12 Dynamics, Range, and Severity of Hydrological …
245
years depends not only on precipitation but also results from the ongoing recession
of groundwater resources in the catchment. Depletion and recharge of these water
reserves show huge inertia that causes droughts of similar properties to group in a
few year-long periods.
12.5.2 Degree of Seasonality and Concentration Date
of Hydrological Drought
The degree of hydrological drought severity depends mainly on hydrometeorological
conditions in the period preceding its occurrence. Deficit of water resources may be
determined by seasonal anomalies associated with a lack of supply in typical time
of alimentation or multiannual disturbances evoked, e.g. by a series of dry years.
Both groups of factors overlap in a random and practically inseparable way, making
droughts and low flows highly variable events with unpredictable frequency and
intensity.
This strong irregularity in the appearance of drought streamflow deficits considerably hampers seasonal analyses based on time series analyzed in monthly steps (see
Sect. 5.1). A solution to this problem may be applying characteristics that describe
the seasonal variability comprehensively. The variability of the annual course of
drought streamflow deficits and their concentration date may be identified using
angular measures introduced to the literature by Markham [29]. Originally, they
served to analyze seasonal variability of precipitation in the US, but following a few
methodological transformations, two seasonality measures of hydrological drought
were proposed: seasonality index (SI) and seasonal concentration date index (CI)
[7].
Both measures assume that monthly volume of drought streamflow deficit is represented by a vector (r i ) of a length proportional to the volume of this deficit and an
angle of inclination (α i ) depending on the position of the middle of a given month
relative to the beginning of the hydrological year:
α i =
360 · S
365
(12.5)
where:
S number of days between the beginning of the hydrological year and the middle
of the month.
As a result, 12 vectors are obtained for which a resultant vector R of the module
|R| and direction ω is determined (Fig. 12.9). The quotient of the resultant vector
|R| and the total length of the partial vectors |r i | allows for the calculation of the
seasonality index SI:
245
years depends not only on precipitation but also results from the ongoing recession
of groundwater resources in the catchment. Depletion and recharge of these water
reserves show huge inertia that causes droughts of similar properties to group in a
few year-long periods.
12.5.2 Degree of Seasonality and Concentration Date
of Hydrological Drought
The degree of hydrological drought severity depends mainly on hydrometeorological
conditions in the period preceding its occurrence. Deficit of water resources may be
determined by seasonal anomalies associated with a lack of supply in typical time
of alimentation or multiannual disturbances evoked, e.g. by a series of dry years.
Both groups of factors overlap in a random and practically inseparable way, making
droughts and low flows highly variable events with unpredictable frequency and
intensity.
This strong irregularity in the appearance of drought streamflow deficits considerably hampers seasonal analyses based on time series analyzed in monthly steps (see
Sect. 5.1). A solution to this problem may be applying characteristics that describe
the seasonal variability comprehensively. The variability of the annual course of
drought streamflow deficits and their concentration date may be identified using
angular measures introduced to the literature by Markham [29]. Originally, they
served to analyze seasonal variability of precipitation in the US, but following a few
methodological transformations, two seasonality measures of hydrological drought
were proposed: seasonality index (SI) and seasonal concentration date index (CI)
[7].
Both measures assume that monthly volume of drought streamflow deficit is represented by a vector (r i ) of a length proportional to the volume of this deficit and an
angle of inclination (α i ) depending on the position of the middle of a given month
relative to the beginning of the hydrological year:
α i =
360 · S
365
(12.5)
where:
S number of days between the beginning of the hydrological year and the middle
of the month.
As a result, 12 vectors are obtained for which a resultant vector R of the module
|R| and direction ω is determined (Fig. 12.9). The quotient of the resultant vector
|R| and the total length of the partial vectors |r i | allows for the calculation of the
seasonality index SI: