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All content in this area was uploaded by Elena Bojilova on Dec 17, 2015
Content may be subject to copyright.
WATER RESOURCES IN BULGARIA
DURING THE 1982 - 1994
DROUGHT PERIOD
Strahil Gerassimov Georgiev
Marin Gospodinov Genev
Elena Kirilova Bojilova
Tatiana Vasilevna Orehova
2
FOREWORD
The main idea of the research is to investigate water resources in Bulgaria during the drought
period 1982-1994. The aim is to take in consideration climate variability, economical and
social development and their influence to the natural processes. The research is in direction
to evaluate sustainable development in conditions of limited water resources. Due to this the
extensive research
of 13-year drought period is performed applying specific scientific
methods. The probabilistic scenarios for safety of water resources in conditions of future
drought periods are created.
The objectives of the research are to investigate is it possible to use this 13-year period of
drought as a model for the future state of precipitation and river discharge. The authors are
convinced that using different method of investigation and probabilistic scenarios it will be
possible to obtain a good picture for the depression in precipitation and runoff. The research
has regional and global aspects. The study considered natural and anthropogeneous factors
and their influence to the studied elements. In the study the regression and correlation
analyses are widely used. Data for representative hydrometric stations for the three main
hydrological zones – Danube, Black Sea and Aegean zones, are used. The authors performed
extension of chronological data series for runoff, precipitation and temperature. Data for
precipitation in England and Wales, solar activity, radiation and temperature anomalies in
the Northern hemisphere are executed.
It was performed multi-varied probabilistic analysis of multi-annual variations of river
discharge. Water balance was prepared. Trend values for rainfall and runoff and difference
between them for previous and present centuries for 50-year periods with beginning 1900
were obtained.
For the drought period three different structures are prepared: probabilistic-chronological,
chronological in relative units for runoff and rainfall and statistical one. The investigations
are made for three different 13-year periods selected from the 106-year study period. Four
short periods were selected and executed during the 1982-1994 drought period.
The research was made for to investigate the variations of the groundwater and its state
during the studied drought period. Examples for selected springs and observations wells are
presented and discussed. The chosen data are from the National Hydrogeological Network.
Six probabilistic scenarios for natural development of hydrological process with different
level of pessimism are described (part 6). Part 7 of the study concerns definition and analysis
of scenarios with considerations of natural and anthropogeneous factors. Different empirical
relationships, double-mass curve analysis are applied and combined with four scenarios with
fixed amount of CO
2
and different level of solar activity during XIX and XX centuries. The
study plan is finalized with selection of the most possible scenario for Bulgaria – weekly
pessimistic scenario for development of water resources.
The research and interpretation of the received results are presented in a number of tables and
figures. The present research can be considered as a methodological work, applicable for
water balance study, design and construction of hydrotechnical works. It can be used for a
future scientific research.
Prof. Dr Michail Angelov
Sofia, March 2004
Water Resources in Bulgaria During the 1982-1994 Drought Period
3
ACKNOWLEDGMENT
The authors wish to thank all those who directly or indirectly have helped in building the
manuscript to what is felt to be a fairly complete work. It is not possible to mention here all
the names. However, the leadership of National Institute of Meteorology and Hydrology
(NIMH) – Bulgarian Academy of Sciences (BAS) should be thanked in the first place. This
research will be not possible without wide range of different information for meteorological,
hydrological and hydrogeological data kindly supplied from the Institute.
Thanks are extended to Prof. Gregory Knight (Pennsylvanian University, USA) and Prof.
Ivan Raev (Forest Research Institute, BAS, Bulgaria) for the invitation of the team to
participate in creation of monograph “Drought in Bulgaria: contemporary analogue for
future climate change” in the chapter 4.1. “Quantitative investigations of the water
resources during the drought period”. The presented here study is extended research on that
topic. The obtained results from the investigations give information for the water resources
in Bulgaria.
No less thank are extended to Dr. Eng. Todorina Bojkova, associate professor at the NIMH
and leader of Department of Hydrology for her encouragement
and co-operation. Without
her continuous support it is very likely that it would have been impossible to publish this
work.
Last but not least the authors acknowledge with sincere appreciation the effort spent by Prof.
Dr Michail Angelov, while reviewing the manuscript of the book.
Sofia, Bulgaria Str. Gerassimov G.
June 2004 M.G. Genev
E.K. Bojilova
T.V. Orehova
4
Table of Content
FOREWORD
ACKNOWLEDGMENT
PREFACE
INTRODUCTION TO THE TOPIC
1. INFORMATION DATA BASE…………………………………………………………..…………….…….
2. ANALYSIS OF THE MULTIANNUAL VARIATIONS OF THE RIVER RUNOFF FOR
DETERMINATION OF FUTURE PROBABILISTIC STATE…………………………….…………
3. DEFINITION OF THE DROUGHT PERIOD FOR PRECIPITATION AND RIVER RUNOFF………….
4. COMPARATIVE AND PROBABILISTIC ESTIMATIONS FOR THE DROUGHT DEEPNESS…………
5. PROBABILISTIC SHIFTING IN THE FUTURE OF THE DEPRESSION OF 1982-1994
(PRECIPITATION AND RUNOFF) OVER THE SCENARIOS FOR DEVELOPMENT OF THE
HYDROLOGICAL PROCESSES IN BULGARIA…………………………………………………...…
6. MODELS FOR NATURAL DEVELOPMENT OF THE HYDROLOGICAL PROCESSES IN BULGARIA
WITH A DIFFERENT LEVEL OF PESSIMISM. COMPARISON WITH THE MODELS FOR
ANTHROPOGENEOUS DEVELOPMENT BY GREEN HOUSE GASES
EFFECT.………………………………………………………………………………
7. SCENARIOS WITH CONSIDERATION OF NATURAL AND ANTHROPOGENEOUS
FACTORS………………………………………………………………………………………………
8. CONCLUSION FOR MOST POSSIBLE SCENARIOS FOR BULGARIA…………………………………
BASIC RESULTS…………………………………………………………………………………………….
RECOMMENDATIONS…………………………………………………………………………………….
VARIATIONS OF THE HUMIDITY DURING THE DROUGHT PERIOD 1982-1994 …………………
REFERENCES…………………………………………………………………………………………………..
SCIENTIFIC PROFILES OF THE AUTHORS……………………………………………………………
Water Resources in Bulgaria During the 1982-1994 Drought Period
5
PREFACE
From different chronological sources it is evident that river discharge respectively water
resources, are deviating in increasing and decreasing directions. In our past the information
for river basins are relatively limited (1935-1955 years) even that we have knowledge for
positive and negative periods for quantitative characteristics of river discharge in river
basins. Using old sources we can estimate that water resources in the past were richer
comparing to our present. We have this type of evidences for Struma, Iskar and Iantra Rivers
in lower river branches. For example Maritza River was navigable to Pazardjuk town.
From our investigations during last decade we can estimate presence of negative and positive
phases that form cycles with different water availability. The aim of presented here research
is to investigate the water resource in the phase of drought period beginning from 1980. The
obtained results will give information for water resources in Bulgaria during 1982-1994
period. The results can be useful for practice in solving engineering and design problems,
and also for educational purposes.
6
INTRODUCTION TO THE TOPIC
The main objective of this research is to make an accurate evaluation of the quantitative and
qualitative conditions of the water resources in Bulgaria during the drought period
(1982-1994). The aim of this study is to estimate the potential of this period to be a model for
the future, taking into consideration the global climate changes. The basic element of this
investigation is the quantitative aspect due to its strong natural relations to the climatic
conditions. The qualitative aspect is related mainly to anthropogeneous impacts. The volume
of the historical information for the quantitative investigation is much more abundant
concerning river discharges, precipitation totals and air temperature. These data are in
correlation between them and also with astronomical parameters such as solar activity and
radiation. This fact allows the objective assessments to be made for a long historical period.
Water Resources in Bulgaria During the 1982-1994 Drought Period
7
1. INFORMATION DATA BASE
The longest historical period with available real data for the solar activity characterized with
the number of solar spots (number of Wolf) is available since 1700 – i.e. around 300 years
(Waldmeick, 1961). The existing data for the global temperature anomalies are present since
1854, and data for the precipitation over the big areas – since 1766 for England & Wales
(Trends'93, 1994).
In Bulgaria the observations and measurements of the precipitation and air temperature start
in 1892, for the river – water levels since 1909 and systematical water discharge
measurements – 1936. The number of meteorological and hydrological stations grows from
the beginning of early 50’s and 60’s, but in the last two decades is decreasing. Due to this
fact, the information availability is rather non-homogeneous – large amount of data for few
decades and insufficient data in the beginning and in the end of the hydrometeorological
observations. It is necessary to take into consideration the non-homogeneity of the data for
the river discharge due to anthropogeneous impacts – after 1950 the main hydro-technical
constructions (water reservoirs, pumping stations, channels, hydropower stations, irrigation
and drainage systems, etc.) were built.
The water regime in Bulgaria is described over main territorial and temporal units in relation
to climate, hydrographic conditions and natural chronological variations. The three main
territorial units are:
1. Danube hydrological zone – all Bulgarian tributaries to the river Danube (45% of
the territory of Bulgaria) – is characterized with temperate climate (European-Continental);
2. Black sea zone - all Bulgarian tributaries with direct discharge to the Black sea
(13.8%) - is characterized with temperate climate and climate with Mediterranean influence
South of the Stara Planina mountain;
3. Aegean hydrological zone - all South Bulgarian rivers with direct discharge to the
Aegean Sea in the territory of Greece and/or Turkey (rivers Struma, Mesta and Maritza) –
41.2% of the territory of the country. This zone is characterized with transition from
temperate to Mediterranean (Continental-Mediterranean) climate in North – South direction.
The investigation of the river discharge variations in respect to the large territorial units
allows avoiding big errors, non-assessment of anthropogeneous impacts over the extended
territorial and temporal distributions of river runoff. The river runoff is assessed as a total
runoff of all river estuaries or at country boundaries of Bulgaria for minimal temporal
interval of one calendar year. The total registered discharge of the groundwater flow from
ephemeral and perennial rivers of the Dobroudzha region to the Danube River and Black Sea
was taken into consideration. The large river intakes for water supply of the Black Sea
villages and towns (Varna, Devnja, Bourgas) from reservoirs Kamchia and Tzonevo were
also taken into consideration. Some small river transfers between Danube and Aegean basins
were not considered. The evapotranspiration from the irrigation systems was not taken into
consideration due to lack of reliable information for irrigation water schemes. No correction
was made for restoration of the natural regime for the river runoff to the individual river
basins due to reservoir operations. The example for this is river Arda that is tributary of river
Maritza in the territories of Greece and Turkey. It is assumed that water regulation (mostly in
8
seasonal reservoirs) and transfer from one calendar year to the other using large hydrograph
zones do not affect the total annual runoff.
A full investigation of the total annual runoff for the three main hydrological zones for the
1960-1996 period was made. This investigation is used as a model for all other investigations
with non-sufficient information. Directly and indirectly the data from hydrometeorological
stations in the estuaries zones and boundary regions were used: 16 hydrometeorological
stations for the Danube basin, 10 stations for Black sea region, and 18 - for the Aegean zone.
Time series from representative hydrometeorological stations with longest period of
observation and high-quality data with small anthropogeneous impacts were used for
assessment of the total river runoff for the three main hydrological zones. For the period
before 1936, the hydrological annual data series for some selected stations were extended
using rating curves
(
)
hfQ =
(based on some episodic experimental river discharge
measurements, longitudinal and cross-sectional profiles of the river body and analysis of
variations of water stage for different stations and periods). The comparison with
precipitation and air temperature was used. As a result of this investigation the long data
series for 16 hydrometeorological stations were obtained (for more comprehensive summary
see Table 1).
The runoff for the stations mentioned above is given in relative units in relation to
discharge areas (runoff in mm), it is averaged to major hydrological zones and the correction
was made for the standard estimation (h) for the period 1960-1996. The comparison of the
two types of assessments shows very good results and it is acceptable also for the Black sea
basin with the lower number of stations as is seen from Table 2.
The data series for the precipitation and air temperature for the main territorial zones
were obtained from observations of all rain gauges and meteorological stations. The series
were extended using correlative relationships up to 1892. The number of the received and
used time-series is given in Table 3.
The average values for precipitation (P, mm) and air temperatures (T,
C
0
) for the three
main hydrological zones for every year were received using linear averaged interpolation for
mean altitude above sea level (H, m), in relation to mean altitude of the hydrological zones
(
(
)
(
)
zonezonezonezone
HTTHPP
~
,
~
==
).
Water Resources in Bulgaria During the 1982-1994 Drought Period
9
Table 1. Extended data series for river runoff from hydrometeorological stations in the three
main hydrological zones.
Drainage basin Hydrometeorological stations Since year
Danube Ogosta river near Misia town
Vit river near Teteven town
Rosiza river near Sevlievo town
Iantra river near Cholakovzy village
Rusenski Lom river near Bojichen village
16850
21650
23500
23700
31830
121
51a
69
77
399
1914
1922
1922
1922
1922
Black Sea Provadia river near Sindel
Kamchia river near Grozdevo village
Veleka river near Zvezdech (no
extension)
42850
43800
14
11
1922
1922
1937
Aegean Struma river near Rajdavyza
Sovolanska Bustriza near Sovolano
village
Struma river near Marino-pole village
Topolniza river near Poibrene village
Maritza river near Plovdiv town
Striama river near Bania village
Chepelarska river near Bachkovo village
Maritza river near Harmanli town
51700
51380
51880
201
182
220
1919
1919
1919
1914
1909
1914
1909
1909
*Data series are extended by Str. Gerassimov.
Table 2. Comparison between assessment for annual runoff in Bulgaria using limited
number of stations (h’) with standard estimation (h) for the period 1960-1996.
Annual Runoff
Drainage basin
h
h
'
=
h
σ
hh
R
'
h∆
σ
P
M
P±
max
h∆
mm mm mm % % mm
Danube (h’ from 5 stations, h
from 16 stations)
145.0 52.2 0.941 17.66 10.0 31.0 37
Black Sea (h’
from 3 stations, h
from 10 stations)
141.0 62.1 0.803 37.0 16.2 81.2 170
Aegean (h’ from 8 stations, h
from 18 stations)
209.4 77.5 0.914 31.4 12.7 43.1 71
Total for Bulgaria (h’ from 16
stations, h from 44 stations)
174.4 61.9 0.957 18.0 8.5 24.5 47
* Data for the river runoff are prepared by Str. Gerassimov;
**
hh
R
'
are coefficient of correlation.
10
Table 3. Number of Used Annual Data Sets with Rainfall and air Temperature.
Data Sets
Drainage basin For precipitation (P),
number
For air temperature (T),
number
Danube 134 71
Black Sea 52 29
Aegean 114 69
Total for Bulgaria 300 169
*After M. Genev, 1998-1999.
Water Resources in Bulgaria During the 1982-1994 Drought Period
11
2. ANALYSIS OF THE MULTIANNUAL VARIATIONS OF THE RIVER RUNOFF FOR
DETERMINATION OF FUTURE PROBABILISTIC STATE
The objective of the analysis of the chronological variations of the river runoff and the
elements of the water balance – precipitation and evaporation, is to obtain the tendency and
some possible changes in the future under different scenarios for global climate change. This
is the base for reliable estimation of the river runoff and water resources for the last drought
period. For the evaluation of some details some additional information was used from the
National Hydrological Network. Comparison of the tendencies of the chronological
variations of the precipitation and air temperature with the river runoff gives us possibility
for estimation in general of the additional losses from evapotranspiration in the irrigation
systems. The results received by these investigations allow us to propose some possible
scenarios for the future conditions of the water resources in Bulgaria. In the preliminary
analysis of the data for the annual river runoff, the regression and correlation analysis
between annual and total chronological values of discharge, precipitation and air temperature
for the three main hydrological zones widely were used. As a result some systematic errors
from anthropogeneous and information characters were eliminated. Time series were
extended up to 1892.
We assumed that the hydrometeorological processes are in close relation with the global
processes such as solar radiation and activity. Data for the full 10 solar cycles (from the 12-th
up to 22-nd) – 1890-1996 (Waldmeiсk, 1961 - treating the number of Wolf) - were used.
From these data we obtain the regression values for precipitation, air temperature and river
runoff for the years 1890 and 1891 using the long data series for the precipitation over
England & Wales and temperature anomalies in the Northern hemisphere (Trends’ 93,
1994). In the Figures 1÷3 the chronological graphs of the annual values of the solar activity
(number of Wolf), solar radiation (Si, S-1366.9 W/m², Foukal, P. & J. Lean 1990),
temperature anomalies in the Northern Hemisphere (dT), precipitation over England &
Wales (P-GB), precipitation, air temperature and runoff over Bulgaria (P-BG, T-BG and
h-BG respectively) are presented. In the same graphs corresponding linear trend lines and
polynomial trends of power 5 are given. The results show:
1. For the whole observation period over the sunspots (number of Wolf) since 1700
up to now the increasing linear trend of the solar activity were observed.
2. The same tendency of increasing linear trend was observed for the solar radiation
and temperature anomalies over the Northern Hemisphere.
3. The precipitation over England & Wales also shows positive trend as well due to
its location near the Atlantic Ocean coast. The position of England close to the ocean carrier
of accumulated solar energy from the tropical zone of the ocean ensures more intensive
atmosphere circulation and rising of precipitation in Western Europe. Similar correlation
between solar radiation and precipitation with time delay for the Pacific Ocean circulation
was obtained from the American hydrologist Perry (Perry, 1992) for the Western coast of
North America.
4. The air temperature over Bulgaria since 1890 shows small positive trend in
accordance with temperature over the Northern Hemisphere.
12
5. The precipitation and river runoff for Bulgaria (since 1890) show negative trends;
i.e. they are in anti-phase with precipitation in England that is determined as a statistical
appearance for atmospheric circulation as well.
The chronological graphs of the annual precipitation over Bulgaria and England & Wales are
presented in the Figure 4. In the Figure 5 the graphs of annual values of solar activity, solar
radiation, and temperature anomalies in the Northern Hemisphere for the period after 1890
are given. In the same graphs corresponding linear trend lines are presented. In the Figures 6,
7 and 8 the chronological graphs of air temperature, precipitation and runoff for the three
main hydrological zones are shown. From the graphs we can conclude that for the 107-year
period including 10 full sun-cycles the tendencies are:
1. Increasing average annual air temperature for the three main hydrological zones.
2. Decreasing of the precipitation and river runoff in Danube and Aegean
hydrological zones and increasing of both for the Black Sea zone.
In the Table 4 the coefficients (
ba,
) of the linear trend equations for the three parameters –
temperature (T,
C
0
), precipitation (P, mm) and river runoff (h, mm) are given, where x is the
number of the year since 1890 up to now (
0
=
x
for 1890). The results of Table 4 show:
3. The strongest decrease of precipitation and river runoff is obtained for the
Danube hydrological zone (North Bulgaria) with gradients:
3166.0;4115.0
−
=
−
=
hp
aa
.
4. In the Aegean hydrological zone (South Bulgaria) the decrease of precipitation
and river runoff is weaker:
2929.0;349.0
−
=
−
=
hp
aa
.
5. In the Black Sea zone the precipitation and river runoff have small positive trend:
0698.0;1817.0
=
=
hp
aa
.
6. The main direction in changing trend for humidity (P, h) for Bulgaria is from
West-Northwest to East-Southeast; this fact corresponds with the main direction of the
atmospheric circulation and transfer of humid Atlantic air over the country.
7. The neutral zone with zero gradients is possibly located around boundary line
between the both climatic zones in Bulgaria: European-Continental and
Continental-Mediterranean. In the Climatic Atlas of Bulgaria this boundary line is situated
along the Black Sea coast and Southwest Bulgaria with main direction from Northeast to
Southwest.
8. In case of extrapolation in West-Northwest direction from the territory of
Bulgaria, such neutral zone with zero gradients obviously may be found, taking into account
the anti-phases in the humidity in England.
9. The trend values of the humidity obviously can be considered as estimates of the
norms (precipitation and runoff) using a 50-year period while taking into consideration that
the basic period is 107 years and that the straight line of the trend can be defined using two
central points from the two half periods (53½ years).
Water Resources in Bulgaria During the 1982-1994 Drought Period
13
Table 4. Linear Trends for Temperature, Precipitation and Runoff in the Three Main
Hydrological Zones and Total for Bulgaria.
Linear Trends
Drainage basin Temperature –
T, ° C
Precipitation –
P, mm
Runoff -
h, mm
T
a
T
b
P
a
P
b
h
a
h
b
Danube 0.0059 7.8431 -0.4115 754.8 -0.3166 171.13
Black Sea 0.0042 10.247 0.1817 633.66 0.0698 151.05
Aegean 0.004 7.4189 -0.349 773.47 -0.2929 270.51
Total for Bulgaria
0.00481 7.9840 -0.3015 746.48 -0.2526 213.08
Source: Gerassimov, Str. G., M.G. Genev, E.K. Bojilova & T.V. Orehova 2003.
“Quantitative investigations of the water resources during the drought period”.
Using as a basis the conclusion (7), for full century orientation we may calculate the trends
values (
hP,
) for the following years: 1900, 1950, 2000, 2050 и 2100. The results are
presented in Table 5. The values of the difference
EhP
=−
are also presented there. These
values present losses of atmospheric water for evaporation.
The variations of the norms for precipitation, runoff and losses (mainly evaporation) for the
three main hydrological zones and for Bulgaria for the periods of 100 years are shown in
Table 5. The first 100-year period is given as available information, and the second one – as
forecast if the tendency in the factors such as solar activity and radiation, atmospheric
circulation, anthropogeneous impact will remain the same. The most important element is
the content of green house gases in the atmosphere; different actions are undertaken to
decrease it. There is small possibility the decrease in trends of precipitation and runoff to
show strong tendency of decreasing for the coming 100-years. The estimates of the norms for
the future 100 years using the available trend may be used as a pessimistic scenario for
changes in the water resources and the elements of the water balance.
The simplest idea is the reverse in the sign of trends for the next century and preservation of
the absolute values; i.e. the norms for 2050 and 2100 will obtain the values of 1950 and 1900
respectively. This can be used as an optimistic scenario.
14
Table 5. Water Balance Using Trend Estimates for the Two Centuries.
Water Balance
Drainage basin
Elements 1900 1950 2000 2050 2100
P
y
∆
∆
y
∆
,
mm
Danube
D
P
, mm 751 730 710 689 668 1 -83
D
h
, mm 168 152 136 120 105 0.759 -63
DD
Ph /
=
α
0.224 0.208 0.192 0.175 0.157 - -
DDD
hPE
−
=
583 578 574 569 563 0.241 -20
Black Sea
B
P
, mm 635 645 654 663 672 1 37
B
h
, mm 152 155 159 162 166 0.378 14
BB
Ph /
=
α
0.239 0.241 0.243 0.245 0.247 - -
BBB
hPE
−
=
483 490 495 501 506 0.622 23
Aegean
E
P
, mm 770 753 735 718 700 1 -70
E
h
, mm 268 253 238 224 209 0.843 -59
EE
Ph /
=
α
0.348 0.336 0.324 0.312 0.298 - -
EEE
hPE
−
=
502 500 497 494 491 0.157 11
Total for
Bulgaria
BG
P
, mm 743 728 713 698 683 1 -60
BG
h
, mm 211 198 185 173 160 0.850 -51
BGBG
Ph /
=
α
0.283 0.272 0.260 0.247 0.234 - -
BGBGBG
hPE
−
=
532 530 528 525 523 0.150 -9
39
10, mW
h
23.42 21.98 20.53 19.20 17.76 - -
Source: Gerassimov, Str. G. & E.K. Bojilova 2003.
Water Resources in Bulgaria During the 1982-1994 Drought Period
15
Solar activity - W
y = 0.1112x + 33.187
0
50
100
150
200
1700
1740
1780
1820
1860
1900
1940
1980
Year
W
Solar radiation - Si
y = 0,0025x + 0,0623
0
0,2
0,4
0,6
0,8
1
1875
1885
1895
1905
1915
1925
1935
1945
1955
1965
1975
1985
Year
Si
Temperature anomalies in Northern Hemisphere
y = 0,0042x - 0,4365
-0,6
-0,4
-0,2
0
0,2
0,4
1854
1864
1874
1884
1894
1904
1914
1924
1934
1944
1954
1964
1974
1984
1994
Year
dT, degree C
Fig. 1. Chronological graphs of the annual values for solar activity (number of Wolf - W),
Solar radiation (Si), temperature anomalies in the Northern Hemisphere (dT) and respective
trend lines.
16
Precipitation over Bulgaria
y = -0,3015x + 746,48
500
600
700
800
900
1000
1890
1900
1910
1920
1930
1940
1950
1960
1970
1980
1990
Year
Precipitation, mm
Precipitation over England &Wales
y = 0,0808x + 902,9
600
800
1000
1200
1400
1766
1786
1806
1826
1846
1866
1886
1906
1926
1946
1966
1986
Year
Precipitation, mm
Fig. 2. Chronological graphs of the annual values of the precipitation over Bulgaria, and over
England & Wales and respective trend lines.
Water Resources in Bulgaria During the 1982-1994 Drought Period
17
Tendencies
Air temperature, T-BG, y = 0.0048x + 7.9839
Runoff depth, h-BG, y = -0.2958x + 214.64
0
100
200
300
400
500
600
1890 1910 1930 1950 1970 1990
Year
River runoff depth, mm
4
5
6
7
8
9
10
Air temperature, degree C
Fig. 3. Chronological graphs of the annual values for air temperature and runoff (layer in
mm) for Bulgaria and respective trend lines.
Tendencies
England&Wales, P-GB, y = 0.2098x + 903.95
Bulgaria, P-BG, y = -0.3015x + 746.48
0
200
400
600
800
1000
1200
1890 1910 1930 1950 1970 1990
Year
Precipitation, P-GB, mm
500
600
700
800
900
1000
1100
1200
Precipitation, P-BG, mm
Fig. 4. Chronological graphs for the annual values of the precipitation over Bulgaria, over
England & Wales and respective trend lines.
18
Tendencies
Solar radiation, Si, y = 0.003x + 0.0691
dT, y = 0.0055x - 0.3717
W, y = 0.5266x + 31.601
0
50
100
150
200
250
300
350
1890 1910 1930 1950 1970 1990
Year
W
-1,6
-1,4
-1,2
-1
-0,8
-0,6
-0,4
-0,2
0
0,2
0,4
0,6
0,8
1
dT in degree C, Si
Fig. 5. Chronological graphs of the annual values for solar activity (number of Wolf - W),
solar radiation (Si), temperature anomalies in the Northern Hemisphere (dT) and respective
linear trends.
Danube basin
Air temperature, T, y = 0.0059x + 7.8431
Precipitation, P, y = -0.4115x + 754.8
Runoff depth, h, y = -0.3166x + 171.13
0
200
400
600
800
1000
1200
1890 1910 1930 1950 1970 1990
Year
Annual values, mm
0
1
2
3
4
5
6
7
8
9
10
Air temperature, degree C
Fig. 6. Chronological graphs of the annual values for air temperature, precipitation and river
runoff (layer in mm) for the Danube hydrological zone and respective linear trend lines.
Water Resources in Bulgaria During the 1982-1994 Drought Period
19
Black Sea basin
Air temperature, T, y = 0.0042x + 10.247
Precipitation, P, y = 0.1817x + 633.66
Runoff depth, h, y = 0.0698x + 151.05
0
200
400
600
800
1000
1200
1890 1910 1930 1950 1970 1990
Year
Annual values, mm
0
2
4
6
8
10
12
Air temperature, degree C
Fig. 7. Chronological graphs of the annual values for air temperature, precipitation and river
runoff (layer in mm) for the Black Sea hydrological zone and respective linear trend lines.
Aegean basin
Precipitation, P, y = -0.349x + 773.47
Runoff depth, h, y = -0.2929x + 270.51
Air temperature, T, y = 0.004x + 7.4189
0
200
400
600
800
1000
1200
1890 1910 1930 1950 1970 1990
Year
Annual values, mm
0
1
2
3
4
5
6
7
8
9
Air temperature, degree C
Fig. 8. Chronological graphs of the annual values for air temperature, precipitation and river
runoff (layer in mm) for the Aegean hydrological zone and respective linear trend lines.
20
3. DEFINITION OF THE DROUGHT PERIOD FOR PRECIPITATION AND RIVER
RUNOFF.
From Figures 4, 5, 6, 7 and 8 it is obvious that after 1980 in Bulgaria the continuous decrease
in the precipitation in combination with increasing air temperatures appear. This situation
leads to depression in the river runoff. The period in which both runoff and precipitation in
Bulgaria are under the norms is 1982-1994 (n=13 years). Before this period the long wet
period beginning since 1954 with precipitation and runoff above the norms was observed. In
this wet period only during one or two years the values of runoff and precipitation are under
the norms.
These cycles of high and low values are most clearly observed using integral curves of the
standardized deviations
int
K
(
( )
( )
( )
∑∑
=
−
=
t
t
t
tf
X
XX
K
int
) - Figures 9, 10 and 11. The last
drought period on these Figures is shown with steep depression curve (Genev, 1998).
In Table 6 the chronological structure of the drought period 1982–1994 is presented in the
relative units - the norms for runoff and precipitation for Bulgaria and for three main
hydrological zones (1982-1994) are divided to the values of 106-year norms. For the
completeness in the last column data for year 1995 are presented as well. The year 1995 is the
beginning of increase of humidity and probably the beginning of the new wet cycle (this is
the tendency observed up to the year of this study).
During 1981 the precipitation over the country shows a tendency for decreasing – 99% from
the norm, but the runoff is still over the norm with +29%. This delay with one year in the
behavior of the runoff is observed also in the end of the drought period: the precipitation in
1995 is with 12% over the norm, but the long lasting drought especially in 1993-1994 does
not give possibility of the runoff to go above the norm.
In column 3 of Table 6 the values of lower quartile (25% probability of non-exceedance or
75% probability of exceedance) that may be used as stronger limit of drought are given.
The river runoff for the Danube basin forms with use of strong criterion one more 13-year
competitor drought period 1983-1985 as is shown in Table 6. For the runoff in the Aegean
basin and for all Bulgaria, the strongest limit gives the 11-year drought period 1985-1995.
This shortening of the drought period from the North to South and especially from West to
East (the drought period in the strong limits for the Black Sea zone is only 6-year long – from
1989 up to 1994), is probably related to availability of spatial waves with anti-phases of the
atmospheric circulation in the most important direction of the humidity transfer for Bulgaria
- from West-Northwest to East-Southeast. Similar conclusion was made above during the
analysis of trend gradients.
From Table 6 we may conclude that the shorter periods with more severe drought (for the
runoff) with mainly significance for the whole country, are the periods 1983-1994 (n=12
years) and 1985-1994 (n=10 years). In the frames of the 14-year period 1982-1995 there are
two weak rises of precipitation and runoff in some cases: during 1984 – the highest value for
the runoff is obtained for the Black sea zone (+44%) and during 1991 – the highest value for
the precipitation is received for the Danube zone (+13%).
Water Resources in Bulgaria During the 1982-1994 Drought Period
21
The probable chronological characteristics of the period 1982-1994 along with estimates of
theoretical approximation of empirical distribution curves
(
)
P
XXP 〈
are presented in Table
7, based on the 106-year period 1890-1995. The theoretical probabilistic curves are
lognormal with correction
(
)
(
)
P
XXNax 〈=±ln
.
From Table 7 we may notice that the 13-year period has rather large availability of dry years
with low probability of occurrence. For example, for this short period with probability fewer
than 20% only three cases are normally expected. Besides precipitation for the Black Sea
zone where such cases are two and for the runoff they are five, for the last zones and for the
whole country such cases are from 6 to 8. Moreover, in this period some years with very low
runoff occur (1990, 1993 and 1994) having excessively small probability of occurrence –
from 0.19 to 0.9 %, i.e. with return period 526-111 years, surpassing the basic period of
approximations (106 years). These unfavorable combinations lead to strong decrease of
runoff during the investigated period.
The statistical structure of the period using two main statistical parameters – average
X
and
standard deviation
X
σ
, with comparison to long period of 106 years are presented in Table 8.
From this Table it is possible to conclude that during the drought period the average values
and standard deviations are considerably lower compared to these of the long series. The
reductions of these values are bigger for the Danube basin ( 638.0=
h
K ), smaller – for
Aegean and the smallest for the Black Sea basin ( 733.0=
h
K ).
Fig. 9. Integral curves of the standardised deviations for the temperature - (1) Danube
hydrological zone, (2) Black Sea hydrological zone and (3) Aegean hydrological zone.
22
Fig. 10. Integral curves of the standardized deviations for the precipitation - (1) Danube
hydrological zone, (2) Black Sea hydrological zone and (3) Aegean hydrological zone.
Fig. 11. Integral curves of the standardised deviations for the river runoff - (1) Danube
hydrological zone, (2) Black Sea hydrological zone and (3) Aegean hydrological zone.
23
Table 6. Chronological Structure of the Drought Period in Relative Unit (K) for River Runoff (h) and Precipitation (P) with Comparison to
106-Year Norms (
Ph ,
)
XXK
K
=
.
Chronological Structure
Drainage basin * ** Years
1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Bulgaria H .80 .94 .82 1.11 .58 .71 .75 .72 .59 .46 .90 .63 .43 .41 .81
P .90 .96 .92 .87 .83 .88 .98 .94 .89 .77 1.04 .78 .72 .83 1.12
Aegean H .82 .97 .94 1.08 .65 .74 .77 .72 .65 .50 .89 .57 .43 .44 .80
P .89 .99 .95 .84 .87 .90 .98 .90 .88 .82 .94 .78 .67 .81 1.09
Danube H .71 .85 .65 1.05 .38 .64 .71 .67 .49 .42 .96 .79 .38 .31 .69
P .89 .91 .89 .84 .79 .86 .98 .94 .88 .71 1.13 .77 .71 .84 1.13
Black Sea h .72 1.00 .71 1.44 .76 .75 .81 .84 .60 .34 .73 .46 .55 .55 1.30
P .87 1.01 .92 1.05 .84 .85 .97 1.08 .93 .76 1.1 .81 .90 .88 1.20
*Element; **Boundary quartile 25%.
(Source: Gerassimov, Str. G., M.G. Genev, E.K. Bojilova & T.V. Orehova 2003. “Quantitative investigations of the water resources during the
drought period”)
Table 7. Probabilistic Chronological Structures for the Drought Period 1982-1994 -
(
)
,%
P
XXP 〈
.
Probabilistic Chronological Structures
Drainage basin Years
* 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994
Bulgaria h 47 32 70 4.6 16 21 17 5.6 0.76 40 8.2 0.44 0.26
P 41 31 18 12 21 46 36 24 4.3 61 5.0 1.4 11
Aegean h 51 46 65 7.7 17 21 15 7.5 0.9 38 3.1 0.19 0.23
P 50 39 13 17 25 48 26 21 9.0 37 5.0 0.4 8.0
Danube h 41 20 62 1.7 18 25 23 5.9 2.7 54 34 1.7 0.44
P 28 26 16 7.8 18 47 38 23 3.1 81 6.3 1.9 16
Black Sea h 58 26 86 32 31 38 41 15 0.83 28 5.0 11 11
P 55 34 62 17 21 45 70 36 7.6 74 12 31 25
*element
24
Table 8. Statistical Structure of the Drought Period 1982-1994:
CvX
X
,,
σ
and Their Deviation
with Comparison for the Period 1890-1995.
Statistical Structure
Drainage basin Element
13
X
106
13
X
X
K
X
=
13
σ
106
13
σ
σ
σ
=K
13
Cv
Cv
K
mm mm
Danube h 98.3 0.638 36.1 0.642 0.367 1.005
P 633.6 0.865 82.2 0.791 0.130 0.844
Black Sea h 112 0.733 41.9 0.710 0.374 0.969
P 598.7 0.93 68.1 0.600 0.114 0.648
Aegean h 182.6 0.719 52.7 0.791 0.289 1.103
P 658.7 0.873 67.2 0.626 0.102 0.718
Total for Bulgaria h 138.2 0.695 41.7 0.747 0.302 1.075
P 640.2 0.877 66.5 0.639 0.104 0.732
Water Resources in Bulgaria During the 1982-1994 Drought Period
25
4. COMPARATIVE AND PROBABILISTIC ESTIMATIONS FOR THE DROUGHT
DEEPNESS
Using deviation from 106-year average values we can’t receive a full overall view for the
deepness of the drought. For this reason some comparisons were made. In Table 9 some 13-year
periods are presented chosen from the 106-year data series concerning runoff and precipitation.
In them the minimum values of the average negative deviations are mentioned. It is seen that
competitive periods for Bulgaria and for the Danube and Aegean zones are 1982-1994 - for
precipitation and 1983-1995 – for the river runoff (with one year's delay). For the Black Sea
basin, the minimum value of the runoff is for 1982-1994, and for precipitation - 1942-1954. In
Table 9 the estimate for the probable period is given in which is possible to observe the average
value
X
(n=13 years). The sustainable standard deviation of the average value is assessed -
(
)
nN
X
n
X
/106==
σσ
and for the standardized deviations
(
)
n
X
Nn
XXt
σ
/−=
based on
Student distribution are received the probable samples of size 13 years
(
)
tP
, and also the return
period
(
)
tPnN /=
.
From the Table 9 we can conclude that for Bulgaria and for two of the main hydrological zones
the minimum values
m
ε
and respectively the maximum values
M
N
are observed during the
period 1983-1995 for the runoff and 1982-1994 for the precipitation. In the Black Sea zone
values of
m
ε
and
M
N
are obtained for the runoff - 1982-1994, and for the precipitation – the
period 1942-1954.
The assessments for the period N are overestimated for the regions with higher trend. The most
correct and reliable estimates for the period are received when influence of the trend is taken
into consideration
106,,
QQQQ
trendiiicor
+−=
. This gives the possibility to filter the deterministic
part of the random processes
i
X
.
This type of investigation is accomplished for four different periods: 1982-1995 (n=14 years),
1982-1994 (n=13 years), 1983-1994 (n=12 years) and 1985-1994 (n=10 years). The received
results are presented in Table 10.
From Table 10 it is visible that the estimates for the return periods for the sample of the 13 years
are considerably lower, due to filtering of additional deviations from the trend, and they are
more homogeneous for all basins. These estimates are assumed to be more reliable. Along with
shortening of the periods the weak increase of the absolute values
m
ε
and for N in most of
cases the decrease is observed. There are opposite cases also – related to predominant intensity
of changes for
n
X
X
σ
,
∆
, which influence in both directions.
26
Table 9. Deviation of 13-Year Average Values (
13
X ) for the Runoff and Precipitation in
Relation to Their 106-Year Average Values (
106
X ) -
%1001
106
13
−=
X
X
ε
and Return Period N
(years).
Deviations
Drainage basin Element
1982-1994 1983-1995 1942-1954 Extremes
ε
N
ε
N
ε
N
m
ε
M
N
%
years
%
years
%
years
%
years
Danube h -36 3250 -38 4333 -14 65 -38 4333
P -14 1560 -12 765 -5 50 -14 1560
Black Sea h -28 433 -26 310 -15 38 -28 433
P -6 68 -7 44 -9 138 -9 138
Aegean h -28 4810 -30 7222 -4 20 -30 7222
P -13 1730 -12 1182 -3 30 -13 1730
Total for Bulgaria h -31 3333 -32 4063 -8 33 -32 4063
P -12 1444 -11 812 -5 48 -12 1444
Table 10. Percent Deviation of Average Measured Values
( )
∑
−=∆
n
itin
XX
n
X
1
,
1
from Their Trend
Line Values
( )
∑
=
n
it
t
X
n
nX
1
,
1
;
( )
%1001
−
∆
=
nX
X
n
n
ε
and Return Periods N (in years) Determined by
using Student Distribution.
Percent Deviations
Drainage basin * 1982-1995 1982-1994 1983-1994 1985-1994
ε
N
ε
N
ε
N
ε
N
%
years
%
years
%
years
%
years
Danube h -29.3 824 -30.0 722 -31.4 667 -36.2 571
P -9.3 326 -11.3 542 -11.7 444 -11.7 244
Black Sea h -25.3 538 -29.1 813 -31.0 750 -38.0 909
P -6.2 65 -8.0 104 -8.6 104 -9.9 96
Aegean h -13.9 2258 -24.3 1733 -26.8 1935 -32.9 2273
P -9.2 424 -10.9 684 -11.8 706 -12.4 417
Total for Bulgaria h -15.1 1795 -25.7 1444 -27.2 1277 -28.2 625
P -9.6 500 -10.7 520 -10.6 462 -11.0 250
* Element
In addition to investigations of the river runoff and precipitation, the analyses in the
groundwater variations and their state during the drought period are considered. For the aim of
Water Resources in Bulgaria During the 1982-1994 Drought Period
27
this study the examples for springs and observational wells from the National Hydrogeological
Network are used. The effect of drought period 1982-1994 to the regime of the groundwater
from the chosen representative springs and wells with long observation period from the three
main hydrological zones are executed (Orehova, T.V. & E.K. Bojilova 2001). Most of the
observational stations refer to shallow groundwater with recharge from rainfall and snowmelt
mainly. Limited number of stations represents deep aquifers. In this study the influence of the
drought period on shallow porous and karstic aquifers are investigated. These aquifers are
potentially most vulnerable to droughts.
Many karstic basins in different parts of Bulgaria characterized with different geological ages
and lithological composition, drained by springs, are reach material for analysis. The karstic
springs have specific regime that represent the dynamics of the recharge and structure of the
basin. In Upper Thracian valley and kettles filled by alluvial and proluvial deposits are formed
important resources of porous groundwater in Bulgaria. Observational wells in kettles and into
alluvial deposits along the rivers are subject to investigations of groundwater regime.
For quantitative comparative estimations the 37-year average values for the longer period
1960-1996 and included in it shorter periods: n=23 years (1960-1981), n=13 years
(1982-1994) and n=10 years (1985-1994) are obtained. The percent deviations for the shorter
period in comparison with the longer period are calculated. To make a comparison, deviations
for the same periods for the river runoff in the respective three main hydrological zones are
obtained. In Table 11 some results for the chosen karstic springs are given.
The chronological structure of the investigated periods in relative deviations
X
XX
σ
ψ
−
=
with
reference to the values of mean and standard deviation (
X
X
σ
,
) is presented in Figures 12÷14.
From Table 11 and Figures (12÷14) we can conclude:
The deviations of spring discharge and groundwater levels have the same sign as the river
runoff. The deviations are in the same range.
The drought period 1982-1994 and especially the shorter component 1985-1994 are
characterized with deep depression of the groundwater.
The chronological structure of the drought for the groundwater is similar to this of the river
runoff. The depression for the two biggest hydrological zones is mostly shown in 1994 and
the two previous years. The depression for the Black Sea zone is mostly presented in 1989
and 1994 (Bojilova E.K. & T.V. Orehova 2000).
28
Fig. 12. Chronological graphs for spring near Musina village and Danube hydrological zone.
Fig. 13. Chronological graphs for spring near Kotel town and Black Sea hydrological zone.
Water Resources in Bulgaria During the 1982-1994 Drought Period
29
Fig. 14. Chronological graphs for spring "Jazo" and Aegean hydrological zone.
Fig. 15. Chronological graphs for observational wells in Kazanlak kettle.
30
Fig. 16. Chronological graphs for observational wells in Upper Thracian valley.
Water Resources in Bulgaria During the 1982-1994 Drought Period
31
Table 11. Deviations of Average Values for Spring Discharge and River Runoff for the Three
Main Hydrological Zones for the Periods 1960-1981, 1982-1994 and 1985-1994 in Relation to
their 37-Year Values.
Deviations of Average Values
Object 1960-1996 1960-1981 1982-1994 1985-1994
ε
ε
ε
ε
%** % % %
Spring 25 – Zlatna Panega - 13.5 -21.7 -22.2
Spring 396 – Musina village - 16.8 -28.1 -45.0
Danube basin -6.0 20.5 -32.3 -39.0
Spring 48 – Kotel town - 12.5 -20.5 -24.9
Spring 63 – Malko Tarnova - 9.9 -15.9 -20.0
Black Sea basin
4.9 14.8 -29.9 -38.9
Spring 59 "Jazo" - Razlog town - 14.7 -22.9 -25.9
Spring 39а - Beden village - 14.6 -19.3 -30.0
Aegean basin -3.9 14.6 -25.3 -34.0
Bulgaria – total -3.9 17.0 -27.7 -35.8
* The analyses are made by T.V. Orehova and E.K. Bojilova;
** In relation to the period 1890-1995 (106 years).
The impact of drought on groundwater is observed all over the territory of the country. The
consequences of drought for the springs were comparable: 20-30% reduction of discharge. The
results are similar for the springs draining karstic massifs of different geological ages:
Precambrian marbles, Triassic dolomite and limestone, Upper Jurassic and Cretaceous
limestone. For observational wells in proluvial-alluvial deposits the declining of water level is
in the range between 0.2 to 0.8 m in average for the period. In Table 12 the examples of
observational wells in Kazanlak kettle and Upper Thracian valley are presented. In column 5 the
absolute deviations for selected representative wells are presented, and in column 6 - the
relative ones. The values in column 5 depend not only on reduction of recharge to the aquifer.
They are subject of influence of variable hydrogeological setting and parameters as well.
According to column 6 the comparable results are obtained. This feature can be seen in Figures
15÷16. In them the chronological structure of water level for selected observation wells are
presented in relative deviations.
The executed data give evidence for major drop of the recharge to the aquifers in Bulgaria
during the 1982-1994 drought period and also for decrease of groundwater resources. In
situation of water shortage the interest to available groundwater resources is increased for
domestic and industrial water supply. It must be taken into consideration that groundwater
similar to surface water is vulnerable to droughts. The idea of this type of research is to
introduce the problems for negative impact of drought periods to the society. It is necessary to
investigate the formation of groundwater resources and to be studied their vulnerability to
droughts. The rational utilization of water resources should take into consideration surface and
groundwater together.
32
Table 12. Characteristics of Water Level for Selected Observation Wells and Standard
Deviations of Water Levels During the Drought Period.
H
av
- average value of water level for the period 1960-1996;
σ - standard deviation for the period 1960-1996;
∆H - multi-annual average deviation for water level during the period 1982-1994.
Water Level Characteristics
Region Observational
well N:
H
av
, m
σ, m ∆H, m ∆H /σ
1 2 3 4 5 6
Kazanlak kettle 271 2.08 0.350 -0.30 -0.857
273 6.96 0.993 -0.76 -0.765
Upper Thracian valley
526 7.85 0.351 -0.23 -0.655
287a 2.92 0.663 -0.51 -0.769
Water Resources in Bulgaria During the 1982-1994 Drought Period
33
5. PROBABILISTIC SHIFTING IN THE FUTURE OF THE DEPRESSION OF 1982-1994
(PRECIPITATION AND RUNOFF) OVER THE SCENARIOS FOR DEVELOPMENT OF
THE HYDROLOGICAL PROCESSES IN BULGARIA
One pessimistic scenario for future development of hydrological processes was drafted in part
2 under assumption that the trend observed during the past century will continue for the next
century as well. This scenario will have its logical basis if the solar activity continues its raising
trend and the influence of the green house gases is restricted. However, in all cases this scenario
gives possibility to model consequences and to propose necessary counteractions against
unfavorable global changes.
Let us assume that the norms for precipitation and runoff will reach their trend values in 2100.
We suppose that the average absolute and relative deviations of the drought period 1982 – 1994
(the so-called drought model) may occur according with their absolute annual minimum values.
Thus we received two variants for the drought periods as pessimistic scenarios. These
pessimistic scenarios must be used for adequate preventive and compensatory actions for
utilization and management of water resources.
In Table 13 the absolute and relative variations in the river runoff and the precipitation are given
for the pessimistic scenario with conservation of the absolute and relative deviations of the
drought model
obs
X
.
From Table 13 it is visible that the so-called pessimistic scenario is the same for the two main
hydrological zones: more pessimistic for the runoff in the Danube basin and for the
precipitation in the Aegean zone. For the Black Sea zone, this scenario is optimistic – an
increase of precipitation and runoff is expected. It is possible this situation to lead to some
changes in the agricultural plans in Bulgaria – creation of wet plant formations.
The probability of the mentioned above model for a future big drought is very small. It can be
calculated as a product of probability of the drought period using the trend line (once in 1444
years for the runoff and once in 520 years for the precipitation) and the trend for which we
assume three main possibilities (conservation, zero and opposite direction); i.e. the probability
is 1:3, consequently the probability of the runoff will be not bigger that once in 4332 years and
for precipitation once in 1560 years. These estimates are in the near domain of the norms given
in Table 9.
The lower boundary of the probability can be determined using the following considerations. If
the standard deviation of the parent entity X is
X
σ
, then the estimate of the last parameter for the
long period (N years) and short period (n years) are:
(
)
NN
X
X
/
σσ
=
and
(
)
nn
X
X
/
σσ
=
.
Consequently, the value of
(
)
N
X
σ
theoretically is lower than
(
)
n
X
σ
or
(
)
(
)
NnnNk
XX
// ==
σσ
times, in our case we received
36.0100/13 ==k
times. In this case with k times we need to
decrease the probable return period for the dry 13-year period 0.36*1444=520 years for the
river runoff and 0.36*520=187 years for the precipitation, and we can receive the value of the
norms after 100 years. The lower boundaries of the calculated probabilities are
%1033.1
520
1
1444
1
4−
×=× for the river runoff and %1003.1
187
1
520
1
3−
×=× for the
precipitation. Furthermore, the theoretically dry pessimistic model has almost zero probability
of appearance in the future.
35
Table 13. Model for Future Strong Drought - Pessimistic Scenario,
obsX
XXK
model
= .
Model for Future Strong Drought - Pessimistic Scenario
Drainage
basin
*
Using absolute deviations
1
K
Using relative deviations
2
K
( )
1410−
X
min
X
X
K
min
X
K
X
m
X
X
K
m
X
K
mm mm - - mm mm - -
Danube h 62.8 12.5 0.639 0.260 73.4 35.9 0.745 0.748
P 587 474 0.926 0.910 593 487 0.936 0.935
Black Sea h 119.8 59.8 1.070 1.15 117.5 54.5 1.049 1.048
P 620 513 1.036 1.043 618 508 1.032 1.033
Aegean h 149.8 75.2 0.717 0.696 157.8 93.4 0.755 0.865
P 619 468 0.837 0.923 624 480 0.844 0.947
Total for
Bulgaria
h 109.8 55.4 0.795 0.684 117.5 69.8 0.863 0.862
P 606 489 0.947 0.935 610 499 0.953 0.954
39
10, mW
12.5 6.15 - - 13.2 7.75 - -
* Element
36
6. MODELS FOR NATURAL DEVELOPMENT OF THE HYDROLOGICAL PROCESSES
IN BULGARIA WITH A DIFFERENT LEVEL OF PESSIMISM. COMPARISON WITH THE
MODELS FOR ANTHROPOGENEOUS DEVELOPMENT BY GREEN HOUSE GASES
EFFECT
In the previous part 5 the scenario for development of the hydrological processes as a natural
phenomenon with very low (almost zero) probability of appearance is presented. This one can
be used as a model with high level of pessimism. Between this one and one neutral scenario we
can define a whole range of intermediate variants as follows:
1. Neutral scenario – model 0 – basic model for comparison. The main elements of the
water balance – runoff
(
)
h
and precipitation
(
)
P
are received as average values from the widely
used model investigations for global and regional climate changes for the 30-year period
1961-1990.
2. Model 1 – weakly pessimistic scenario – using the trend values for river runoff and
precipitation in the middle of the next century (2050).
3. Model 2 – medium pessimistic scenario – using the trend values for river runoff and
precipitation in the end of the next century (2100).
4. Model 3 –pessimistic scenario using the average values h and P for the period of the
deepest depression 1982-1994 ("the drought model") - depression.
5. Model 4 – strong pessimistic scenario – using the trend values for 2050 (model 1)
plus absolute deviations from the deepest depression (1982-1994) with trend values in the
middle of depression period - 1988.
6. Model 5 – the strongest pessimistic scenario – using the trend values in 2100
(model 2) plus absolute deviations from the deepest depression (1982-1994) with trend values
in the middle of depression period - 1988.
Obviously, the last two models are combinations of the models 1+3 and 2+3 respectively. The
last one was discussed in the previous part as a model of absolute deviations.
In Table 14 the relative deviations
(
)
,%
δ
for the river runoff and precipitation for the mentioned
above models in relation to the basic values
(
)
0
X
for the three main hydrological zones and
totals for Bulgaria are presented. For the further comparisons the absolute deviations of the
mean annual air temperature
(
)
T
are given.
From the list of scenarios it is visible that the drought period 1982-1994 is located in the middle
of the gradation of the pessimistic variants using deviations for river runoff and precipitation
from the basic values.
In the present range of pessimistic scenarios for natural development of the hydro-processes we
will try to find the place for some scenarios for the anthropogeneous impact through green
house gases choosing from the large number of available models created in the last years. The
global and regional models for atmospheric circulation and induction from the green house
gases are widely used with the chosen level of
2
2 CO×
; that mentioned level of concentration is
applied for the next century with basic period 1961-1990.
Water Resources in Bulgaria During the 1982-1994 Drought Period
37
From the results of the used for Bulgaria CFDL-T model (Sharov, V. & E. Koleva 1999) for
2060 the decrease of the precipitation with –7% and –10% for North and South Bulgaria
respectively was received. According to the same authors, the received increase in the
temperature is about +3.9
C
0
. This value is very high, so they refer to the other researchers
(study of IPCC) from which is the information that for the next century we can expect increase
in the temperature with “only” +2
C
0
. So, we can conclude that for the model of next century we
can expect the increase of the temperature of about +1
C
0
.
For comparison in the list of models in Table 14 for natural development of the hydro-processes
the increase in air temperature does not exceed 1
C
0
(+0.87
C
0
for the Danube basin from the
model 5).
The received decrease in the precipitation -7÷-10% obviously corresponds to the models 2 and
3 or to the average model between them.
We can receive approximate estimate for the river runoff using conformity between the trend
equations from Table 4 for the modeled precipitation over North and South Bulgaria, which can
be used with some assumptions for the Danube and Aegean hydrological zones. We received
decrease in the river runoff -28% and -34% respectively, that is in the range of model 3 using
Table 14 but with changes in the places of zones. In the mentioned above climatic model, the
authors used different preliminary information – South Bulgaria includes some parts of Danube
and Black Sea basins. Also in the climatic study the average values of air precipitation (
PP
i
/
)
from the stations most often located in the lower parts of the country – below 800 m a. s. l is
often used. In our study, the average values are corresponding to the average altitude above sea
level of the basins. However, these differences in the estimates for the river runoff and the
climatic anthropogeneous model lead us to the model 3 for the natural development of the
hydro-atmosphere processes.
According to the used from Arnell (Arnell, 1999) the simulation hydrological model in the basis
of four scenarios (UKHI, UKTR, CCC и GHGx) of climatic changes in Europe, for the territory
of Bulgaria we can expect decrease in the river runoff in the range between -25% up to -50%. In
Table 14 we made the comparison and we can conclude that the models with anthropogeneous
changes in the water resources due to green house gases effect are in the range between model 2
and model 5 for natural development – from medium pessimistic up to strongest pessimistic
scenarios.
Approximate probabilistic estimates for the values of runoff and precipitation assumed to be
average values for the different time intervals are given for to make orientation easier in the
possible realization of the mentioned scenarios.
(1) The deviations in Table 14 concern the average values for one year. All the models
are realistic in extremely dry years such as 1993, 1994 and 1990 from the studied dry period -
see Table 6 and Table 7. There the deepest drought is characterized with 69% for the runoff in
1994 – Danube basin. The lowest probability is 0.19 (once in 526 years) for the annual river
runoff in the Aegean zone during 1993.
(2) The deviations in Table 14 refer to the average values for the 13-year period. From
Table 9 and Table 10 we received orientation that the model 3 has very small probability of
occurrence: once in 1444 – 3333 years for the runoff of Bulgaria and once in 520 – 812 years for
38
the precipitation. The model 5 has very low probability of occurrence – less than once in 4333
years for the runoff and once in 1560 years for the precipitation.
(3) The deviations in Table 14 use the average values for the 30-year “climatic” period.
The probability for the central pessimistic scenario (model 3) is negligible and is almost zero.
Using the Student distribution, there are no available tables for such big standard deviations, but
using extrapolations the probability of the 30-year independent sets is lower than once in 1
million cases (30 million years).
We can conclude that the scenarios for such drastic decrease of the climatic norms formulate
one apocalyptic picture for the drought of Bulgaria. Such severe drought is difficult to accept for
appearance in the coming 50-100 years.
39
Table 14. Pessimistic Scenarios for Future Natural Development of the Hydrological Processes in Bulgaria. Deviations
(
)
,%
δ
of the Basic Elements
of the Water Balance
(
)
X
in Relation to their Basic Values
(
)
0
X
and for Air Temperature (T) in
C
0
.
Pessimistic Scenarios
Drainage
basin
* Model 0
(basic period 1961-1990)
Model 1
Trend 2050
Model 2
Trend 2100
Model 3
Depression
1982-1994
Model 4
Model 5
T
1
T
2
D T
1
+D T
2
+D
X
o
0
δ
*
0
δ
1
δ
2
δ
3
δ
4
δ
5
δ
% % % % % % %
Danube h, mm 152.0 1.4 -5.1 -20.7 -31.2 -35.3 -48.8 -58.7
P, mm 725.8 1.0 -0.8 -5.1 -8.0 -12.7 -16.2 -19.1
T,
C
0
8.24 -0.08 0.1 0.54 0.84 0.21 0.57 0.87
Black Sea h, mm 165.5 -7.7 -5.1 -2.0 0.1 -32.3 -29.7 -27.6
P, mm 660.9 -2.6 -1.8 0.3 1.7 -9.4 -7.6 -6.3
T,
C
0
10.52 -0.05 0.09 0.4 0.61 0.10 0.36 0.57
Aegean h, mm 254.9 -0.4 -3.6 -12.3 -18 -28.4 -35.5 -41.2
P, mm 749.9 0.7 -0.8 -4.3 -6.7 -12.2 -15.0 -17.4
T,
C
0
7.63 0.0 0.13 0.43 0.63 0.20 0.45 0.65
Total for
Bulgaria
h, mm 200.2 -0.7 -4.3 -13.6 -20.1 -31.0 -38.6 -45.1
P, mm 727.7 0.4 -0.9 -4.1 -6.1 -12 -14.6 -16.7
T,
C
0
8.28 -0.04 0.11 0.47 0.71 0.19 0.49 0.73
*Abbreviation
∑
=
=
30
1
0
1
i
i
X
n
X
;
,%1001
0
106
0
×
−=
X
X
δ
;
,%1001
0
*
0
×
−=
X
X
Tr
δ
;
5,...2,1,%;1001
0
=×
−= i
X
X
i
i
δ
.
Source: Gerassimov, Str. G. & E.K. Bojilova 2003.
40
7. SCENARIOS WITH CONSIDERATION OF NATURAL AND ANTHROPOGENEOUS
FACTORS
The mathematical examination of the double-mass curves analysis was widely used. This
apparatus for the number of Wolf (Waldmeick, 1961)
∑
=
i
ii
WX
1
and global temperature
anomalies (
T
∆
) of the Northern hemisphere (Lean, J. & P. Foukal 1988):
( )
∑
+∆=
i
ii
Ty
1
100.1
was applied. On its basis five periods with linear approximation applying constant gradients
∑∑
=
k
j
ji
k
j
jiij
XyG
,,,
/
were estimated. These five periods are presented in Table 15
(Gerassimov, Str.G., E.K. Bojilova, T.V. Orehova & M.G. Genev 2001).
Table 15.
Periods with constant gradients.
Periods With Constant Gradients
Periods
n
W
C G
1854-1877 24 49.8 288 1.367
1878-1937 60 38.6 296 1.928
1938-1947 10 65.0 308 1.663
1948-1960 13 100.8 318 1.141
1961-1995 35 71.9 338 1.501
The number of the years in the different periods is from ten to sixty. Respectively, the value of
the solar activity is between 38.6 up to 108.8. The values of the gradients are in the range from
1.141 to 1.928. On the basis of the points, the empirical relationship is estimated:
( )
200. −+= Cb
W
a
G
α
(1)
in which C is the concentration of CO
2
in ppm;
α
,,
ba
are empirical constants.
Furthermore, after estimation of the empirical constants and with application of
(
)
GfT =∆
with weighting factor of the errors one can receive the next relationship:
( )
175.1200..10.052.7
086.1
5
15.0
−−+=∆
−
CW
W
T
. (2)
Using the trend linear estimations between the precipitation over the Great Britain
( 904.2098.0 +=
xP
GB
) and global temperature anomalies (
317.0.0055.0
−
=
∆
xT
) is obtained:
1.9182.38 +∆×=
TP
GB
. (3)
Water Resources in Bulgaria During the 1982-1994 Drought Period
41
On the basis of the trend linear estimations between air temperatures, precipitation on Bulgaria
with global temperature anomalies and precipitation over Great Britain the following
relationships are received:
66.78727.0 +∆×= TT
BG
, (4)
GBBG
PP ×−= 437.153.2045 . (5)
The average discharge layer for Bulgaria is estimated in the same manner with equation:
7.517981.0 −×=
BGBG
Ph . (6)
In the present research, four different scenarios of the Intergovernmental Panel of Climate
Change IPCC (A, B, C, CD) with chosen levels of concentration of CO
2
in 2100 are used. In this
study everyone from these scenarios will be combined with different level of solar activity –
such as 91;78;8.38;2.63 andW = . These different scenarios are presented in the following
part of this paper.
SCENARIOS WITH DIFFERENT LEVELS OF SOLAR ACTIVITY
Using the equations given above, the received results for chosen scenarios are presented in
Тable 16. The group of scenarios are with average level of solar activity W . The basic values
for comparison are in fact the trend values in the middle period 1961-1990. The values are
presented in Table 17.
Comments for scenarios with 2.63=W . The scenario A with extremely high concentration of
green house gases would have catastrophic influence over the water resources of Bulgaria and is
almost impossible to expect. The scenario B is an average scenario and is possible to appear.
The scenarios C and CD with the most probable increase of the concentration of CO
2
- they
show possible decrease of the precipitation and river runoff even with the lower temperatures
compared with the basis values.
Comments for scenarios with 8.38=W . These are optimistic scenarios in direction of solar
activity. In them the air temperatures over Bulgaria are lower then the basis values excluding
scenario A. The last scenario CD is with increase in precipitation and river runoff even in
context of increasing concentration of CO
2
. Only in these optimistic scenarios for case of CD
we can expect increase in precipitation
%2.0=
p
δ
and discharge
%8.0=
h
δ
.
42
Тable 16. Scenarios with different levels of solar activity W .
Scenarios With Different Levels of Solar Activity W
Scenarios
2
CO
C
T
∆
GB
P
p
δ
BG
T
BG
P
p
δ
BG
h
h
δ
ppm
C
0
mm
%
C
0
mm
%
mm
%
2.63=W
A 820 2.17 1001 8.6 9.55 607 -15.8 78 -58.9
B 560 1.01 957 3.8 8.54 670 -7.0 140 -26.2
C 490 0.70 945 2.5 8.27 688 -4.6 157 -17.3
CD 430 0.43 933 1.4 8.04 702 -2.6 171 -9.8
8.38=W
A 820 1.15 962 4.4 8.66 663 -8.0 133 -29.9
B 560 0.44 935 1.4 8.04 702 -2.6 171 -9.8
C 490 0.25 928 0.7 7.88 712 -1.6 181 -4.5
CD 430 0.08 921 -0.1 7.73 722 0.2 191 0.8
78=W
A 820 2.42 1011 9.7 9.77 593 -17.2 64 -66.2
B 560 1.19 963 4.5 8.70 662 -8.2 132 -30.3
C 490 0.85 951 3.2 8.40 679 -5.8 148 -21.9
CD 430 0.57 940 2.0 8.16 695 -3.6 164 -13.5
91=W
A 820 3.36 1046 13.5 10.59
542 -24.8 14 -93
B 560 1.69 983 6.6 9.13 633 -12.2 103 -45.6
C 490 1.24 965 4.7 8.74 659 -8.6 129 -31.9
CD 430 0.86 951 3.2 8.41 679 -5.8 148 -21.9
*
,
2
CO
C
ppm
is the concentration of CO
2
;
CT
0
,∆ are global temperature anomalies for Northern hemisphere;
CT
BG
0
, is air temperature over Bulgaria;
mmPP
BGGB
,, are the precipitation sums over Great Britain and Bulgaria respectively;
mmh
BG
, is discharge layer over Bulgaria;
,%
P
δ
is relative deviation for the precipitation;
,%
h
δ
is relative deviation for the discharge layer.
Comments for scenarios with 78=W . The presented scenarios are pessimistic ones for
precipitation and discharge for Bulgaria, especially scenario A. The scenario A has no analogy
in the natural development of hydrological processes up to now.
Water Resources in Bulgaria During the 1982-1994 Drought Period
43
Comments for scenarios with 91=W . The scenarios A with 13.5 % increase of precipitation
over Great Britain is drastically for water resources of Bulgaria - 93% decrease of the river
runoff and – 24,8 % of precipitation. These scenarios with increased level of solar activity are
the strongest pessimistic (black) scenarios for water resources in Bulgaria.
Table 17. Basic values for comparison.
Basic Values
Parameters Values
CT
00
,∆
0.10
mmP
GB
,
0
921.8
CT
BG
00
,
8.39
mmP
BG
,
0
720.9
mmh
BG
,
0
189.5
44
8. CONCLUSION FOR MOST POSSIBLE SCENARIOS FOR BULGARIA
From the presented four scenarios for different concentrations of CO
2
(A, B, C and CD) we can
exclude the extreme scenario A for all chosen levels of W . The other three scenarios we can
accept as equally probable. We need to take into consideration one scenario CD with the lowest
values of CO
2
concentration and solar activities giving slight increase of the precipitation and
river discharge (+ 0,2 % and + 0,8 %, respectively).
We received wide range of nine possible scenarios for Bulgaria (models 0÷
÷÷
÷5 plus B, C and CD).
The first group are scenarios for tendencies and analogies to the natural development (models
0÷
÷÷
÷5). The second group includes scenarios with consideration of natural and anthropogeneous
factors with application of empirical relationships (scenarios B, C and CD). So, we can
conclude that from nine scenarios only CD has slight increase in precipitation and river runoff.
The other eight scenarios show low values of these parameters. Furthermore, we have
probability of 89% for decrease in the precipitation and discharge over Bulgaria, so there are no
reasons for optimistic expectations.
The most possible for the near future development is the model 1 - weakly pessimistic
scenario, using the trend values for river runoff and precipitation in the middle of new
century (2050). The model 1 shows similar results as scenario C with moderate values of CO
2
concentration. This scenario has the biggest frequency and probability of occurrence in the
context of normal distribution of the probabilities. This is a medium pessimistic estimation,
and it can be proposed for future hydro-technical design, construction works and utilization
of the water resources.
Water Resources in Bulgaria During the 1982-1994 Drought Period
45
BASIC RESULTS
1. Since 1980 in Bulgaria long lasting decrease in the precipitation combined with
increase of the air temperature were observed, which lead to deep depression in the river runoff
and groundwater flow.
2. During the 1982-1994 period the runoff and precipitation in Bulgaria are below the
norms. This period is characterized with 31% decrease of runoff for Bulgaria with comparison
to the norms to the period 1890-1996 (with -26% from the trend norm and -31% from the
average of the basic period 1961-1990). This period is characterized with very low probability
of occurrence: once in 3333, or once in 1444 years for the runoff and precipitation respectively.
The relative decreases from the trend with accordance to the absolute value are biggest for the
Danube hydrological basin (-31%) and the lowest in the Black Sea zone (-29%). The return
periods are between 1733 years for the Aegean and 722 years for the Danube basins
respectively.
3. The drought period 1982-1994 was preceded by the long wet period 1954-1981, in
which only some years are below the norms. This wet period is included in the study of the
106-year (1890-1995) research period with negative trends for the precipitation and the runoff
in Bulgaria.
4. The long 108-year period 1890-1997 is characterized with the next tendencies for
the three main hydrological zones:
Increase of the average annual air temperature for the three zones in accordance to the
trend lines for the solar activity and radiation and temperature anomalies in the Northern
Hemisphere.
Decrease of the precipitation and river runoff for Danube and Aegean zones and
increase for both elements for the Black Sea zone; with accordance to asynchrony and
synchrony with precipitation over England & Wales were obtained.
5. The depressions in the drought period are most visible during 1990, 1993 and 1994
when the absolute minimums of the longer period 1890-1995 are observed. The values are from
0.31 to 0.43 in accordance to the norm and probability of occurrence once in 526 years for the
Aegean basin up to once in 120 years for the Black Sea zone.
6. Hydrogeological time-series during the 1982-1994 drought period showed clear
vulnerability of groundwater in Bulgaria to drought. Due to economic significance of
groundwater resources, extensive research should be projected concerning impact of droughts
on recharge to aquifers.
7. The drought period 1982-1994 is used as a model for the future possible deep
drought – the strongest pessimistic scenario. We made the next assumptions:
Preservation of the absolute and relative deviations around the trend values;
Preservation of all trend lines from the 1890-1997 period during the next century;
Accepting the trend values from 2100 as norms for the model.
46
8. The numerical values of the strongest pessimistic scenario have very low probability
of appearance for future realization – not bigger than once in 4333 years for the runoff and once
in 1560 years for the precipitation.
9. Using the trend pessimistic assessments for the middle and the end of the next
century and the 1982-1994 depression, five different scenarios for pessimistic natural
development for the hydro-processes in Bulgaria were proposed. For these scenarios the
estimates of the deviations for the river runoff, precipitation and air temperature in relation to
the average values for the chosen basic period 1961-1990 are obtained.
10. The depression of precipitation and runoff for the 1982-1994 drought period takes
central place amongst the proposed five pessimistic scenarios (model 3).
11. The models of anthropogeneous process impact in Bulgaria, which take into
consideration the green house gases effects are between models 3 and 5. They have very low
probability of occurrence of the climatic norms (average values for the 30-year period).
12. The drought period of 10-14 years with similar parameters of the discussed one
(1982-1994) is possible to appear during the next century, although it has very low probability
of occurrence.
Water Resources in Bulgaria During the 1982-1994 Drought Period
47
RECOMMENDATIONS
For diminishing the negative impact from the possible future droughts, we can give some
recommendations for the utilization of the water resources:
Construction of reservoirs with annual (seasonal) and multi-annual reservoir regulation
of the runoff as a long lasting prevention action.
Water transfer from the wet water basins to the dry regions – as long lasting and short
lasting actions, after deep hydrological, ecological and economic investigations.
Rational utilization of the available water resources accumulated in the reservoirs,
natural lakes and groundwater reservoirs during the wet periods/years, as constant issues in
the management of the water resources.
Economical water utilization from all consumers using legislative and economic actions
and stimulus.
Restriction actions in water users' policy taking into consideration the priority of the
different groups of water users, as an extreme action to overpass the water crisis.
48
VARIATIONS OF THE HUMIDITY DURING THE DROUGHT PERIOD 1982-1994
1982. Precipitation and river runoff for the Black sea rivers – around and little bit lower
compared with the norms; South Bulgarian rivers – around the norms; Danube tributaries –
up to 30÷90% of the norms, the probability of non-exceedance - 30÷60%. The driest months
– September and October.
1983. The drought period continues during the whole year, in many regions the minimum
river runoff is in May and in the other ones it is from June to November. The annual runoff
is up to 60÷90% from the norm with probability of non-exceedance 20÷50%. The runoff is
lower in Danube and Black Sea zones. The rainfalls are rare and are distributed over the
year. Annual sum of precipitation is 85÷95% of the norms with probability of occurrence
25÷50%. Extreme flood events are observed on the Southeastern rivers caused by intensive
rainfalls (up to 350mm precipitation for 2-3 days).
1984. The drought period continues in January. During February – May there are sufficient
precipitation sums and spring seasonal flood. The drought begins from August and
continues up to the end of the year. The driest months are from September to December.
Annual river runoff is over the norms with 5÷45%. Seasonal flood is observed especially in
the rivers from the Black Sea zone. The probability of non-exceedance is around 70%
(60÷90 %). Annual sum of precipitation for Bulgaria is below the norm (-10%÷ -15%). For
the Black Sea zone it is slightly above the norm (5÷10%) with probability of
non-exceedance around 20% (10÷65%).
1985. The drought period continues during January and February with low flow in the
mountain regions up to April. Seasonal flood is from March to April. After this the drought
period continues. The drought is strong in Danube basin (up to 30÷40% from the norm) and
has weak appearance in the Black Sea zone (up to 70÷80% from the norm). The
precipitation sums are 80÷90% from the norm.
1986. The drought period is lasting during January with interruption of 2-3 months and after
this the force of the drought increases. The river runoff for total area of the country
decreases up to 70% of the norm, for the Danube zone - 60% of the norm, with probability
of occurrence 15% (10÷35%). The precipitation sums are 80÷90% of the norm, with
probability of occurrence 20%.
1987. The river runoff and precipitation are below the norm (70÷80%, 96÷98% from the
norm respectively). The humidity in winter and spring is in the normal range. The drought
begins from June and continues up to the end of the year. The driest months are August and
September.
1988. Also a dry year. The river runoff decreases up to 72% of the norm (65-85%), the
precipitation diminishes up to 94% of the norm (65÷110%). The Black Sea zone is more
humid. The probability values of non-exceedance are 17÷36% in average for the runoff and
precipitation sums. Normal seasonal low-flow and drought from July to October.
Water Resources in Bulgaria During the 1982-1994 Drought Period
49
1989. A dry year - 59% of the norm for the runoff and 89% for the precipitation (45÷65%
and 80÷95% respectively). The driest basin is the Danube drainage basin followed by the
Black Sea zone and the Aegean hydrological area. The drought period is mainly from July to
September and seasonal flood is rather weak.
1990. An extremely dry year. The runoff decreases in average for the country up to 46% of
the norm, and for the precipitation up to 77%. The driest it is in the Black Sea zone (34% for
runoff and 76% for precipitation from the norms), followed by Danube and Aegean
hydrological zone (50% and 82%). The probability of non-exceedance is below 1% for the
runoff and around 4% for the precipitation. The drought continues during all year except
January and December. During August – September some rivers get dry.
1991. For Bulgaria totally the humidity is around the norm (90% for the runoff and 104%
for the precipitation). The Black Sea zone is the driest (73%), and the wettest – the Danube
zone. Extreme flood along the river Jantra in its upper part around Gabrovo and Tarnovo
towns from intensive rainfalls on 6-7 of July occurs (up to 100mm for two days).
1992. The runoff and precipitation below the norms (63% and 78% of the norms in average
for the country) with probability of occurrence 8.2% and 5.0% respectively. During all year
it is dry but seasonal flood and low flow have normal appearance with maximum discharge
in April and minimum river flow in August – September.
1993. An extremely dry year. The runoff decreases in average for the country up to 43% of
the norms, and for precipitation – 72%. The low flow period is strongly visible in Danube
zone (up to 35-40% of the norms) and is weakly represented in the Black Sea zone – up to
55% of the norms. The runoff has very small seasonal flood only during March and May.
During August and September some of the river bodies get dry.
1994. The driest year from the 1982-1994 drought period. The runoff decreases to 41% of
the norm in average for Bulgaria. The precipitation sums show slight increase – 83% of the
norm. The low flow is lasting during almost the whole year. The small seasonal flood is
observed during March – May. During August and September some of the river bodies get
dry.
1995. The precipitation sums are in average over the norms for the country – 12% above the
norms, but the runoff is still under the norms because of the memory of the process (19% in
average). The runoff is the lowest in the Danube zone – 69% of the norm, and the biggest
discharge is for the Black Sea zone – 130% of the norm.
50
EXTENDED LIST OF REFERENCES
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perspective. Global Environmental Change, 9: 5-23.
Anderson, T.W. 1971. The Statistical Analysis of Time Series. John Willey & Sons, New York.
Bojilova, E.K. & T.V. Orehova 2000. Influence of Drought Period of 1982-1994 to the
Groundwater Regime in the Danube Hydrological Zone. 20th Conference of the Danube
Countries, The Slovak Republic.
Bojilova, E.K., Str.G. Gerassimov, T.V. Orehova & M.G. Genev 2002. Scenarios for Natural
development of the hydrological processes in Bulgaria considering anthropogeneous
impact. 6 p., 21st Conference of the Danube counties, 2-6 Sep., Bucharest, Romania, full
text on CD-ROM.
Bojilova, E.K., Str.G. Gerassimov, T.V. Orehova & M.G. Genev 2002. Natural state of
development of the river runoff in Bulgaria considering anthropogeneous impact. 15 p.,
ICHE-2002 Conference, 18-21 Sep., Warsaw, Poland, full text on CD-ROM.
Box, G.E.P. & G.M. Jenkins 1970. Time Series Analysis - Forecasting and Control. San
Francisco, California: Holden-Day, U.S.A.
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Water Resources in Bulgaria During the 1982-1994 Drought Period
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SCIENTIFIC PROFILES OF THE AUTHORS
1
STRAHIL GERASSIMOV GEORGIEV - Professor, Dr., Eng. Areas of professional
interests: general and engineering hydrology, hydrometry, hydrological networks, hydrological
analysis and estimations, hydrological forecasting, hydro-ecology, water resources and their
variations due to natural and anthropogeneous impact, flood events, drought studies.
1
MARIN GOSPODINOV GENEV - Associate Professor, Dr., Eng. Areas of professional
interests: general and engineering hydrology, water resources, hydrological forecasting, global
climate change studies, ecology and environment protection studies, artificial lakes.
1
ELENA KIRILOVA BOJILOVA - Researcher Scientist, Dr., Eng. Areas of professional
interests: general and engineering hydrology, hydrological analysis and estimations,
hydrological modeling, stochastic and disaggregation modeling, river analysis system
modeling, soil physics, mathematical ecology and ecological economics.
2
TATIANA VASILEVNA OREHOVA - Senior Research Associate, Dr., Hydrogeologist,
Institute of Geology – Bulgarian Academy of Sciences. Areas of professional interests:
groundwater and climate variability; groundwater monitoring and modeling; groundwater
dynamics.
1
National Institute of Meteorology and Hydrology, Bulgarian Academy of Sciences,
Tzarigradsko chaussee 66, 1784 Sofia, Bulgaria
E-mail: marin.genev@meteo.bg; elena.bojilova@meteo.bg
2
Geological Institute, Bulgarian Academy of Sciences,
Str. Acad. G.Bonchev, bl. 24, 1113 Sofia, Bulgaria
E-mail: orehova@geology.bas.bg
