11 Low-Flows in Polish Rivers
207
an exceptionally long period of severe drought depleting seriously Polish water
resources in the 1940s and 1950s. New methodical approaches and extending data set
of discharge series allowed to do research on the scale of the whole country [9–15].
Presented authors analysed the spatial pattern and temporal dynamics of low-flows
in different time scales, identifying hydrometeorological and physiographical determinants of this process. Very interesting studies concerning regional scale, e.g. [6,
16–22], discussed widely relations between factors and processes important for the
low-flow formation and very often gave the base for local modelling of water circulation under drought condition. Much attention was given to case studies of simple or
group of rivers. On their base, many methodological and applicable solutions were
presented. Significant progress was made in a matter of probability assessment of
low-flow maximum duration and deficit [6, 23–26]. As a result, a very interesting
algorithm and software were made by Jakubowski & Radczuk [27]. It allows to
estimate automatically all important parameters of low-flow distributions, applying
the best fitted theoretical functions. In recent times, two-dimensional distributions to
extreme low-flows assessment are discussed and tested—copula function and generalized Pareto distribution. Many interesting conclusions gave studies on the impact
of dammed and natural lakes on low-flow [28–30], analyses of low-flow formation in
transit rivers [31, 32] or multidirectional studies on anthropogenic impact [33–35].
Low-flows were also a component of investigations on river regime typology. In
analyses presented by Dynowska [36] or Wrzesi´ nski [37], low-flow periods constitute
a few seasonal phases of flow. In recent times, research in the field of drought risk
management operational systems supporting is developed. It includes characteristics
and models of river low-flows [38]. This approach, based on strategical management
in pursuit of sustainable assurance of water safety for social and natural systems,
ensures the durability of freshwater ecosystems services responsible for maintaining
biodiversity, life processes, and environment reclamation as well as providing people
with economic benefits.
11.2 Methods and Data
Periods of drought streamflow deficits are usually defined on the base of the threshold
where deviations of daily runoff from truncation level are estimated. One of the
most common methods for the low-flow episode delimitation is the threshold level
method, also known as a peak over the threshold. In this approach, a period during
which discharge attains values below an established limit is defined as a streamflow deficit period. Its two basic parameters are the low-flow duration and deficit
volume (Fig. 11.1). Threshold values are estimated based on second degree main
flows (maximum, mean or median value of annual minimum flows), periodic flows
from the flow duration curve such as the percentage of exceedance from the range of
between 70 and 95% or analysis of annual minima distribution. There also conventional flows can be applied. They refer to specific challenges of water management
207
an exceptionally long period of severe drought depleting seriously Polish water
resources in the 1940s and 1950s. New methodical approaches and extending data set
of discharge series allowed to do research on the scale of the whole country [9–15].
Presented authors analysed the spatial pattern and temporal dynamics of low-flows
in different time scales, identifying hydrometeorological and physiographical determinants of this process. Very interesting studies concerning regional scale, e.g. [6,
16–22], discussed widely relations between factors and processes important for the
low-flow formation and very often gave the base for local modelling of water circulation under drought condition. Much attention was given to case studies of simple or
group of rivers. On their base, many methodological and applicable solutions were
presented. Significant progress was made in a matter of probability assessment of
low-flow maximum duration and deficit [6, 23–26]. As a result, a very interesting
algorithm and software were made by Jakubowski & Radczuk [27]. It allows to
estimate automatically all important parameters of low-flow distributions, applying
the best fitted theoretical functions. In recent times, two-dimensional distributions to
extreme low-flows assessment are discussed and tested—copula function and generalized Pareto distribution. Many interesting conclusions gave studies on the impact
of dammed and natural lakes on low-flow [28–30], analyses of low-flow formation in
transit rivers [31, 32] or multidirectional studies on anthropogenic impact [33–35].
Low-flows were also a component of investigations on river regime typology. In
analyses presented by Dynowska [36] or Wrzesi´ nski [37], low-flow periods constitute
a few seasonal phases of flow. In recent times, research in the field of drought risk
management operational systems supporting is developed. It includes characteristics
and models of river low-flows [38]. This approach, based on strategical management
in pursuit of sustainable assurance of water safety for social and natural systems,
ensures the durability of freshwater ecosystems services responsible for maintaining
biodiversity, life processes, and environment reclamation as well as providing people
with economic benefits.
11.2 Methods and Data
Periods of drought streamflow deficits are usually defined on the base of the threshold
where deviations of daily runoff from truncation level are estimated. One of the
most common methods for the low-flow episode delimitation is the threshold level
method, also known as a peak over the threshold. In this approach, a period during
which discharge attains values below an established limit is defined as a streamflow deficit period. Its two basic parameters are the low-flow duration and deficit
volume (Fig. 11.1). Threshold values are estimated based on second degree main
flows (maximum, mean or median value of annual minimum flows), periodic flows
from the flow duration curve such as the percentage of exceedance from the range of
between 70 and 95% or analysis of annual minima distribution. There also conventional flows can be applied. They refer to specific challenges of water management