will result in
higher evapotranspiration and adverse structure of water balance. Moreover, there is
a predicted higher density and intensity of hydrometeorological extremes, especially
in interior zones [1–3]. Wide and multidirectional analyse of the low-flow regime,
in the context of presented facts, seems to be very important for research and practical purposes. Results should improve low-flow prediction methods and support the
implementation of water shortage effect mitigation strategies, especially in the field
of hydropower, water supply, agriculture, inland water transport, etc.
The river low-flow is commonly defined as a period of low flows (water levels)
in a river or flows during prolonged dry weather [4, 5]. This process is usually initiated by rainfall shortage—meteorological drought (see chapter Dynamics, Range,
and Severity of Hydrological Drought in Poland). Prolonged lack of precipitation
combined with intense evapotranspiration results in gradual loss of soil moisture
within the vadose zone—agricultural drought. It may lead to significant depletion
of groundwater resources in the hydrologically active zone (groundwater low-flow)
and in hydraulically connected river beds (surface water low-flow)—hydrological
drought. Thus, the streamflow drought that occurs at the end of the chain of events
(adverse in terms of water management) can be considered as a reliable indicator of
hydrological drought development [5, 6]. Winter low-flows in rivers follow a different
course. Limited runoff is then due to temporary water retention in the snow cover
often combined with riverbed freezing during severe frosts that stop all forms of
drainage. It is worth noticing that the low-flow episode has a highly seasonal nature
in this case because all of the water entrapped in snow cover will break the drought
period during spring snowmelt alimentation. Low-flow appearance may temporary disorder river and valley ecosystems existence. Low-flow episodes can seriously hinder realization of water management tasks and during severe hydrological
drought may lead to serious consequences of social, economic, and environmental
matter—socioeconomic drought [7].
Observations of river low-flows have been conducted from the down of history.
The oldest notes about dramatically low water stages on Polish lands concern
years: 988, 1121, 1332, 1473 [8]. However, the contemporary methodology of
this phenomenon assessment not until 50. of XX century appeared. Its origin was