12 Dynamics, Range, and Severity of Hydrological …
235
Finally, for each of 87 investigated catchments the study identified the years and
months with hydrological drought described by: the number of days with hydrological drought (N D ), the volume of drought streamflow deficit during drought (V D ),
and a coefficient of severe hydrological drought contribution calculated as a quotient
of severe streamflow deficit and total streamflow deficit during drought:
C S D =
V S D
V D
· 100%
(12.2)
where:
CS D coefficient of severe hydrological drought contribution (%),
VS D volume of severe streamflow deficit during drought (m
3 ),
V D volume of total streamflow deficit during drought (m
3 ).
A comprehensive assessment of the hydrological drought at the country-wide
scale required a definition of spatial measures that enabled a comparative analysis
on a multiannual scale. The first measure is a range of hydrological drought that
indicates the part of the country affected by hydrological drought in a given year or
month [21]:
R D =
A D
A
· 100%
(12.3)
where:
R D
a range of hydrological drought (%),
A D total area of the catchments affected by the drought (km
2 ),
A total area of all investigated catchments (km
2 ).
Relative discharge deficits for individual low flow periods in a catchment (RVn)
may be used to work out this deficit associated with drought (RV D ), which also serves
as an estimator of the drought severity. The weighted average can then be used to
calculate the hydrological drought severity index for entire Poland in monthly or
yearly intervals [21]:
S D =
N
i=1 (RV Di · A i )
N
i=1 A i
(12.4)
where:
S D
hydrological drought severity index (%),
RV Di a relative deficit of streamflow during a drought in catchment i (%),
A i
catchment i area (km
2 ),
N
number of catchments affected by hydrological drought.
235
Finally, for each of 87 investigated catchments the study identified the years and
months with hydrological drought described by: the number of days with hydrological drought (N D ), the volume of drought streamflow deficit during drought (V D ),
and a coefficient of severe hydrological drought contribution calculated as a quotient
of severe streamflow deficit and total streamflow deficit during drought:
C S D =
V S D
V D
· 100%
(12.2)
where:
CS D coefficient of severe hydrological drought contribution (%),
VS D volume of severe streamflow deficit during drought (m
3 ),
V D volume of total streamflow deficit during drought (m
3 ).
A comprehensive assessment of the hydrological drought at the country-wide
scale required a definition of spatial measures that enabled a comparative analysis
on a multiannual scale. The first measure is a range of hydrological drought that
indicates the part of the country affected by hydrological drought in a given year or
month [21]:
R D =
A D
A
· 100%
(12.3)
where:
R D
a range of hydrological drought (%),
A D total area of the catchments affected by the drought (km
2 ),
A total area of all investigated catchments (km
2 ).
Relative discharge deficits for individual low flow periods in a catchment (RVn)
may be used to work out this deficit associated with drought (RV D ), which also serves
as an estimator of the drought severity. The weighted average can then be used to
calculate the hydrological drought severity index for entire Poland in monthly or
yearly intervals [21]:
S D =
N
i=1 (RV Di · A i )
N
i=1 A i
(12.4)
where:
S D
hydrological drought severity index (%),
RV Di a relative deficit of streamflow during a drought in catchment i (%),
A i
catchment i area (km
2 ),
N
number of catchments affected by hydrological drought.