11 Low-Flows in Polish Rivers
219
At the beginning of low-flow episode, rivers may be alimented in mixed way by quick
and base flow. At the end of the low-flow event, the period of runoff rise is usually
determined by direct runoff as an effect of effective precipitation or snowmelt alimentation. A very wide variety of intensity of separate form of alimentation modified
by current hydrometeorological conditions and seasonal factors result in different
reactions to feeding. Average low-flow recession rate was highly varied (Fig. 11.7).
Low-flow episodes develop very slowly in lake catchments (buffering function of
lake basins) an in uplands (good retention in groundwater reservoirs). In mountainous catchments low-low recession rate is relatively high and similar; the highest
value is characterized by Dunajec river—0.452 dm
3 km
−2 s
−1 d
−1 . The average rate
of low-flow rise is not fully proportional to the recession. Its value varied in a very
wide range from 0.023 dm
3 km
−2 s
−1 d
−1 (Note´ c river), up to 0.882 dm
3 km
−2 s
−1 d
−1
(Dunajec river). Mountainous catchments much quicker terminate low-flow episodes
than the others (rRi = 0.498-0.882 dm
3 km
−2 s
−1 d
−1 ) because of rapid reaction for
precipitation or snowmelt intensified by quick surface flow over steep slopes. It
is worth noticing that along transit rivers rate of low-flow recession and the rise
decreases gradually which should be connected with a continuous increase of basin
water resources as well as lack of synchronicity between low-flows occurrence in
the main river and its tributaries.
The multiannual course of low-flow duration is characterized by wet and dry group
of years (Fig. 11.9). Periods with severe hydrological droughts, common for whole
country, covered the first half of the 1950s, the first half of 1960s, the middle part of
1980s, beginning of 1990s, the middle part of 2000s and beginning of 2010s. It is easy
to calculate that serious water shortage periods occur in Poland every 10-15 years
on average. However, long low-flows with huge streamflow deficit appeared in all
rivers at the same time very rarely. For example in the lower course of transit rivers,
longer low-flows occurred in the Vistula river for first 20 years of the investigated
period, and then the Oder river was characterized by longer duration of these episodes
(Fig. 11.9a).
Moreover, low-flows in the Vistula river are generally shorter and occur more
often than in the Oder river. It is worth noticing that big rivers are deprived of lowflows in the late 1970s and early 1980s because of the flood years. However, very
high floods which covered the majority of Poland territory in 1997 and 2010 very
often accompany low-flow episodes in the same year.
In smaller catchments, low-flow periods are more evenly distributed on the time
scale (Fig. 11.9b). Hydrological droughts in the 1960s and the 1970s were more
severe for mountainous rivers whereas 1980s and 1990s impacted more seriously
lowland catchments. In every case, the multiannual course of low-flow duration looks
more like fluctuation or oscillation than a linear trend or chaotic distribution. Severe
low-flow duration distributions also confirm the above observations (Fig. 11.9c, d).
Common long water shortage periods are connected with macro hydrometeorological
conditions. However, the others seem to be under local factors influence, especially
hydrogeological determinants and some water management installations.
In the series of low-flow duration, statistically significant trends exist very rarely
(Fig. 11.10a). Observed tendency very often was driven by the separate group of long
219
At the beginning of low-flow episode, rivers may be alimented in mixed way by quick
and base flow. At the end of the low-flow event, the period of runoff rise is usually
determined by direct runoff as an effect of effective precipitation or snowmelt alimentation. A very wide variety of intensity of separate form of alimentation modified
by current hydrometeorological conditions and seasonal factors result in different
reactions to feeding. Average low-flow recession rate was highly varied (Fig. 11.7).
Low-flow episodes develop very slowly in lake catchments (buffering function of
lake basins) an in uplands (good retention in groundwater reservoirs). In mountainous catchments low-low recession rate is relatively high and similar; the highest
value is characterized by Dunajec river—0.452 dm
3 km
−2 s
−1 d
−1 . The average rate
of low-flow rise is not fully proportional to the recession. Its value varied in a very
wide range from 0.023 dm
3 km
−2 s
−1 d
−1 (Note´ c river), up to 0.882 dm
3 km
−2 s
−1 d
−1
(Dunajec river). Mountainous catchments much quicker terminate low-flow episodes
than the others (rRi = 0.498-0.882 dm
3 km
−2 s
−1 d
−1 ) because of rapid reaction for
precipitation or snowmelt intensified by quick surface flow over steep slopes. It
is worth noticing that along transit rivers rate of low-flow recession and the rise
decreases gradually which should be connected with a continuous increase of basin
water resources as well as lack of synchronicity between low-flows occurrence in
the main river and its tributaries.
The multiannual course of low-flow duration is characterized by wet and dry group
of years (Fig. 11.9). Periods with severe hydrological droughts, common for whole
country, covered the first half of the 1950s, the first half of 1960s, the middle part of
1980s, beginning of 1990s, the middle part of 2000s and beginning of 2010s. It is easy
to calculate that serious water shortage periods occur in Poland every 10-15 years
on average. However, long low-flows with huge streamflow deficit appeared in all
rivers at the same time very rarely. For example in the lower course of transit rivers,
longer low-flows occurred in the Vistula river for first 20 years of the investigated
period, and then the Oder river was characterized by longer duration of these episodes
(Fig. 11.9a).
Moreover, low-flows in the Vistula river are generally shorter and occur more
often than in the Oder river. It is worth noticing that big rivers are deprived of lowflows in the late 1970s and early 1980s because of the flood years. However, very
high floods which covered the majority of Poland territory in 1997 and 2010 very
often accompany low-flow episodes in the same year.
In smaller catchments, low-flow periods are more evenly distributed on the time
scale (Fig. 11.9b). Hydrological droughts in the 1960s and the 1970s were more
severe for mountainous rivers whereas 1980s and 1990s impacted more seriously
lowland catchments. In every case, the multiannual course of low-flow duration looks
more like fluctuation or oscillation than a linear trend or chaotic distribution. Severe
low-flow duration distributions also confirm the above observations (Fig. 11.9c, d).
Common long water shortage periods are connected with macro hydrometeorological
conditions. However, the others seem to be under local factors influence, especially
hydrogeological determinants and some water management installations.
In the series of low-flow duration, statistically significant trends exist very rarely
(Fig. 11.10a). Observed tendency very often was driven by the separate group of long