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E. Tomaszewski and M. Kozek
of the western part of the lowland belt show a small and spatially variable contribution of severe droughts. In the rest of the country, severe hydrological droughts
play a vital role in catchments that discharge water directly into the sea. However,
considering a relatively efficient precipitation supply and favourable structure of the
water balance in the coastal zone, water management should not be seriously affected
by severe hydrological droughts.
The considerations presented above refer to a typical year and may differ from a
situation when after a series of dry years (see Sect. 12.4) a particularly severe hydrological drought appears and causes damage to the environment and serious losses in
the water management. This problem was assessed based on the maximum observed
coefficients of severe drought contribution (Fig. 12.4c). The average maximum
CS D in relation to a typical year increased by seven times (19.7%). The distribution of the analyzed variable shows outliers that indicate particularly strong effects
of severe droughts on hydrological systems (above 35%: the San, the Rawka, the
Łeba). As these catchments represent different geographical regions (mountains,
lowlands, coastal areas), it may be hypothesized that extremely severe hydrological droughts strongly depend not only on hydrometeorological conditions but also
local catchment-related factors and result from both natural phenomena and human
activities.
The process of hydrological drought development follows a slightly different
pattern along larger transit rivers (Fig. 12.3). The average annual number of days with
hydrological drought in the upper course of the Vistula is mainly determined by its
Carpathian tributaries. Below the entry of the San, the number of such days increases
to 165 per year, but this does not reflect the features of drought in autochthonous
catchments of direct tributaries of the middle Vistula section. Only after the entry of
the Narew and the Bug, the number of days with hydrological drought drops back
to 150 and remains at this level until the Vistula mouth thanks to gradual increment
in water resources along with the growing basin area. Despite the relatively low
value of N D estimated for the Vistula, the contribution of hydrological droughts
is pretty high and can periodically disturb water management. A reverse situation
occurs for the Oder. An average number of days with hydrological drought rises
along the river course and is determined by its right bank lowland tributaries and
droughts developing in the catchment of the Warta. The effects of these factors are
so strong that the increase in water resources of the basin seems only a secondary
factor determining the duration of hydrological droughts along the Oder.
The analysis of the hydrological drought duration and the contribution of severe
droughts was supplemented by verification of the multiannual variability of these
parameters against a systematic component. To this end, the linear trends verified
with Student’s t-test were identified at the significance level α = 0.05 (Table 12.1,
Fig. 12.3). Interestingly, 11 out of 14 significant, multiannual trends of an annual
number of days with hydrological drought had a negative sign. These trends occurred
in the selected mountain and upland catchments and indicated gradual mitigation of
periods with water deficits within the investigated period. The trends with the best
fit (R
2 : 0.28–0.44) appeared in the rivers where large dam reservoirs were built.
E. Tomaszewski and M. Kozek
of the western part of the lowland belt show a small and spatially variable contribution of severe droughts. In the rest of the country, severe hydrological droughts
play a vital role in catchments that discharge water directly into the sea. However,
considering a relatively efficient precipitation supply and favourable structure of the
water balance in the coastal zone, water management should not be seriously affected
by severe hydrological droughts.
The considerations presented above refer to a typical year and may differ from a
situation when after a series of dry years (see Sect. 12.4) a particularly severe hydrological drought appears and causes damage to the environment and serious losses in
the water management. This problem was assessed based on the maximum observed
coefficients of severe drought contribution (Fig. 12.4c). The average maximum
CS D in relation to a typical year increased by seven times (19.7%). The distribution of the analyzed variable shows outliers that indicate particularly strong effects
of severe droughts on hydrological systems (above 35%: the San, the Rawka, the
Łeba). As these catchments represent different geographical regions (mountains,
lowlands, coastal areas), it may be hypothesized that extremely severe hydrological droughts strongly depend not only on hydrometeorological conditions but also
local catchment-related factors and result from both natural phenomena and human
activities.
The process of hydrological drought development follows a slightly different
pattern along larger transit rivers (Fig. 12.3). The average annual number of days with
hydrological drought in the upper course of the Vistula is mainly determined by its
Carpathian tributaries. Below the entry of the San, the number of such days increases
to 165 per year, but this does not reflect the features of drought in autochthonous
catchments of direct tributaries of the middle Vistula section. Only after the entry of
the Narew and the Bug, the number of days with hydrological drought drops back
to 150 and remains at this level until the Vistula mouth thanks to gradual increment
in water resources along with the growing basin area. Despite the relatively low
value of N D estimated for the Vistula, the contribution of hydrological droughts
is pretty high and can periodically disturb water management. A reverse situation
occurs for the Oder. An average number of days with hydrological drought rises
along the river course and is determined by its right bank lowland tributaries and
droughts developing in the catchment of the Warta. The effects of these factors are
so strong that the increase in water resources of the basin seems only a secondary
factor determining the duration of hydrological droughts along the Oder.
The analysis of the hydrological drought duration and the contribution of severe
droughts was supplemented by verification of the multiannual variability of these
parameters against a systematic component. To this end, the linear trends verified
with Student’s t-test were identified at the significance level α = 0.05 (Table 12.1,
Fig. 12.3). Interestingly, 11 out of 14 significant, multiannual trends of an annual
number of days with hydrological drought had a negative sign. These trends occurred
in the selected mountain and upland catchments and indicated gradual mitigation of
periods with water deficits within the investigated period. The trends with the best
fit (R
2 : 0.28–0.44) appeared in the rivers where large dam reservoirs were built.