A Large International River: The Danube. Summary of Hydrological Conditions and Water Management Problems in the Danube Basin

W O R K I N G P A P E R

A LARGE INTERNATIONAL

RIVER

THE DANUBE SUMMARY OF HYDROLOGICAL CONDITIONS

A N D

WATER MANAGEMENT PROBLEXS IN THE DANUBE BASIN

B.

Hock G. Kovdcs

January

1987 WP-87-11

I n t e r n a t i o n a l I n s t i t u t e for Applied Systems Analysis

NOT FOR QUOTATION WITHOUT PERMISSION OF THE AUTHOR

A LARGE INTERNATIONAL

RIVER:

THE DANUBE SUMMAEW OF HYDROLOGICAL CONDITIONS AND WATER MANAGEXENT PROBLEXS IN THE

DANUBE BASIN

B. Hock G. Kovdcs

January

1987 WP-87-11

Working _Papers are interim r e p o r t s on work of t h e I n t e r n a t i o n a l I n s t i t u t e f o r Applied Systems Analysis a n d h a v e r e c e i v e d only lim- i t e d review. Views o r opinions e x p r e s s e d h e r e i n d o not neces- s a r i l y r e p r e s e n t t h o s e of t h e I n s t i t u t e or of i t s National Member Organizations.

INTERNATIONAL INSTITUTE FOR APPLIED SYSTEMS ANALYSIS 2361 Laxenburg, Austria

PREFACE

The demand of policy makers and managers to find environmentally sound a n d sustainable economic development i s obvious. At t h e s a m e time, v a r i o u s b r a n c h e s of s c i e n c e s dealing with environmental issues have become more and more specialized. The solution to problems

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o f t e n of a global c h a r a c t e r

-

r e q u i r e s t h e interdisciplinary analysis of v e r s a t i l e sys- tems consisting of n a t u r a l , economic and social elements of t h e environment.

Within t h e long s e r i e s of water r e l a t e d topics of IIASA1s Environment Programme, a new p r o j e c t D e c i s i o n S u p p o r t S y s t e m s f o r M a n a g i n g L a r g e I n t e r n a t i o n a l R i v e r s

(Zm)

w a s r e c e n t l y launched. The formulation of environmentally sound management policy f o r land-use and water r e s o u r c e s development r e q u i r e s t h e r e l i a b l e prediction of t h e impacts of d i f f e r e n t human interventions in o r d e r t o eliminate conflicts between d i f f e r e n t i n t e r e s t groups, and t o p r e s e r v e t h e quality of life in both t h e b i o s p h e r e and society. S e v e r a l models f o r t h e assessment of various environmental impacts a l r e a d y exist, b u t t h e l a r g e s c a l e of r i v e r basins and t h e amount of d a t a

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t h e availability of which is even limited in some cases

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r e q u i r e t h e development of a g g r e g a t e d systems of models t h a t c a n provide decision makers with easily understandable information at various h i e r a r c h i c a l lev- els. Considering t h i s requirement, t h e objective of t h e p r o j e c t i s t o con- s t r u c t a computer-based i n t e r a c t i v e d a t a and information system to ^facili- tate t h e effective participation of policy making a u t h o r i t i e s in determining c u r r e n t conditions and expected changes in hydrological systems.

The outline of LIR emphasizes t h e importance of t h e p r e p a r a t i o n of case studies. Their r o l e is not only t o check t h e applicability on t h e system f o r solving actually o c c u r r i n g problems, but t h e analysis of t h e basins as cases will a s s i s t in selecting t h e c r u c i a l questions t h a t should b e answered by t h e Decision S u p p o r t System. The Danube basin w a s chosen as t h e f i r s t

c a s e study t o b e investigated in t h e framework of LIR. The r e a s o n f o r t h i s choice i s p a r t l y t h a t IIASA is located in t h e basin, and, t h u s all information easily accessible. The international c h a r a c t e r of t h e r i v e r ( t h e r e a r e 8 r i p a r i a n countries and 3 o t h e r s s h a r i n g a small p a r t of t h e catchment), t h e rapidly developing problems of t h e utilization of water (canalization, increasing transboundary pollution, seasonal water s h o r t a g e ) , and t h e e f f o r t s of t h e r i p a r i a n countries t o improve t h e conditions of water r e s o u r c e s development within t h e basin (which i s c l e a r l y indicated by t h e f a c t t h a t a joint declaration was signed) a r e a l s o r e a s o n s supporting t h e selection of t h e Danube as t h e f i r s t c a s e study.

Naturally, t h e hydrological conditions of t h e r i v e r system and t h e water management problems o c c u r r i n g within t h e catchment a r e well known f o r t h e e x p e r t s in water sciences working in t h e r i p a r i a n countries. F o r e x p e r t s participating in t h e p r o j e c t and coming from o t h e r countries, o r , r e p r e s e n t i n g o t h e r scientific disciplines, i t i s necessary, however, t o sum- marize t h e most important information describing t h e water regime of t h e r i v e r system and t h e obstacles hindering t h e development of water r e s o u r c e s in t h e basin. This working p a p e r and t h e detailed list of r e f e r - e n c e s provide more information on w a t e r management in t h e Danube basin.

R.E. Munn

Environment Programme

A LARGE INTERNATIONAL RTVER- THE DANUBE SUMAURY OF HYDROLOGICAL CONDITIONS AND WATER MANAGEMENT PROBLEMS IN THE

DANUBE BASIN

B. Hock* and G. Kovdcs

INTRODUCTION

The Danube basin was selected as t h e f i r s t c a s e study t o b e analysed within t h e framework of t h e IIASA p r o j e c t aiming at t h e development of a decision s u p p o r t system ( h e r e a f t e r DSS) f o r managing l a r g e international r i v e r s ( h e r e a f t e r LIR). I t w a s f e l t necessary t o p r e p a r e a p a p e r which would provide t h e e x p e r t s

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working in t h e p r o j e c t but not being familiar with t h e water regime of t h e Danube

-

with t h e basic information on t h e hydrological condition of t h e r i v e r system as w e l l a s on t h e historical development and t h e p r e s e n t problems of water management within t h e basin.

The compilation of t h e p a p e r w a s assisted by t h e excellent l i t e r a t u r e dealing with t h e v e r s a t i l e problems of water r e s o u r c e s development in t h e basin. The list of r e f e r e n c e s in t h i s p a p e r provides guidance f o r r e a d e r s who are i n t e r e s t e d in obtaining more detailed information on some

* s c i e n t i f i c Adviser, Research Centre f o r Water Resources Development (VITUKI), Budapest

p a r t i c u l a r topic in water management.

One publication t h a t d e s e r v e s p a r t i c u l a r mention is t h e Danube Monog- r a p h y (RZdD, 1986), t h e r e s u l t of a joint p r o j e c t of t h e e i g h t r i p a r i a n coun- t r i e s where e x p e r t s collected lengthy d a t a s e r i e s c h a r a c t e r i z i n g t h e quan- t i t a t i v e regime of t h e Danube and i t s main t r i b u t a r i e s . The d a t a were h a r - monized by t h e F e d e r a l Hydrometeorological Institute in Belgrade and t h e R e s e a r c h Institute f o r Water Management in Bratislava, who a c c e p t e d responsibility f o r coordination of national r e p o r t s .

Finally, t h e e n t i r e material, which contains not only t h e hydrological d a t a , b u t also t h e h i s t o r i c a l summary of water management activity in e a c h c o u n t r y , w a s edited and published by t h e Bayerische Landesamt fiir Wasserwirtschaft in Miinchen. Detailed information o n quantitative condi- tions a n d water r e s o u r c e s development are available in t h i s publication.

Unfortunately, t h i s p r o j e c t w a s limited to traditional hydrology and t h e water quality problems were not dealt with in t h i s framework. T h e r e f o r e , i t w a s n e c e s s a r y to give more detailed information on t h e conditions of water quality in t h i s working p a p e r , because in t h i s field no such comprehensive publication can b e o f f e r e d , where t h e qualitative d a t a are summarized in a similar way.

Analyzing t h e problems hindering t h e utilization of water r e s o u r c e s in t h e basin, i t w a s found t h a t t h e major obstacle at p r e s e n t and in t h e n e a r f u t u r e i s water pollution due t o t h e r e l e a s e of waste waters not sufficiently t r e a t e d from community systems and industries as w e l l as t h e non-point s o u r c e pollution originating from intensified a g r i c u l t u r e and u r b a n areas.

Some qualitative p a r a m e t e r s indicate t h a t even t h e quality of bank f i l t e r e d water

-

providing t h e l a r g e s t amount of water f o r community water supply

-

i s endangered by t h e increasing d e t e r i o r a t i o n of r i v e r waters. This w a s a l s o a motivation r e q u i r i n g more detailed information on quality conditions.

A l s o t h e r e w a s a t h i r d a s p e c t giving r e a s o n s f o r studying t h e water quality problems of t h e Danube. The implementation of a regional UNDP/WHO p r o j e c t will commence in 1987 aiming a t t h e determination and improvement of water quality conditions in t h e Danube basin within t h e framework of which t h e r i p a r i a n countries will make joint e f f o r t s t o solve t h i s s e r i o u s problem of water management. T h e r e a r e s e v e r a l common

elements in t h e two p r o j e c t s (i.e. UNDP/WHO p r o j e c t on t h e Danube water quality and t h e IIASA LIR Danube case study). T h e r e f o r e , i t w a s decided t h a t t h e i r implementation should b e closely coordinated. Hence, while p r e p a r i n g t h i s p a p e r i t w a s considered t h a t t h i s should a l s o s e r v e as a background document f o r t h e UNDP/WHO p r o j e c t .

T h e r e a r e two a n n e x e s t o this r e p o r t . The f i r s t i s t h e Declaration by t h e r i p a r i a n c o u n t r i e s signed in December 1985; and t h e second is a sum- mary evaluation of t h e environmental impact assessment of t h e Gabcikovo-Nagymaros b a r r a g e system.

In t h e Declaration, r e p r e s e n t a t i v e s of t h e e i g h t r i p a r i a n c o u n t r i e s e x p r e s s e d t h e i r willingness t o solve t h e problems of water management in a coordinated manner, as i t w a s recognized t h a t t h e o b s t a c l e s hindering t h e r e a s o n a b l e utilization of water r e s o u r c e s c a n b e removed only by joint e f f o r t s in a n international r i v e r basin. Decision w a s a l s o r e a c h e d concern- ing t h e institutional framework r e q u i r e d t o implement t h e programmes defined in t h e Declaration in t h e fields of water quality c o n t r o l , flood p r o - tection and g e n e r a l w a t e r management. The DSS of LIR and t h e r e s u l t s of t h e Danube case study are o f f e r e d t o t h e r e l e v a n t a u t h o r i t i e s of t h e e i g h t c o u n t r i e s as usable tools t o achieve t h e objectives p u t forward in t h e Declaration.

The modification of environmental conditions in t h e r i v e r and in t h e surrounding region d u e t o t h e construction of b a r r a g e s w a s r a i s e d in some r i p a r i a n c o u n t r i e s as a s e r i o u s environmental concern. The s a m e happened in Hungary in connection with t h e Gabcikovo-Nagymaros b a r r a g e system now under construction. A detailed environmental impact assessment was, t h e r e f o r e , p r e p a r e d t o s u r v e y t h e side-effects of t h e system, t o determine t h e measures needed f o r t h e prevention of undesirable impacts and f o r t h e utilization of positive changes. A brief evaluation of t h i s study i s given in Annex 11.

1. INTERNATIONAL IMPORTANCE OF THE DANUBE BASIN

The Danube is a c h a r a c t e r i s t i c a l l y international r i v e r , passing through eight c o u n t r i e s (FRG, Austria, Czechoslovakia, Hungary, Yugoslavia, Romania, Bulgaria and t h e USSR), and draining v e r y small areas of a f u r t h e r f o u r non-riparian countries: Italy, Switzerland, Poland, Albania.

Due t o t h e relatively low contribution t o t h e t o t a l area of t h e basin, t h e economic conditions of t h e latter c o u n t r i e s and t h e impacts of t h e i r econ- omy on t h e water regime will not b e discussed in t h i s p a p e r . In addition, t h e Danube connects a l s o t h e two d i f f e r e n t socio-economic g r o u p s of West and E a s t European countries.

The size of t h e catchment belonging t o t h e t e r r i t o r y of various coun- t r i e s and t h e number of population living t h e r e (Figure 1 , Table 1 ) demon- s t r a t e t h e importance of t h e r i v e r in t h e economic life of t h e s e countries.

The r a t i o of catchment a r e a t o t h e t o t a l c o u n t r y a r e a a n d catchment popu- lation t o t o t a l c o u n t r y population a r e f a i r l y significant (see Table 1 ) with t h e exception of t h e Soviet Union (WHO, 1976). The multi-purpose utiliza- tion of t h e Danube R i v e r is of vital importance f o r approximately 7 1 million inhabitants living in t h e basin (Ldszldffy, 1967).

With eight r i p a r i a n c o u n t r i e s s h a r i n g t h e w a t e r s of t h e Danube, t h e environmentally sound utilization of w a t e r r e s o u r c e s and t h e development of suitable land u s e policy within t h e catchment are strongly r e q u i r e d t o main- t a i n t h e p r o p e r quantity and quality along t h e whole r i v e r . The achieve- ments of t h e s e goals pose many interesting international considerations and unique opportunities f o r cooperation.

Having recognized t h i s necessity, in 1985 t h e eight Danube r i p a r i a n c o u n t r i e s issued in Bucharest a common d e c l a r a t i o n about t h e cooperation t o b e realized on t h e field of water r e s o u r c e s management. In t h i s d e c l a r a - tion, t h e Governments e x p r e s s e d t h e i r r e a d i n e s s t o c o o p e r a t e in o r d e r t o c o n s e r v e and rationally utilize t h e water r e s o u r c e s of t h e Danube River.

The cooperation t o b e c a r r i e d o u t in t h e framework of t h r e e working groups, will include all t h e e i g h t Danube countries. The p r o t o c o l entitled

"Joint measures t o b e t a k e n in t h e c o u r s e of cooperation of t h e Danube c o u n t r i e s aiming a t t h e solution of t h e water management problems of t h e Danube River with special r e g a r d t o i t s protection from pollutions", drawn

Table

1

C h a r a c t e r i s t i c d a t a of t h e Danube asi in*

(WHO, 1976;

RZdD,

1986)

1

2 3 4 5 6 7

8

Country Country's Area of c o u n t r y Coun- People of % of t e r r i t o r y in Danube catch-

t r y ' s

c o u n t r y

in

Danube

thousand ment popula- Danube bank

h2

t i o n catchment l e n g t h

m i l l i o n i n

people c o u n t r y

thou- % of coun-

mill.

% of

sand

t r y in

peo- coun-

Danube p l e

t r y

c a t chment popula-

t i o n

A u s t r i a

83.9 80.7 96.3 7 7

100

1 2

Czechoslo-

v a k i a 127.9 73.0 57.1 1 4 7 50 3

Yugoslavia 255.8 183.2 71.6 1 9 13 68 1 7

Rumania 237 5 232.2 97.8 1 9 1 9

100

24

B u l g a r i a 110.9 48.2 43.4

8

4 50 7

USSR 22402.2 44.3

Om

2 226

1

0.4 2

Other

c o u n t r i e s 3.0 (I,CH,PL,fi)

h he d a t a a r e e s t i m a t e d , s i n c e n a t i o n a l s t a t i s t i c s do n o t

c o n t a i n such breakdown

a t t h e time t h e Declaration was signed, a l r e a d y contained a c o n c r e t e plan of actions f o r achieving t h e above-mentioned goals.

P a r a g r a p h 7 of t h e Declaration states t h a t

". . .

in o r d e r t o success- fully c a r r y o u t t h e f o r e s e e n measures, t h e governments of t h e Danube coun- t r i e s will t a k e advantage of t h e possible cooperation with t h e United Nations Organisation, with i t s specialized agencies as w e l l as with o t h e r i n t e r e s t e d international organisations" (see Annex I).

In t h i s connection, t h e P r o j e c t P r o p o s a l of t h e UNDP/WHO European Regional Bureau p r e p a r e d in 1986 d e s e r v e s s p e c i a l attention, identifying t h e following goals f o r water quality p r o t e c t i o n of t h e Danube River:

-

t o s a f e g u a r d human health by protecting drinking and i r r i g a t i o n water s o u r c e s against pollution;

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t o develop a common regional s t r a t e g y towards t h e effective c o n t r o l of pollution of a l a r g e international r i v e r intersecting regions belonging t o d i f f e r e n t socio-economic systems;

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t o s t r e n g t h e n existing national water quality management p r a c t i c e s ;

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_to promote t h e t r a n s f e r and exchange of technology in water quality c o n t r o l activities.

S o f a r , 5 Danube c o u n t r i e s have announced t h e i r r e a d i n e s s to p a r t i c i p a t e in t h e p r o g r a m (WHO, 1986).

Similar close cooperation c a n b e established between IIASA and t h e Danubian c o u n t r i e s in t h e implementation of t h e p r o j e c t LIR and, especially of t h e case study dealing with t h e Danube basin. Most of t h e c o u n t r i e s h a v e a l r e a d y e x p r e s s e d t h e i r i n t e r e s t and a l s o t h e i r willingness to p a r t i c i p a t e in t h e development of t h e decision s u p p o r t system. F u r t h e r s t e p s are needed t o create formal connection between t h e LIR p r o j e c t and t h e panel r e p r e s e n t i n g t h e signatory countries of t h e Declaration.

Finally, considering t h e close i n t e r r e l a t i o n s h i p between a n d , th e sup- plementary c h a r a c t e r of t h e two p r o j e c t s (i.e. IIASA/LIR and UNDP/WHO), i t i s advisable to join t h e e f f o r t s of t h e two organisations in t h e implementa- tion.

2. WATER RESOURCES OF THE DANUBE BASIN 2.1 G e o g r a p h i c a l C o n d i t i o n s

The r i v e r Danube i s t h e 21st longest r i v e r in t h e world and t h e second longest in Europe, being almost 3000 km long from i t s s o u r c e at a height of 1078 m in t h e Black Forests (FRG) t o i t s delta on t h e Black S e a (USSR). I t s basin of 817 000 km2 r e p r e s e n t s 8% of t h e area of Europe. The c r e e k s Breg and Brigach, with t h e i r springs in t h e Black Forest, g e t a new name (Danube), downstream of t h e i r confluence at Donaueschingen. Between t h i s point and t h e delta t h e elevation difference i s 678 m and t h e length of t h e r i v e r i s 2857 km (Benedek-Ldszlb, 1980). The bottom slope conditions of t h e r i v e r Danube are shown in Figure 2 (Ldszldffy, 1967).

In t h e s t r e a m system of a huge r i v e r like t h e Danube, t h e r e are a l s o numerous important t r i b u t a r i e s . About 120 r i v e r s flow into t h e Danube itself, from which t h e g r e a t e s t ones a l s o have important t r i b u t a r i e s (of 2nd o r d e r ) . A survey of t h e most important t r i b u t a r i e s of 1st o r d e r i s given in Table 2.

According t o i t s geological s t r u c t u r e and geographic layout t h e Danube basin c a n b e divided into t h r e e regions, namely t h e u p p e r , middle, and lower Danube basin.

a ) The u p p e r Danube b a s i n c o v e r s t h e t e r r i t o r y from t h e s o u r c e s t r e a m s in t h e Black F o r e s t Mountain down t o the Devin Gate eastward from Vienna. I t includes in t h e n o r t h t h e t e r r i t o r i e s of t h e Swabian and Fal- conian mountains, p a r t s of t h e Bavarian F o r e s t and Bohemian F o r e s t down t o t h e Austrian Muhl- and Waldviertel, and t h e Bohemian-Moravian Uplands.

Southward from t h e Danube extends t h e Swabian-Bavarian-Austrian foothill belt, comprising major p a r t s of t h e Alps up t o t h e watershed divide in t h e crystalline Central Alps.

b ) The m i d d l e D a n u b e b a s i n c r e a t e s a magnificent and unique geo- g r a p h i c unit. I t s p r e a d s from t h e Devin Gate, dividing t h e last promontories of t h e Alps (Leitha Mountains) from t h e Little Carpathians downstream of t h e confluence of t h e March/Morava and Danube, t o t h e mighty fault section between t h e Southern Carpathians and t h e Balkan Mountains n e a r t h e Iron Gate Gorge. The middle Danube section is t h e l a r g e s t one. I t i s confined by

Donowschingen

t ¹⁸

Ulm

Kelheim o

Regensbuq

Kre ms V i m 0 Brob slovo Gobtikovo No moros s"%peS+

Adony Fajq

Hohocs

P

Novi Sod Belpmde

Orsovo Turnu Sever in

Gruio

-4 I

Cernovodo

Broilo

4.- 1

Figure 2. Bottom slope conditions of t h e Danube (Liepolt, 1967)

Table

2

The major t r i b u t a r i e s of t h e Danube

(RZdD,

1986)

Mouth a t Side Length Catchment

Danube

Ian

h A722

nl ^{e r}

Lech

81

t m ' i h l Naab Regen

Isar Inn Tr aun Enns

Ybbs

K-P

Mar c h h o r ava

r i g h t r i g h t l e f t l e f t l e f t r i g h t r i g h t r i g h t r i g h t r i g h t l e f t l e f t Mosonyi Duna

(Lajta,R&ba,etc.)

1

794 r i g h t

. l 8

061

v&

Hron I p e l S i 6

D r au/Dr 6va T i s z a / T i s a Sava

Temes

Velika Morava Timok

J i u I s k a r 01 t J a n t r a Vedea Arges

I a l o m i t a S i r e t P r u t

l e f t

l e f t

l e f t

r i g h t

r i g h t

l e f t

r i g h t

l e f t

r i g h t

r i g h t

l e f t

r i g h t

l e f t

r i g h t

l e f t

l e f t

l e f t

l e f t

l e f t

t h e Carpathians in t h e n o r t h , by t h e Karnische Alps and Karawanken, Julische Alps in t h e east and southeast, and by t h e Dinaric Mountains in t h e west a n d south. This closed c i r c l e of mountains embraces t h e South Slo- vakian and E a s t Slovakian Lowland, t h e Hungarian Lowland and t h e Transyl- vanian Uplands.

c ) The Lower D a n u b e basin i s composed of t h e Romanian-Bulgarian Lowland, t h e S i r e t and P r u t r i v e r basins, and t h e surrounding upland pla- t e a u s and mountains. I t is confined by t h e Carpathians in t h e west and t h e n o r t h , by t h e Bessarabian upland plateau in t h e e a s t , and by t h e Dobrogea and t h e Balkan Mountains in t h e south. A t t h e P r u t mouth t h e Dobrogea pro- montories p r o j e c t into t h e Bessarabian upland plateau.

The Danube a c c e p t s waters from high mountains and t h e i r foothills, from highlands, plains, lowlands, and depressions. T h e r e f o r e , i t s c h a r a c t e r v a r i e s from a high-mountainous stream t o a lowland r i v e r .

The distribution of t h e Danube among i t s r i p a r i a n c o u n t r i e s is as fol- lows:

In t h e Federal R e p u b l i c of G e r m a n y , from t h e confluence of t h e s o u r c e s t r e a m s Brigach and Breg down t o t h e Austrian b o r d e r , t h e Danube flows a distance of 580 km. A r e a c h about 180 km long i s constrained by n a r r o w valleys in which t h e Danube c u t s i t s way through mountain ridges. On a s t r e t c h about 400 km long, t h e Danube p a s s e s through wide valleys.

The A u s t r i a n Danube i s about 350 km long, including 21 km as f r o n t i e r r e a c h with t h e FRG and about 8 km with Czechoslovakia. About 150 km in t o t a l consists of sections passing through narrow valleys in which t h e Danube c u t s i t s way through mountain r a n g e s . Over about 200 km t h e Danube flows through t h e flatlands and f o u r g r e a t basins. The descent of t h e Danube in Austria is about 150 m.

The Czechoslovak portion of t h e Danube on t h e l e f t ( n o r t h e r n ) r i v e r bank r e a c h e s from t h e mouth of t h e Mach/Morava River about 172 km down- stream t o t h e mouth of t h e Ipel'/Ipoly. The section on t h e r i g h t (southern) r i v e r bank i s only 22.5 km long, t h e remainder being a n 8 km f r o n t i e r with Austria, and a 142 km b o r d e r with Hungary.

The H u n g a r i a n Danube r e a c h i s 417 km long, within a n 142 km t h e b o r d e r with Czechoslovakia. I t starts on t h e mighty alluvial f a n of t h e stream at t h e u p p e r margin of t h e Pannonian Basin r e a c h i n g a s f a r as t h e c e n t r e of t h a t Basin.

The Y u g o s l a v i a n r e a c h of t h e Danube, which i s about 587 km long l i e s mainly (358 km) in t h e Pannonian Basin. Along this f i r s t r e a c h , t h e s l o p e of t h e r i v e r i s only 0.05-0.04 p e r mill. Upstream from t h e f a u l t g o r g e section at t h e Iron Gate, close to t h e mouth of t h e Nera River, i t creates a common b o r d e r with Romania a n d remains a f r o n t i e r r i v e r down to t h e Timok mouth, a b o u t a 229 km long s t r e t c h .

In t h e downstream direction, t h e Danube i s a f r o n t i e r r i v e r between Romania and Bulgaria on a 472 km long r e a c h .

The Danube flows through a 1075 km long r e a c h of R o m a n i a n t e r r i - t o r y , s t a r t i n g in t h e area of t h e middle Danube above t h e mountainous r e a c h of t h e I r o n Gate down to t h e e s t u a r y , so t h a t Romania h a s t h e l a r g e s t por- tion of t h e Danube c o u r s e . From t h i s total length 229 km i s t h e b o r d e r with Yugoslavia between t h e N e r a and Timok r i v e r s . A f u r t h e r 472 km long sec- tion i s t h e b o r d e r with Bulgaria. Downstream from t h e P r u t mouth t h e r i v e r follows t h e b o r d e r with t h e Soviet Union on about 80 km down to t h e bend of t h e Kilia b r a n c h and f u r t h e r to t h e Black Sea e s t u a r y (RZdD, 1986).

The Danube Delta, covering a n area of 5640 km2, i s t h e second l a r g e s t one in Europe. Eighty p e r c e n t of i t belongs to t h e S o v i e t U n i o n and 20% t o Romania (Liepolt, 1973).

2.2 Climate

Due to t h e elongated s h a p e of t h e Danube Basin in a west-east direction a n d d i v e r s e relief f e a t u r e s , t h e climatic conditions are also r a t h e r vari- able. In t h e western regions of t h e u p p e r Danube, t h e r e i s a n Atlantic influ- e n c e , while e a s t e r n t e r r i t o r i e s are more continental in climate. In t h e u p p e r and middle Danube basin, especially in t h e Drau/Drdva and Sava basins t h e climate i s a l s o influenced by t h e Mediterranean S e a .

The basic c h a r a c t e r of climate, determined by t h e spacious layout of t h e Danube, i s differentiated and modified by t h e mighty mountainous sys- t e m s into n a t u r a l regions.

The dependence of climatic elements on altitude is a f u r t h e r contribu- tion t o t h e diversified climate, s i n c e t h e basin r e a c h e s from high mountain r a n g e s c o v e r e d with g l a c i e r s , o v e r h a r s h mid-mountains and upland plat- forms, t o hot (in summer) lowlands.

The climatic elements most important f o r water r e s o u r c e s management are precipitation and snow c o v e r . In t h e following, a detailed description of t h e s e two elements will b e given. Note t h a t t h e basic l i t e r a t u r e (RZdD, 1986) contains more detailed information about t h e o t h e r climatic elements (radiation, a i r t e m p e r a t u r e , evaporation, wind).

2.2.1 Precipitation

The amount of a v e r a g e annual precipitation f l u c t u a t e s within t h e r a n g e from 3000 mm in high mountains t o 400 mm in t h e d e l t a region.

Already t h e u p p e r Danube b a s i n shows a n astonishingly v a r i a b l e c h a r a c t e r . In t h e high alpine regions t h e values of 2000 mm are sometimes s u r p a s s e d , t h e mountain marginal b e l t s being e x t r a o r d i n a r i l y r i c h in p r e - cipitation. The increment of mean annual precipitation values amounts t o about 5 0 mm p e r 100 m r i s e in height in t h e n o r t h e r n alpine promontories and in t h e Alps. A f u r t h e r influence of mountains, o c c u r s o n t h e windward slopes where p r e c i p i t a t i o n i n c r e a s e s due t o o r o g r a p h i c lifting. Thus t h e isohytes almost p a r a l l e l t h e height contours of t h e mountains. In t h e north- e r n alpine foothills t h e amount of precipitation d e c r e a s e s in t h i s way from about 1500 mm p e r y e a r on t h e p e r i p h e r y of t h e mountains t o 700 mm p e r y e a r in t h e Danube valley. About 1500 mm p e r y e a r of precipitation fall a l s o in t h e Danube s o u r c e a r e a , in t h e Black F o r e s t and in t h e h i g h e r regions of t h e Bavarian and Bohemian Forests.

Other t e r r i t o r i e s show a v e r a g e precipitation values between 600 and 1000 mm, t h e valleys and basins (for instance t h e Naab valley, t h e Vienna Basin) being relatively d r y with about 700 mm of precipitation. Relatively d r y a l s o are t h e intermontane valleys. The u p p e r Inn r i v e r valley, extend- ing in t h e west-east direction (Lower Engadin) is considered as a

c h a r a c t e r i s t i c i n t e r a l p i n e d r y valley with only 600-700 mm of precipita- tion. Generally, t h e mean annual precipitation (annual s e r i e s 1931-1960) in t h e German p a r t of t h e Danube catchment area amounts t o 950 mm p e r y e a r . In t h e m i d d l e D a n u b e b a s i n t h e highest values of mean annual precipi- tation o c c u r on t h e o u t s k i r t s of mountains surrounding t h e lowlands. The highest precipitation values 2000 mm to 3000 mm show t h e effect of t h e southward o r i e n t e d mountain chains of t h e Julische Alps a n d t h e Dinaric system, which are exposed t o t h e humid-warm a i r masses coming from t h e Mediterranean Sea. In t h e Carpathians t h e mean precipitation values a r e between 1000 and 2000 mm. In t h e rain-shadow of t h e mountains of t h e east Bohemian-Moravian Uplands, as w e l l as in t h e Carpathian Foothills, t h e a v e r a g e precipitation amounts t o 600-1000 mm. In t h e s o u t h e r n p a r t of t h e c e n t r a l Danube Lowland t h e annual precipitation d r o p s t o 600-800 mm, in t h e Hungarian Lowland t o 560 mm, and in t h e region of t h e middle Tisza/Tisa River t o 500 mm. The d r y n e s s of t h e climate i n c r e a s e s in t h e middle Danube Lowland in t h e n o r t h e r n and e a s t e r n directions.

In t h e plains of t h e Lower Danube t h e amount of precipitation r e a c h e s also only 500-600 mm, while t h e lowest precipitation values are r e c o r d e d at t h e Danube e s t u a r y with l e s s t h a n 400 mm. In some y e a r s t h e r e is no p r e - cipitation at a l l o v e r t h e e n t i r e summer period. Due t o low precipitation and high summer t e m p e r a t u r e s , t h e region of t h e Danube e s t u a r y may be considered as a t e r r i t o r y with s t e p p e climate (RZdD, 1986).

2.2.2

Snuw

cover

The number of d a y s with snow c o v e r as well as t h e duration and thick- ness of t h e snow c o v e r i n c r e a s e with t h e altitude. In alpine valleys only 58 days p e r y e a r are c h a r a c t e r i z e d with precipitation falling in t h e form of snow ( a v e r a g e o v e r t h e period 1931-1970), while on t h e Zugspitze (3000 m) 1 9 1 d a y s w e r e r e c o r d e d .

The s h o r t e s t duration of snow c o v e r (9-12 days) i s a t t h e Black S e a coast. The snow c o v e r l a s t s for only 20-30 days in t h e plains of t h e middle Danube basin, 40-60 days in t h e u p p e r Danube basin, t h e mean proportion of snow in t h e total annual precipitation being 10-15 p e r cent. In t h e alpine foothills and in high regions of mid-mountains t h e snow c o v e r l a s t s more

than 100 days. In those regions about 20-30 p e r c e n t of t h e annual precipi- tation fall in t h e form of snow. A t a n t h e altitude of 1500 m t h e snow c o v e r s t a y s from f o u r months (Central Alps) t o 6 months (peripheral mountains).

A t a n altitude of 2000 m more t h a n 6-8 months; and at 2500 m more than 8-10 months. The snow line is at a n altitude of 2900 m on humid windward sides, and 3200 m in t h e Central Alps.

The snow c o v e r stays f o r a longer period in t h e Carpathians than in t h e Alps. A t altitudes above 2000 m t h e snow c o v e r lasts 300 days in t h e T a t r a s , Bucegi, F a g a r a s and Retezat Mountains, and on t h e peak Lomnickf S t i t even 324 days.

In t h e highest mountain regions about 80-90% of t h e annual precipita- tion fall in t h e form of snow.

The snow c o v e r thickness i s generally slight in plains and lowlands.

Snow, falling frequently a l r e a d y in October, lasts usually only 1-3 days.

Continuous snow c o v e r i s usually formed in December and January, reaching t h e maximum of 15-20 cm in F e b r u a r y , and melting away in March. In e x t r a o r d i n a r y cold winters, r i c h in snow, as f o r instance t h e period 1941-1942 in t h e u p p e r Danube basin, o r 1953-1954 in t h e lower Danube basin, 40-60 cm snow thickness w e r e r e c o r d e d in t h e lowlands. Such snow conditions are exceptions and are frequently followed by r a p i d warming, causing p a r t i a l o r complete snow melting.

In t h e medium altitudes of 1500-2000 m a snow c o v e r starts to develop a l r e a d y in October-November. In F e b r u a r y t h e a v e r a g e snow c o v e r thick- n e s s may r e a c h 150-250 cm. The snow melt period lasts from March to May.

A t altitudes above 2000 m t h e snow c o v e r maximum is attained in Febru- a r y , t h e snow c o v e r thickness reaching in some places 500 cm o r more.

The reliability and a c c u r a c y of d a t a on snow c o v e r thickness d e c r e a s e with increasing altitude above sea level. Snow c o v e r thickness v a r i e s locally due t o snow-drifts. A t high altitudes of t h e Carpathians and Alps 50-70 days with snow storms are r e c o r d e d , while in t h e plains only 1-3 days with snow s t o r m s o c c u r (RZdD, 1986).

2.3 Hydrological Regime of the Danube Basin

In t h e Danube .Basin, systematic hydrological o b s e r v a t i o n s in t h e r i p a r i a n c o u n t r i e s are available f o r between 1 0 0 and 1 5 0 y e a r s .

A t about t h e end of t h e last c e n t u r y t h e network f o r water level o b s e r - vations developed rapidly in t h e Danube Basin. By t h e beginning of o u r cen- t u r y , a l r e a d y s e v e r a l hundreds of gauging stations had been installed mostly on t h e main r i v e r s . The development of r e g u l a r observation of t h e smaller

-

t h i r d r a n k

-

t r i b u t a r i e s h a s taken place, however, only a f t e r Second World War. In 1950 t h e r e w e r e more t h a n 1000 and today t h e r e a r e consid- e r a b l y more than 2000 gauging s t a t i o n s in t h e Danube Basin (RZdD, 1986).

2.3.1 Mean discharge conditions

The a v e r a g e runoff conditions of t h e Danube Basin are c h a r a c t e r i z e d on t h e basis of d a t a s e r i e s of 48 gauging stations, from which 2 4 are on t h e Danube itself and 24 on i t s most important t r i b u t a r i e s . Of c o u r s e , when selecting t h e s e gauging stations, n e i t h e r all requirements r e g a r d i n g r e p r e s e n t a t i v i t y a n d d a t a quality n o r length of r e c o r d , i.e., t h e full obser- vation period 1931/70, could b e m e t .

Table 3 is a s u r v e y of t h e gauging stations selected and Figure 3 i s a layout showing t h e s i t e s of t h e gauges.

The values M Q , MHQ and MLQ* computed from t h e d a t a s e r i e s of t h e s e 48 gauging stations, t h e r a t i o of t h e latter two values t o MQ as well as t h e corresponding specific runoff values M q , M L q are listed in Table 4.

Figure 4 is t h e hydrological longitudinal profile of t h e Danube River.

The Upper Danube i s definitively c h a r a c t e r i z e d by t h e high runoff sup- plies from t h e Alps. Already t h e t r i b u t a r i e s Iller, Lech a n d I s a r bring con- s i d e r a b l e inflows but t h e g r e a t e s t and definitive change i s d u e t o t h e Inn River. Although t h e catchment a r e a of t h e Inn, 26,130 km2, i s only 52% of t h a t of t h e Danube t i l l t h e mouth of t h e Inn, t h e mean annual d i s c h a r g e of t h e Inn, 743 m3/s, s u r p a s s e s by 8% t h a t of t h e Danube itself. In t h e domain of t h e mean flood d i s c h a r g e s M H Q , this difference i s even h i g h e r (609.) and only in t h e domain of mean low d i s c h a r g e s MLQ is t h e value of t h e Danube

o or

explanation o f t h e s y m b o l s M Q , M H Q , M L Q , M q , M H q ,

MLQ

s e e f o o t n o t e t o Table 4 .

Table 3 List of selected aauges in the Danube Basin

(RZdD,

1986) Nr. Gauge River Irm Catchment Observed

ar ea2 discharge data A.

_km

from the ~ e r i o d

1 2 3 4 ^{5 6}

1 rzlgolstadt Danube 2 458.3 20 001 1931-1970 2 Regensburg Danube 2 376.1 35 399 1931-1970 3 Hofkirchen Danube 2 256.9 47 ^{496 1931-1970} 4 Passau-Ilzstadt Danube 2 225.3 76 597 1931-1970 5 Linz Danube 2 135.2 79 490 1931-1970 6 Stein--ems Danube 2 002.7 96 045 1931-1970

7 Wien-Iiussdorf Danube 1934.1 1 0 1 7 0 0 1931-1970

8

Bratislava Danube 1 868.8 131 338 19 31-1970

10 ~ u n a a l d s Danube 1 751.8 171 720 1948-1970

11 Naggmaros Danube 1 694.6 183 534 1931-1970

12 M o G c s Danube 1 446.8 209 064 1931-1970

13 Bezdan Danube 1 425.5 210 250 1931-1970

14 Bogojevo Danube 1 367.4 251 593 1931-1970

15 ~anEevo Danube 1 153.3 525 009 1931-1970

16 V. ~radiite Danube 1 059.8 570 375 1931-1970

17 @:ova Danube 955.0 576232 1931-1970

18 Novo Selo Danube 833.6 5 8 4 9 0 0 1937-1970

19 Lom Danube 743.3 588 860 1941-1970

20 Svistov Danube 554.3 650 340 1931-1970

21 Zimnicea Danube 554.0 658 400 1931-1970

22 Ruse Danube 495.6 669 900 1931-1970

23 Silistra Danube 375.5 689 700 1941-1970

24 Vadu Oii-Hirsova Danube 252.3 709 100 1931-1970

25 Ceatal Izmail Danube 72.0 807 000 1931-1970

Pas sau-Ingl

ing

Salzburg

S t egr

Moravsky Jan S a l a

B r ehy

Ipelsky Sokolec X?eubr Ucke

Landacha

Donji Miholjac Qilok

Tiszabecs Szeged Senta Csenger Pel sBz a01 ca Mako

S r ,Mitrovica L joblost Orahovica S t o e n e s t i Storozinec Lungoci

C

ernovci

Inn

3.1 26 084

Salzach 64.4 4 427

E X I ~ S

30.88 5 915 March/Morava 67.6 24 129

V a h

3309 1 0 620

Hron 102 05 3

82l

I p e l ' /Ipolg 42.9 4 838 Drau/Dr&va 481.2 1 0 415 Mu/Mura 114.4

8

340

Sornes/Szamos 46.6 1 5 283

~ l a n & / S a j 6 50.1 6 440 Idures/Maros 23.7 30 149

Sava 136.0 87 996

Q.

Morava 39.9 37 320 I s k a r 27 05

8

370

01 t 71

- 2

22 683

S i r e t 448 0 672 S i r e t 74.0 36 036

Prut 772 -0 6 890

Table

4_

Characteristic discharge values for the observation periodx) (RZdD. 1986)

Ingolstadt/Danube 308 Regensburg/Danub e 435 Hofkirchen/Danube 645

Pas sau-Ilz s tadt /Danub e 143 0 Linz/Danube 1509

S

tein-Krems/Danube 1864

Wien-Nussdorf/Danube 1943 Bratislava/Danube 2020

~unaalm6s/~anube 2314

symbols adopted are the following:

- catchment area

- multiannual mean discharge

:

MQ/A -multiannual specific mean discharge

- mean of annual peak discharges

= MHQ/A - multiannual specific flood discharge

- mean of annual lowest discharges

=

MLQ/h multiannual specific low discharge

Nagyrnaros/Danube 2379 Mohdc s/Danub e 2389 Be z dan/Danub e 2479 Bogo j evo/Danube 306 0 Pancevo/Danube 5490

Vadu Oii-Hir sova/ 6216

, Danube

Ceatal Izmail/Danube 6550

Moravsky Jan/Msrch- 110 Morava

Sal a/V& 152

32. Ipelsky Sokolec/Ipel

21

/ I P O ~ Y

208

1

e9 9.90 0.09 4.34 43

*O

0.38

33.

Neubr

Ucke/Drau/Drdv,a 266 910 98.3 3.42 0. 37 25.5 87 *4 9.44 36. Don

j i

Mihol jac/

Dr au/Dr 6va 5 54 1341 23 5 2.42 0.42

LOO

4 _I'

Fimure 4. H v d r o l o n i c a l l o n n i t u d i n a l u r o f i l e of t h e D a n u b e (RZdD, 1986)

16% h i g h e r t h a n t h a t of t h e Inn. Considering t h e d i f f e r e n t specific runoff values, t h o s e of t h e Inn Basin are significantly h i g h e r in a l l t h r e e domains.

Danube b e f o r e Inn 6.46 13.6 39.2

Inn 9.85 28.5 114

Danube a f t e r Inn 7.46 19.0 53.8

The high s p e c i f i c runoff values of t h e Danube a f t e r t h e mouth of t h e R i v e r Inn are somewhat f u r t h e r i n c r e a s e d by t h e Austrian alpine t r i b u - t a r i e s T r a u n , ~ b n s and Ybbs (Mq and MLq), o r , at l e a s t k e p t c o n s t a n t till t h e mouth of t h e March/Morava R i v e r ( M H q ) . The mean annual d i s c h a r g e i n c r e a s e s from 1430 m3/s at t h e mouth of t h e Inn t o a b o u t 1950 m3/s at t h e gauging s t a t i o n of Wien-Nussdorf with a catchment area of 102,000 km2.

Until t h e mouth of t h e Drau/Drdva R i v e r , t h e catchment area redou- b l e s t o a b o u t 210 000 km2 while t h e mean d i s c h a r g e i n c r e a s e s only by 530 m3/s r e a c h i n g t h e value of 2,480 m3/s a b o v e t h e mouth of t h e Drau/Drdva River. E x c e p t t h e West C a r p a t h i a n s (V&I 14.3 l/s.km2 a n d Hron 13.1 l/s.km2), t h i s g r e a t intermediate catchment a r e a is v e r y low in p r e c i p i t a t i o n a n d runoff (March/Morava 4.5 l/s.km2, Ipelt/Ipoly 4.3 l/s.km2). In t h e Hun- g a r i a n Lowland t h e s p e c i f i c runoff v a l u e s fall even below 3 l/s.km2.

Correspondingly, t h e s p e c i f i c runoff values computed f o r t h e whole (accu- mulated) catchment a l s o d e c r e a s e (MLq from 7.9 t o 4.6 l/s.km2, Mq from 19.1 t o 11.8 l/s.km2). The l a r g e s t d e c r e a s e i s t h a t of t h e mean s p e c i f i c flood runoff MHq (from 53 t o 23.4 l/s.km2) due t o t h e lowering of t h e peak d i s c h a r g e s along t h i s long r i v e r r e a c h by t h e l a r g e r e t e n t i o n c a p a c i t y of t h e c o n s i d e r a b l e a r e a s inundated on t h e flood plain.

Within t h e following 270 km long Danube r e a c h in Yugoslavia, t h e t h r e e g r e a t e s t t r i b u t a r i e s join t h e Danube. They i n c r e a s e t h e accumulated c a t c h - ment area of t h e Danube by 150% (from 210,000 km2 at Bezdan t o 525,000 km2 at Pancevo) and t h e mean d i s c h a r g e by 120% from 2480 m3/s t o 5490 m3/s. These t r i b u t a r i e s are: t h e Drau (Drava) r i v e r (A

=

40,000 km2, MQ

=

554 m3/s), t h e Tisza (Tisa) r i v e r (A

=

157, 000 km2, MQ

=

766 m3/s), and t h e Sava r i v e r (A =95, 700 km2, MQ

=

1610 m3/s). The i n c r e a s e rates of t h e mean low d i s c h a r g e (from 976 t o 2,240 m3/s) and of t h e mean flood d i s c h a r g e (from 4,290 m3/s t o 10,070 m3/s) a r e more or less t h e same. The relatively high runoff rates of t h e Drau/Drava (Mq =15 l/s.km2) and t h e Sava River (Mq

=

18 l/s.km2) originate from t h e r a i n y catchments of t h e S o u t h e r n Alps and t h e Dinaric Mountains. On t h e o t h e r hand, t h e Tisza/Tisa r i v e r , collect- ing t h e major p a r t of t h e waters of t h e Hungarian Lowland, achieves at i t s mouth only a mean specific d i s c h a r g e of 5.4 l/s.km2 in s p i t e of i t s many t r i - b u t a r i e s from t h e Carpathians.

C o n t r a r y to t h e absolute values mentioned, t h e changes of specific run- off rates r e l a t e d t o t h e accumulated catchment area of t h e Danube are not significant along t h i s 270 m long r e a c h :

W q from 4.6 t o 4.3 l/s.km2 Mq from 11.8 to 10.4 l/s.km2 and MHq from 23.4 to 19.2 l/s.km2

Along t h e about 1,000 km long r e a c h between t h e I r o n Gate and t h e mouth of t h e Danube in t h e Black S e a , t h e catchment a r e a i n c r e a s e s by almost 300,000 km2 up t o 817,000 km2 while t h e i n c r e a s e of t h e mean annual d i s c h a r g e i s only about 1000 m3/s (from 5,490 t o 6,550 m3/s). The niean specific runoff from t h i s area is, in s p i t e of inflows from t h e Carpathians (Olt 7.1 l/s.km2) and t h e Balkan Mountains (Iskar 7.1 l/s.km2), r a t h e r low:

3.7 l/s.km2, i.e. even lower than t h e specific runoff from t h e relatively poorly contributing a r e a between t h e mouths of t h e March (Morava) and t h e Sava r i v e r s with i t s 4.9 l/s.km2.

Although t h e d e c r e a s e of t h e mean specific runoffs r e l a t e d t o t h e whole (accumulated) Danube Basin seems to b e insignificant (Mq from 10.4 t o 8 . 1 l/s.km2, MLq from 4 . 3 t o 3.6 l/s.km2 and MHq from 19.2 to 13.2 l/s.km2), t h e r e a l e x t e n t of t h e changes will b e c l e a r when considering t h e r e l a t i v e indices of d e c r e a s e , which are: f o r Mq 22%, f o r MLq 15% and f o r MHQ 31%.

The disproportionately s t r o n g d e c r e a s e of t h e mean specific flood runoff a l s o shows t h e impact of t h e long r i v e r bed r e a c h e s and t h e relatively l a r g e inundation areas.

The i n c r e a s e of t h e d i s c h a r g e along a r i v e r c a n also b e visualized by duration c u r v e s . They a r e shown, f o r selected gauging s t a t i o n s of t h e Danube, in Figure 5.

The seasonal changes of t h e multiannual means of t h e monthly d i s c h a r g e s c a n b e applied, in connection with t h e climatic f a c t o r s of t h e runoff e v e n t , f o r t h e description and c h a r a c t e r i z a t i o n of t h e hydrological regime. The variation of t h e monthly mean d i s c h a r g e s along t h e Danube are shown in Figure 6.

The monthly d i s c h a r g e s of various probabilities are shown f o r o n e s e l e c t e d station in Figure 7 (UjvAri, 1967; RZdD, 1986).

2.3.2 Flood conditions

Flood events c a n o c c u r in t h e whole Danube Basin both d u e t o storms o r to snowmelt and rain. In t h e second c a s e , t h e runoff is i n c r e a s e d a l s o when t h e soil i s f r o z e n or water-saturated. The floods caused only by snowmelt a r e h a r d l y dangerous in t h e Upper Danube Basin. However, t h e importance of floods is quite high f o r t h e f l a t catchments along t h e Middle and Lower Danube.

In t h e alpine region, summer cold a i r intrusions are of p a r t i c u l a r importance s i n c e t h e y t r i g g e r low-pressure activities in t h e Mediterranean region. In t h e warm s e c t o r of low-pressure a r e a s moving from t h e Gulf of Genoa in n o r t h e r n or n o r t h - e a s t e r n directions o v e r t h e Alps, t h e v e r y h o t and vapor-saturated a i r of t h e Mediterranean region i s c a r r i e d in t h e n o r t h e r n direction. When t h e migration velocities are slow, t h e p r e c i p i t a - tions c a n l a s t s e v e r a l days and c a u s e considerable floods, beginning in t h e s o u t h e r n alpine region (catchments of t h e Drau (Drbva) and ,of t h e U p p e r

Figure 5. Discharge duration curves of selected gauges on the Danube (RZdD, 1986)

Figure 6. Inter-annual distribution of monthly mean discharges along t h e Danube (RZdD, 1986)

LOO - 4 4 2 I

n

^{I I} I V V V l v l V I I l X X X I X I I

Figure 7. Inter-annual distribution of monthly mean discharges of vari- ous probabilities at t h e gauge Danube/Mohdcs, 1930/70 (RZdD, 1986)

Sava r i v e r s ) , through t h e Bohemian F o r e s t and t h e north-eastern domains of t h e Carpathians, up t o t h e basin of t h e Oder (Odra) r i v e r . On t h e n o r t h e r n edge of t h e Alps, this weather situation is often connected with enhanced precipitation coming from West o r Northwest. Above them, a humid, w a r m a i r from t h e South streams causing additional abundant o r o g r a p h i c precipi- tation. The g r e a t floods of 1940, 1954, 1965 and 1966 were caused by t h i s

t y p e of weather situations. Additionally, a snowmelt in t h e mountains may b e superimposed covering g r e a t height differences from t h e v e r y low up t o t h e v e r y high regions. This happened during t h e floods of 1940 and 1965.

Summer storms c a n lead t o g r e a t o r even v e r y g r e a t floods almost in t h e whole Danube Basin. In t h e s o u t h e r n regions under Mediterranean influ- e n c e t h e r e is a f u r t h e r storm period in l a t e autumn (mostly November) which a l s o c a n c a u s e g r e a t floods (Drau/Drdva, S a v a and t h e t r i b u t a r i e s from t h e Balkan Mountains).

The g r e a t e s t floods known on t h e Danube upstream from t h e mouth of t h e Inn were due t o snowmelt with r a i n onto a more o r l e s s f r o z e n soil s u r - f a c e (March 1945, F e b r u a r y 1862, December/January 1881/1882). During t h e common observation period 1931/70, floods of March 1947 and March 1956 along t h e u p p e r Danube r e a c h gave only a slight idea of what n a t u r e could d o in t h a t a r e a . Along t h e c o u r s e of t h e Danube, both floods men- tioned r e a c h e d r e a l l y considerable dimensions. This t y p e of flood

-

snow melt combined with spring precipitation

-

o c c u r s on t h e middle and lower Danube, p a r t i c u l a r l y downstream from t h e Tisza (Tisa) mouth, relatively often with g r e a t discharges.

The seasons o r months, respectively, in which t h e probability f o r flood e v e n t s is t h e highest, c a n b e r e a d from t h e c u r v e of 1% surpassing (e.g. s e e Figure 7). This does not exclude, of c o u r s e , t h e possibility of g r e a t floods c a n o c c u r during o t h e r periods as well. A s examples of t h i s situation, t h e floods on t h e r i v e r Inn of F e b r u a r y 1862 (4,500 m3/s), September 1899 (6,600 m3/s) and September 1920 (5,310 m3/s) c a n be mentioned.

In e a r l i e r times, along s e v e r a l Danube r e a c h e s t h e r e were a l s o floods due t o i c e jams. They caused g r e a t damages and were t h e r e f o r e v e r y much f e a r e d . The d a n g e r of such floods d e c r e a s e d considerably due t o r i v e r training. The floods are practically eliminated along t h e Upper Danube, due t o t h e closed b a r r a g e chain from Ulm t o Vienna. A s a r e s u l t of t h e o p e r a - tion of t h e b a r r a g e s , no i c e d r i f t t a k e s place any more along t h i s s t r e t c h . R a r e cases can b e mentioned as exceptions when a v e r y s h a r p cold wave o c c u r s during a flood.

The propagation of floods along t h e Danube may follow v e r y different forms. For example, a mighty flood on t h e Upper Danube, such a s t h a t of 1954, c a n f l a t t e n o u t as i t moves toward t h e Lower Danube 'in such a way t h a t i t will b e a b s o r b e d t h e r e by t h e recession hydrograph of a small, belated s p r i n g flood (Figure 8). On t h e o t h e r hand, i t might a l s o happen t h a t s m a l l incipient flood waves on t h e u p p e r r e a c h develop t o g r e a t floods on t h e Middle and Lower Danube. I t a l s o c a n c l e a r l y b e recognized t h a t t h e wave p e a k s c a n considerably f l a t t e n along t h e long t r a n s p o r t r e a c h e s and t h e relatively g r e a t flood plains of t h e Middle and Lower Danube whenever t h e intermediate areas d o not c o n t r i b u t e t o a f u r t h e r sharpening of t h e flood e v e n t (RZdD, 1986).

2.3.3 Low-water conditions

The seasons o r months, respectively, in which low d i s c h a r g e s (stream- flow droughts) often o c c u r , are v e r y w e l l recognizable from t h e form of t h e 99% exceedance c u r v e (e.g. Figure 7). Since t h e d u r a t i o n of low d i s c h a r g e s generally i s longer, t h e i r impact on t h e mean d i s c h a r g e of t h e c o r r e s p o n d - ing month is a l s o s t r o n g e r t h a n t h a t of a s t e e p flood wave with a s m a l l water volume. Thus, t h e g r a p h i c a l r e p r e s e n t a t i o n s are more r e l i a b l e as f a r as t h e o c c u r r e n c e of streamflow d r o u g h t s i s concerned t h a n in r e s p e c t t o t h e o c c u r r e n c e of floods. Low d i s c h a r g e s c a n v e r y often b e o b s e r v e d along t h e whole Danube in autumn and winter. Due to t h e variability of t h e f e a t u r e s of t h e various catchments, t h e minimum values o c c u r in o n e place mostly in autumn, and in winter in o t h e r catchments.

The absolute minimum values almost coincide in time from t h e s o u r c e to t h e mouth of t h e Danube, since t h e e v e n t s resulting in l o w d i s c h a r g e s depend on a synoptic weather situation of long duration.

Such a case happened in t h e autumn of 1947 when extremely low d i s c h a r g e s were o b s e r v e d along t h e e n t i r e Danube. During t h e period from 30 August t o 2 November t h e r e w a s almost no precipitation (its value being in t h e surroundings of Vienna only 13% of t h e multiannual a v e r a g e value of t h a t period), leading to an e x t r e m e drought and v e r y low d i s c h a r g e s in all streams of t h e Danube Basin.

lngolstadt Regensburg Hofkirchen

tim

Pantevo V. ~radigte Orsovo Nwo Selo Lorn Svistov Ruse Si li stra Vodu Oii

MAY

I

JUNE

1

JULY IAUGUST. \\ ISEPT.

I

Figure 8. Flood h y d r o g r a p h s at d i f f e r e n t gauging stations on t h e Danube in 1954 (RZdD, 1986)

In J a n u a r y 1954, a more e x t r e m e low d i s c h a r g e o c c u r r e d along t h e whole Danube, even lower than in 1947. A longer d r y period in t h e autumn of 1953 had been followed by a cold winter s o t h a t t h e whole precipitation was accumulated in t h e snow c o v e r . In such a way t h e y e a r 1954 brought f o r

a long r e a c h of t h e U p p e r Danube two v e r y r a r e extremes: t h e low d i s c h a r g e s in January a n d t h e mighty flood in July.

In t h e y e a r s 1933, 1934, 1948 and 1964 f u r t h e r v e r y c l e a r - c u t stream- flow d r o u g h t p e r i o d s were o b s e r v e d along t h e whole Danube. The low d i s c h a r g e s along t h e Danube f o r all t h e periods mentioned are graphically r e p r e s e n t e d in Figure 9. The inhomogeneities, recognizable in t h e f i g u r e might b e due t o t h e problems of d i s c h a r g e measurements, which c a n b e con- s i d e r a b l e not only in t h e r a n g e of high d i s c h a r g e s but in t h a t of low ones as well.

The low d i s c h a r g e s of a n area (base flow) depend mostly on t h e amount of water s t o r e d e i t h e r on t h e s u r f a c e (in lakes) o r in t h e aquifers. Along t h e Upper Danube, t h e r e are two contrasting extremes: t h e foothill b e l t of t h e Alps, r i c h in l a k e s a n d extended g r a v e l fields on t h e s o u t h e r n s i d e , while in t h e n o r t h , t h e Bavarian and Bohemian F o r e s t s are made of basement r o c k s , which have p o o r s t o r a g e capacity.

For g e n e r a l c h a r a c t e r i z a t i o n of t h e low d i s c h a r g e conditions of a r i v e r o r a catchment area, one could use, besides t h e a v e r a g e low d i s c h a r g e p e r i o d , a l s o t h e r a t i o MLQ:MQ (see Table 4, RZdD, 1986).

2.3.4 Water balance of the Danube basin

The water balances t o b e compiled f o r t h e whole Danube basin as well as f o r i t s balance a r e a s (subcatchments and national a r e a s ) is confined t o t h e most simple v a r i a n t of t h e water balance: only t h e long-term mean values of t h e t h r e e b a s i c balance elements:

-

precipitation (P)

-

evaporation (E)

-

runoff (R)

will b e investigated. In t h i s c a s e , i t is not n e c e s s a r y t o s e p a r a t e t h e com- ponents of precipitation ( r a i n and snow), a e r i a l evaporation (soil evapora- tion, f r e e water s u r f a c e evaporation and t r a n s p i r a t i o n of plants), and run- off ( s u r f a c e runoff, intermediate flow, groundwater runoff). Since t h e water consumption of water u s e r s i s considerable only in t h e case of i r r i g a - tion and t h e a r e a of i r r i g a t e d fields is relatively low in t h e basin, t h e

d i f f e r e n c e between water i n t a k e a n d r e t u r n flow i s a l s o negligible in t h e long t e r m a n d l a r g e s c a l e water balance. T h e r e i s a f u r t h e r simplification a c c e p t a b l e when long-term mean values are c o n s i d e r e d in t h e b a l a n c e equa- tion: t h e c h a n g e s of t h e w a t e r volume s t o r e d in t h e soil a n d on t h e t e r r a i n c a n a l s o b e n e g l e c t e d (Domokos-Sass, 1986).

The b a l a n c e elements mentioned a b o v e h a v e b e e n investigated f o r t h e 47 sub-catchments of t h e Danube basin.

The w a t e r b a l a n c e elements of t h e most important sub-catchments of t h e Danube basin are l i s t e d in Table 5, g r o u p e d a c c o r d i n g t o t h e physico- g e o g r a p h i c a l c h a r a c t e r of t h e sub-catchments: high mountains, mid- mountains a n d hilly r e g i o n s .

The sub-catchments of t h e high mountainous r e g i o n , all of them in t h e Alps (with t h e e x c e p t i o n of t h e Vdh c a t c h m e n t ) p r o v i d e a v e r a g e t e r r i t o r i a l p r e c i p i t a t i o n values between 900 a n d 1500 mm. The runoff c o e f f i c i e n t is between 0.44 a n d 0.68.

While Table 5 yields information a b o u t t h e w a t e r b a l a n c e a n d runoff c o e f f i c i e n t conditions of t h e t r i b u t a r i e s , Table 6 c o n t a i n s s t a t i s t i c s on w a t e r b a l a n c e elements a n d runoff c o e f f i c i e n t s f o r t h e accumulated c a t c h - ments of 5 d i f f e r e n t Danube sections.

The c o n t r i b u t i o n of e a c h Danube c o u n t r y t o t h e total d i s c h a r g e leaving t h e c o u n t r y ' s downstream b o r d e r c a n b e c a l c u l a t e d from t h e d a t a of t h e water balance:

1. F e d e r a l Republic of Germany 90.5%

2. Austria 62.2%

3. Yugoslavia 34.8%

4. Czechoslovakia 32.5%

5. Romania 17.4%

6. S o v i e t Union 9.5%

7. Bulgaria 7.4%

8. Hungary 5.0%

Water balances f o r s e l e c t e d r e g i o n s of t h e Danube Basin

(RZdD,

1986)

-- ^-- - - - - - - -

Subcat chment Areal mean value of Runoff coef f

i-

r . River Catchment P r e c i p i - &apora- Runoff tation c i e n t

ezea t i o n

E R ^R

P & = -

b2

mm/a mm/a mm/a P

H i g h m o u n t a i n s

2 Lech 4 398

1

349 536 769 0- 57

6 Iser

8

369

1

174 536 614 0.52

8

livl 26 976

1

315 436 868 0.66

1 0 Ezlns 5 940

1

483 483

¹

016 0.68

1 4 V& 9 714 9 06 53 4 41 5 0.46

24 Drau/Drdva 41 810 998 575 435 0 44

28 Sava 94 778

1

091 634 514 0.47

Mid-mountains and h i l l y l a n d

4 Naab 5 645 767 49 0 2

88

0.37

1 2 lldarch 27 633 640 483 1 4 1 0.22

1 3 ~ a a b / R d b a 1 4 702 738 59 2 1 3 0 0.18

1 6 N i t r a 5 415 694 549 157 0.23

18

Hron 5 251 809 514 290 0.36

20 I p e l '/Ipoly 4 594 -661 531 139 0.21

22 sib 1 5 129 663 572

81

0.12

26 Tisza/Tisa 158 182 744 560 177 0.24

30 V. Morava 38 233 746 540 216 0.29

32 J i u 1 0 731 831 576 278 0. 33

34

Isk~r

7

811

7 25 455 230 0.32

36

O l t

2 4 8 1 0 873 59 2 234 0.27

38 Lorn 3 380 599 51 2 6 6 0.11

40 Arges

11

814 800 577 190 0.24

42 Ialomita 1 0 305 738 556 146 0.20

44 S i r e t 45 420 757 550 1.57 0.21

46 P r u t 28 945 606 470 96 0.16