242
E. Tomaszewski and M. Kozek
The contribution of severe droughts strongly correlates with total drought duration during the year (Fig. 12.6b). The resulting linear dependence does not show a
good fit (R
2
= 0.33), which indicates a large impact of stochastic hydrometeorological conditions that affect the relationship. However, a strong linear dependence
(R
2
= 0.92) could be observed for the envelope following the maximum points of
severe drought contribution. This function indicates regional limitations of severe
drought development in Poland and may be helpful in water management planning
that considers the predicted annual number of days with hydrological drought. The
severe drought contribution coefficient partly correlated with drought range index
(Fig. 12.6c). Although graphic representation of the dependence shows a cloud of
points impossible to approximate with a linear function, the envelope composed of
points with a maximum contribution of severe droughts can be described with the
exponential equation of high degree of fit (R
2
= 0.74). In practice, it is possible to
determine regional limits of the maximum coefficient of severe droughts contribution
in relation to the area affected by hydrological drought.
The multiannual course of the analyzed parameters did not reveal any linear trends
statistically significant at α = 0.05. It can, therefore, be concluded that in Poland,
the long-term variability of hydrological drought parameters depends primarily on
the natural fluctuation of hydroclimatic conditions.
12.5 Seasonal Variations
12.5.1 Monthly Variability
Dynamics of the hydrological system reveals significant variability of the water
balance structure and the size of water resources within the seasonal cycle. Monthly
intervals are basic units for determining disposable resources for water management
and assessing water needs at a regional and national scale. This section provides a
statistical analysis only for the months in which the number of days with hydrological
drought exceeded 10 (see Sect. 12.2).
Monthly distribution of the average number of days with hydrological drought
reflects a simple, seasonal variability (Fig. 12.7). Between November and March, this
parameter systematically decreases until the minimum of 18 days. This is the effect
of a gradually weakening evapotranspiration determined by temperature changes and
disappearance of vegetation. The gradually decreasing drought severity index (from
8.6 to 2.8%) confirms the hypothesis that in recent years, catchment water resources
are recharged in winter. This is mainly due to milder weather conditions during
this season, resulting in high rainfall retention and relatively small evapotranspiration [7]. Winter hydrological droughts demonstrate relatively stable range (20–25%),
and the local maximum of the severe drought contribution coefficient falls in January.
This indicates stability of winter droughts in selected mountain catchments in which
regular and periodic storage of water in the snow cover is often intensified by frost
E. Tomaszewski and M. Kozek
The contribution of severe droughts strongly correlates with total drought duration during the year (Fig. 12.6b). The resulting linear dependence does not show a
good fit (R
2
= 0.33), which indicates a large impact of stochastic hydrometeorological conditions that affect the relationship. However, a strong linear dependence
(R
2
= 0.92) could be observed for the envelope following the maximum points of
severe drought contribution. This function indicates regional limitations of severe
drought development in Poland and may be helpful in water management planning
that considers the predicted annual number of days with hydrological drought. The
severe drought contribution coefficient partly correlated with drought range index
(Fig. 12.6c). Although graphic representation of the dependence shows a cloud of
points impossible to approximate with a linear function, the envelope composed of
points with a maximum contribution of severe droughts can be described with the
exponential equation of high degree of fit (R
2
= 0.74). In practice, it is possible to
determine regional limits of the maximum coefficient of severe droughts contribution
in relation to the area affected by hydrological drought.
The multiannual course of the analyzed parameters did not reveal any linear trends
statistically significant at α = 0.05. It can, therefore, be concluded that in Poland,
the long-term variability of hydrological drought parameters depends primarily on
the natural fluctuation of hydroclimatic conditions.
12.5 Seasonal Variations
12.5.1 Monthly Variability
Dynamics of the hydrological system reveals significant variability of the water
balance structure and the size of water resources within the seasonal cycle. Monthly
intervals are basic units for determining disposable resources for water management
and assessing water needs at a regional and national scale. This section provides a
statistical analysis only for the months in which the number of days with hydrological
drought exceeded 10 (see Sect. 12.2).
Monthly distribution of the average number of days with hydrological drought
reflects a simple, seasonal variability (Fig. 12.7). Between November and March, this
parameter systematically decreases until the minimum of 18 days. This is the effect
of a gradually weakening evapotranspiration determined by temperature changes and
disappearance of vegetation. The gradually decreasing drought severity index (from
8.6 to 2.8%) confirms the hypothesis that in recent years, catchment water resources
are recharged in winter. This is mainly due to milder weather conditions during
this season, resulting in high rainfall retention and relatively small evapotranspiration [7]. Winter hydrological droughts demonstrate relatively stable range (20–25%),
and the local maximum of the severe drought contribution coefficient falls in January.
This indicates stability of winter droughts in selected mountain catchments in which
regular and periodic storage of water in the snow cover is often intensified by frost