240
E. Tomaszewski and M. Kozek
12.4 Multiannual Variations
In multiannual perspective, the average annual number of days with hydrological
drought in Poland ranged from 104 to 183 (Fig. 12.5). The longest droughts occurred
in the years 1990–1994 when long-term and severe low flows in rivers were observed
[7], caused by a series of dry years [28]. The lowest number of days with hydrological drought was noticed in 1998 and 2010 when water levels in Polish rivers were
exceptionally high, resulting in catastrophic floods in some catchments. An extreme
year in this respect was 1997, when droughts were followed by particularly high
flood waves with peak flows exceeding 0.001 probability of exceedance (so-called
1,000-year flood) at some water gauges.
The groups of dry and wet years are also visible along the course of hydrological
drought range index (Fig. 12.5). The periods with the large annual number of days
with hydrological drought are usually characterized by an extensive range of the
phenomenon. However, drought has never affected 100% of the analyzed area in one
year for the entire period of the study. Droughts with the range exceeding 95% of
the country area occurred in the years 1990–1994, 2003–2004, 2006 and 2012. The
hydrological drought of the smallest range of 15% occurred in 2010.
Average annual index of drought severity in Poland ranged in the investigated
multi-year period between 16.6 and 32.2% (Fig. 12.5). Years with high drought
Fig. 12.5 Multiannual course of selected hydrological drought parameters in Poland. N D – mean
annual number of days with hydrological drought, R D – range of hydrological drought, S D –
hydrological drought severity index (%): 1 – 0–4, 2 – 5–10, 11–16, 17–21, 5 – CS D – coefficient
of severe drought contribution; parameters N D , S D and CS D were calculated based on weighted
average where the weight was the catchment area (see Sect. 12.2)
E. Tomaszewski and M. Kozek
12.4 Multiannual Variations
In multiannual perspective, the average annual number of days with hydrological
drought in Poland ranged from 104 to 183 (Fig. 12.5). The longest droughts occurred
in the years 1990–1994 when long-term and severe low flows in rivers were observed
[7], caused by a series of dry years [28]. The lowest number of days with hydrological drought was noticed in 1998 and 2010 when water levels in Polish rivers were
exceptionally high, resulting in catastrophic floods in some catchments. An extreme
year in this respect was 1997, when droughts were followed by particularly high
flood waves with peak flows exceeding 0.001 probability of exceedance (so-called
1,000-year flood) at some water gauges.
The groups of dry and wet years are also visible along the course of hydrological
drought range index (Fig. 12.5). The periods with the large annual number of days
with hydrological drought are usually characterized by an extensive range of the
phenomenon. However, drought has never affected 100% of the analyzed area in one
year for the entire period of the study. Droughts with the range exceeding 95% of
the country area occurred in the years 1990–1994, 2003–2004, 2006 and 2012. The
hydrological drought of the smallest range of 15% occurred in 2010.
Average annual index of drought severity in Poland ranged in the investigated
multi-year period between 16.6 and 32.2% (Fig. 12.5). Years with high drought
Fig. 12.5 Multiannual course of selected hydrological drought parameters in Poland. N D – mean
annual number of days with hydrological drought, R D – range of hydrological drought, S D –
hydrological drought severity index (%): 1 – 0–4, 2 – 5–10, 11–16, 17–21, 5 – CS D – coefficient
of severe drought contribution; parameters N D , S D and CS D were calculated based on weighted
average where the weight was the catchment area (see Sect. 12.2)