12 Dynamics, Range, and Severity of Hydrological …
237
In the belt of highlands and lowlands, south of the Note´ c and latitudinal section
of the Vistula and the Bug, the average duration of hydrological drought enhances
markedly. This zone is additionally separated by a longitudinal transect along the
first-order watershed divide isolating the Vistula and the Oder basins (Fig. 12.3).
West of this line, the duration of low flows in the years with hydrological drought
reached five to six months, and its spatial variability was negligible. This indicates
a similar water balance structure, hardly beneficial for water management, and poor
water resources that are due to low retention capability of hydrogeological structures.
The eastern part of the analyzed area is much more diverse. It harbours catchments
with the small annual number of days with drought in the ´
Swi˛ etokrzyskie Mountains
as well as the basin of the Wieprz, where droughts last up to 178 days. This is
associated with a slow rate of groundwater recharge in well-fissured carbonate rocks
of the Lublin Upland. Within the lakeland regions, the average annual number of
days with hydrological drought is shortened and usually does not exceed 150. This
is obviously due to increased rainwater supply in the moraine elevation zone and the
inflow of wet polar-marine air masses from the north-west sector. Interestingly, in
the catchments with flow-through lakes of considerable size (e.g. of the Pisa or the
Łyna), the average number of days with hydrological drought increases up to 180.
This is probably due to enhanced evaporation from the water surface and not too
large retention capacity of such lake harbouring catchments.
Severe hydrological drought markedly disturbs the functioning of facilities and
water management systems. It is caused by a loss of seasonally renewable water
resources and often results in the degradation of water-dependent ecosystems.
Riverbeds then contain limited amounts of water supplied by groundwater reservoirs
renewed in the multiannual cycle. Prolonged severe hydrological drought causes
gradual drying of ever-larger watercourses. This phenomenon was assessed based
on an annual number of days with low flow below Q 95% , assumed as the severe
drought estimator. The contribution of streamflow deficit during a severe drought in
the total volume of the deficit during drought (CS D ) provided data necessary for the
analysis of the structural properties of these events.
Average annual coefficient of severe droughts contribution in Poland equals about
2.7% (Fig. 12.4b). Variability of the parameter is small, as half of the values oscillating around the median fall within a relatively narrow range between 2.3 and
3.3%. The empirical distribution of the parameter is close to normal, which proves a
stationary character of severe drought formation at the country-wide level. In a significant number of cases, the coefficient of severe droughts was inversely proportional
to an annual number of days with hydrological drought (Fig. 12.3). This indicates
some limitations in the development of this drought phase determined mostly by
hydrogeological conditions. The coefficient of severe droughts contribution in the
Carpathian catchments is close to the average for Poland but also more diversified.
These catchments show a lower importance of severe droughts in the rivers below
dam reservoirs (guaranteed flow), and variable CS D determined by the severity of
winter hydrological droughts and variable retention capacity of the catchments that
depends on geostructural and geofiltration factors. The above is true also for the
Sudeten catchments, except for lower importance of winter droughts. The catchments
237
In the belt of highlands and lowlands, south of the Note´ c and latitudinal section
of the Vistula and the Bug, the average duration of hydrological drought enhances
markedly. This zone is additionally separated by a longitudinal transect along the
first-order watershed divide isolating the Vistula and the Oder basins (Fig. 12.3).
West of this line, the duration of low flows in the years with hydrological drought
reached five to six months, and its spatial variability was negligible. This indicates
a similar water balance structure, hardly beneficial for water management, and poor
water resources that are due to low retention capability of hydrogeological structures.
The eastern part of the analyzed area is much more diverse. It harbours catchments
with the small annual number of days with drought in the ´
Swi˛ etokrzyskie Mountains
as well as the basin of the Wieprz, where droughts last up to 178 days. This is
associated with a slow rate of groundwater recharge in well-fissured carbonate rocks
of the Lublin Upland. Within the lakeland regions, the average annual number of
days with hydrological drought is shortened and usually does not exceed 150. This
is obviously due to increased rainwater supply in the moraine elevation zone and the
inflow of wet polar-marine air masses from the north-west sector. Interestingly, in
the catchments with flow-through lakes of considerable size (e.g. of the Pisa or the
Łyna), the average number of days with hydrological drought increases up to 180.
This is probably due to enhanced evaporation from the water surface and not too
large retention capacity of such lake harbouring catchments.
Severe hydrological drought markedly disturbs the functioning of facilities and
water management systems. It is caused by a loss of seasonally renewable water
resources and often results in the degradation of water-dependent ecosystems.
Riverbeds then contain limited amounts of water supplied by groundwater reservoirs
renewed in the multiannual cycle. Prolonged severe hydrological drought causes
gradual drying of ever-larger watercourses. This phenomenon was assessed based
on an annual number of days with low flow below Q 95% , assumed as the severe
drought estimator. The contribution of streamflow deficit during a severe drought in
the total volume of the deficit during drought (CS D ) provided data necessary for the
analysis of the structural properties of these events.
Average annual coefficient of severe droughts contribution in Poland equals about
2.7% (Fig. 12.4b). Variability of the parameter is small, as half of the values oscillating around the median fall within a relatively narrow range between 2.3 and
3.3%. The empirical distribution of the parameter is close to normal, which proves a
stationary character of severe drought formation at the country-wide level. In a significant number of cases, the coefficient of severe droughts was inversely proportional
to an annual number of days with hydrological drought (Fig. 12.3). This indicates
some limitations in the development of this drought phase determined mostly by
hydrogeological conditions. The coefficient of severe droughts contribution in the
Carpathian catchments is close to the average for Poland but also more diversified.
These catchments show a lower importance of severe droughts in the rivers below
dam reservoirs (guaranteed flow), and variable CS D determined by the severity of
winter hydrological droughts and variable retention capacity of the catchments that
depends on geostructural and geofiltration factors. The above is true also for the
Sudeten catchments, except for lower importance of winter droughts. The catchments