Studies and reports in hydrology

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Studies and reports in hydrology 39

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Recent titles in this series:

Quadriiinguai publication: Engüsh-French-Spanish-Russian.

20.

21:

22.

23.

24.

25.

26.

27.

28.

29.

30.

31.

32.

33.

34.

35.

36.

37.

38.

39.

40.

41.

Hydrological maps. Cosdition Unesco- WMO.

World catalogue of very large floods/Répertoire mondial des très fortes crues.

Floodflow computation. Methods compiled from world experience.

Water quality surveys.

Effects of urbanization and industrialization on the hydrological regime and on water quaiity. Proceedings of the Amsterdam Symposium. October 1977/Effets de l’urbanisation et de l’industrialisation sur le régime hydrologique et sur l a qualité de l’eau.

Actes du Colloque d’Amsterdam. Octobre 1977. Co-edition IAHS-Unesco

-

Coédition AISH-Unesco.

World water balance and water resources of the earth. (English edition).

Impact of urbanization and industrialization on water resources planning and management.

SociMconomic aspects of urban hydrology.

Casebook of methods of computation of quantitative changes in the hydrological regime of river basins due to human activities.

Surface water and ground-water interaction.

Aquifer contamination and protection.

Methods of computation of the water balance of large lakes and reservoirs.

Vol. I Methodology

Vol. II Case studies (in preparation)

Application of results from representative and experimental basins.

Groundwater in hard rocks.

Groundwater Models.

Vol. I Concepts, problems and methods of analysis with examples of their application.

Sedimentation Problems in River Basins.

Methods of computation of low stream flow.

Roceedings of the Leningrad Symposium on specific aspects of hydrological computations for water projects (Russian).

Methods of hydrological computations for water projects.

Hydrological aspects of drought.

Guidebook to studies of land subsidence due to groundwater withdrawal.

Guide to the hydrology of carbonate rods.

For details of the complete series please see the list printed at the end of this work.

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Hydrological aspects of drought

A contribution to the International Hydrological Progr amme

Prepared by a joint Unesco/WMO panel M.A. Beran and

J.A. Rodier, rapporteurs

Unesco - WMO

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The designations employed and the presentation o f material throughout t h i s publication do not imply the expression o f any opinion whatsoever on the part of the publishers Concerning the legal status o f any country, territory, city or

area or of i t s authorities, or concerning the delimitation of i t s frontiers or boundaries.

Published in 1985 by the United Nations

Educational, Scientific and Cultural Organization 7, place de Fontenoy, 75700 Paris, and the World Meteorological Organization, 41, avenue Giuseppe-Motta, Geneva

Printed by: Presses Universitaires de France, Vendôme

O

Unesco/WMO 1985

ISBN 92-3-102288-1

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Preface

Although the total amount o f water on earth i s generally assumed t o have remained virtually constant, the rapid growth o f population, together with the extension o f irrigated agriculture and industrial development, are stressing the quantity and quality aspects o f the natural system. Because o f the increasing problems, man has begun t o realize that he can no longer follow a “use and discard” philosophy - either with water resources or any other natural resources. As a result, the need for a consistent policy o f rational management o f water resources has become evident.

Rational water management, however, should be founded upon a thorough understanding o f water availability and movement. Thus, as a contribution t o the solution o f the world’s water problems, Unesco, in 1965, began the first world-wide programme o f studies o f the hydrological cycle - the International Hydrological Decade (IHD). The research programme was complemented by a major effort in the field o f hydrological education and training. The activities undertaken during the Decade proved t o be o f great interest and value t o Member States. B y the end o f that period, a majority o f Unesco’s Member States had formed IHD National Committees t o carry out relevant national activities and t o participate in regional and international co-operation within the IHD programme. The knowledge o f the world’s water resources had substantially improved. Hydrology became widely recognized as an independent professional option and facilities for the training o f hydrologists had been developed.

Conscious of the need t o expand upon the efforts initiated during the Intemational Hydrological Decade and, following the recommendations o f Member States, Unesco, in 1975, launched a new long-term intergovernmental programme, the International Hydrological Programme (IHP), t o follow the Decade.

Although the IHP i s basically a scientific and educational programme, Unesco has been aware from the beginning o f a need t o direct i t s activities toward the practical solutions o f the world’s very real water resources problems.

Accordingly, and in line with t h e recommendations o f the 1977 United Nations Water Conference, the objectives o f the International Hydrological Programme have been gradually expanded in order t o cover not only hydrological processes considered. in interrelationship with t h e environment and human activities, but also the scientific aspects o f multi- purpose utilization and conservation of water resources t o meet the needs o f economic and social development. Thus, while maintaining IHP’s scientific concept, the objectives have shifted perceptibly towards a multidisciplinary approach t o the assessment, planning, and rational management o f water resources.

As part o f Unesco’s contribution t o the objectives o f the IHP, two publication series are issued: “Studies and Reports in Hydrology” and “Technical Papers in Hydrology.” In addition t o these publications, and in order t o ex- pedite exchange o f information in the areas in which it i s most needed, works o f a preliminary nature are issued in the form o f Technical Documents.

The purpose o f the continuing series “Studies and Reports in Hydrology” t o which this volume belongs, i s t o pre- sent data collected and the main results o f hydrological studies, as well as t o provide information on hydrological research techniques. The proceedings o f symposia are also sometimes included. I t i s hoped that these volumes will furnish material o f both practical and theoretical interest t o water resources scientists and also t o those involved in water resources assessments and the planning for rational water resources management.

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.

INTRODUCTION

. . .

₁

1.1 1.2 1.2.1 1.2.2 1.2.3 2

.

2.1 2.2 2.2.1 2.2.2 2.2.3 2.2.4 2.2.4.1 2.2.4.2 2.2.4.3 2.2.4.4 2.2.5 2.2.5.1 2.2.5.2 2.2.6 2.3 2.3.1 2.3.2 2.3.2.1 2.3.2.2 2.3.2.3 2.3.2.4 2.3.2.5 2.3.3 2.3.3.1 2.3.3.2 2.3.3.3 2.3.4 2.4 2.4.1 2.4.2 2.4.3 2.4.4 2.4.5 2.4.6 2.4.7 2.5

General

. . .

H y d r o l o g i c a l d r o u g h t

. . .

V a r i o u s aspects o f d r o u g h t

. . .

The d e f i n i t i o n o f d r o u g h t

. . .

V a r i o u s aspects o f h y d r o l o g i c a l d r o u g h t s

. . .

CHARACTERISTICS OF HYDROLOGICAL DROUGHT

. . .

General

. . .

The p e r s i s t e n c e phenomenon

. . .

Introduction and summary

. . .

Sources o f p e r s i s t e n c e

. . .

The evidence f o r and a g a i n s t i n t e r a n n u a l p e r s i s t e n c e C a n t h e c o n f l i c t i n g evidence b e r e c o n c i l e d ?

Can processes w i t h l o w p l d i s p l a y l o n g r u n s ?

. . .

Evidence f o r p e r s i s t e n c e b e i n g more e v i d e n t a f t e r extreme y e a r s

W i t h i n y e a r p e r s i s t e n c e

. . .

Atmosphere and g r o u n d l a i r i n t e r f a c e systems

Conclusions about p e r s i s t e n c e

. . .

Space and t i m e h e t e r o g e n e i t y

. . .

I n t r o d u c t i o n

. . .

S p a t i a l h e t e r o g e n e i t y

. . .

The cause of s p a t i a l v a r i a b i l i t y

s p a t i a l e x t e n t o f d r o u g h t

. . .

Mapping drought i n t e n s i t y by means o f r a i n f a l l data

. . .

Problems i n mapping h y d r o l o g i c a l v a r i a b l e s

S t a t i s t i c a l t r e a t m e n t o f s p a t i a l v a r i a b i l i t y

. . .

Temporal h e t e r o g e n e i t y

. . .

'General f e a t u r e s o f t e m p o r a l h e t e r o g e n e i t y

. . .

Case s t u d i e s o f temporal v a r i a b i l i t y

Runoff v a r i a b i l i t y

. . .

Conclusion

. . .

Droughts and c l i m a t i c changes

. . .

General

. . .

D r o u g h t occurrence s i n c e 1850

. . . . . .

&e d r o u g h t s p e r i o d i c ?

. . .

Droughts and c l i m a t e i n m o t i o n

Anthropogenic c l i m a t e change

. . .

C l i m a t e h i s t o r y

. . .

C l i m a t e change and drought i n c i d e n c e

References t o Chapter 2

. . . . . . . . . . . .

Do d r i e r and more n o r t h e r l y Sahel s t a t i o n s d i s p l a y more P e r s i s t e n c e ?

. . . .

Do s t a t i o n groups d i s p l a y more p e r s i s t e n c e t h a n i n d i v i d u a l s ?

. . .

H y d r o l o g i c a l subsystem

. . . . . .

. . .

. . . . . .

5 5 5 5 5 6 7 8 9 9 10 10 10 11 11 11 12 12 1 2 1 3 15 1 6 1 6 1 6 1 7 17 17 19 19 1 9 20 2 1 22 22 23 23

-7

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3

.

FACTORS RESPONSIBLE FOR THE DROUGHTS

. . .

²⁷

3.1 3.1.1 3.1.2 3.1.2.1 3.1.2.2 3.1.2.3 3.1.2.4 3.1.2.5 3.1.2.6 3.1.2.7 3.1.3 3.1.3.1 3.1.3.2 3.1.3.3 3.1.3.4 3.1.3.5 3.2 3.2.1 3.2.2 3.2.2.1 3.2.2.2 3.2.2.3 3.3 3.3.1 3.3.1.1 3.3.1.2 3.3.1.3 3.3.1.4 3.3.1.5 3.3.1.6 3.3.1.7 3.3.1.8 3.3.2 3.3.2.1 3.3.2.2 3.3.2.3 3.3.3 3.4 4

.

4.1 4.2 4.2.1 4.2.2 4.2.3 4.3 4.3.1 4.3.2 4.3.2.1 4.3.2.2 4.3.2.3 4.3.2.4 4.3.3 4.3.4 4.3.5 4.4 4.4.1 4.4.2 4.4.3

P a r t i c u l a r aspects o f t h e dynamics o f t h e a i r masses I n t r o d u c t i o n

. . .

^;

. .

Some p h y s i c a l f a c t o r s a s s o c i a t e d w i t h m i d - l a t i t u d e dr Subsidence and i t s causes

. . .

The r o l e o f h i g h p r e s s u r e c e l l s

. . .

Teleconnections between p r e s s u r e anomalies

. . .

S e l f g e n e r a t i n g mechanisms

. . .

Long waves i n t h e w e s t e r l i e s

. . .

E x t e r n a l f a c t o r s r e s p o n s i b l e f o r d r o u g h t

. . .

T r o p i c a l d r o u g h t s

. . .

West A f r i c a

. . .

Drought i n I n d i a

. . .

Drought i n South America

. . .

A u s t r a l i a n d r o u g h t

. . .

Drought i n South A f r i c a

. . .

I n f l u e n c e o f man

. . .

I n d i v i d u a l mechanisms

. . .

Drought e f f e c t

. . .

L o c a l e f f e c t s on t h e c l i m a t e

. . .

P o s s i b i l i t i e s o f f o r e c a s t i n g drought

. . .

M e t e o r o l o g i c a l methods o f f o r e c a s t i n g

. . .

Analogue methods

. . .

L i n e a r r e g r e s s i o n methods

. . .

Teleconnections

. . .

S t a t i s t i c a l and k i n e m a t i c methods

. . .

Contingency t a b l e s

. . .

Use o f a i r - s e a i n t e r a c t i o n s

. . .

S t a t i s t i c a l t i m e s e r i e s f o r e c a s t s

. . .

E x t r a p o l a t i o n i n t i m e u s i n g c y c l i c i t i e s

. . .

H y d r o l o g i c a l methods o f f o r e c a s t i n g drought

. . . .

Recession based methods

. . .

Regression methods

. . .

Cycles i n annual streamflow

. . .

Review o f accuracy

. . .

The r o l e o f sea s u r f a c e temperature

. . .

Anthropogenic e f f e c t s on c l i m a t e

. . . . . . . . .

ought

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

METHODOLOGY FOR THE STUDY OF DROUGHT AND EXCEPTIONAL LOW RIVER FLOWS

. . . .

Choice o f i n d i c e s f o r depth o f d i s c h a r g e and p r e c i p i t a t i o n

. . .

Use o f h i s t o r i c a l and geomorphological i n f o r m a t i o n

. . .

H i s t o r i c a l documents and f o l k memory

. . .

Geomorphological and o t h e r palaeoenvironmental i n d i c a t o r s

. . .

Droughts o f t h e immediate p a s t from i n d i r e c t evidence

. . .

A n a l y s i s o f p r e c i p i t a t i o n depth and o t h e r c l i m a t i c v a r i a b l e s

General

. . .

Annual r a i n f a l l

. . .

P l u v i o s i t y

. . .

P e r c e n t i l e s

. . .

D i s t r i b u t i o n f i t t i n g

. . .

R u n l e n g t h and run sum

. . .

S p a t i a l d e s c r i p t i o n o f drought r a i n f a l l

. . .

More complex c l i m a t e based i n d i c a t o r s

. . .

A n a l y s i s o f d i s c h a r g e

. . .

A n a l y s i s o f discharges o f r i v e r s w i t h one d e f i n e d r a i n y and one d e f i n e d d r y season

. . .

A n a l y s i s o f d i s c h a r g e f o r r i v e r s w i t h o u t w e l l d e f i n e d r a i n y o r d r y seasons

. . . .

A n a l y s i s o f r a i n f a l l i n o t h e r d u r a t i o n s

. . .

S p e c i a l purpose measurements d u r i n g l o w f l o w p e r i o d s

. . .

27 27 27 27 33 33 35 35 35 43 44 44 48 5 1 52 52 52 52 52 52 55 55 55 55 55 56 56 56 57 57 58 58 58 58 59 59 59 60 65 65 66 66 66 67 67 67 68 68 68 68 70 70 7 1 7 1 72 72 72 73

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4.5 4.5.1 4.5.2 4.6 4.6.1 4.6.2 4.6.3 4.6.4 4.6.5 4.6.6 4.7

F i t t i n g s t a t i s t i c a l d i s t r i b u t i o n s t o drought d a t a

. . .

Choice o f d i s t r i b u t i o n s

. . .

Methods o f f i t t i n g d i s t r i b u t i o n s

. . .

P o s s i b i l i t i e s o f d r o u g h t p r e d i c t i o n by c o r r e l a t i o n w i t h g e o l o g i c a l

and o t h e r c h a r a c t e r i s t i c s o f b a s i n s

. . .

General

. . .

The UK scheme f o r e s t i m a t i n g l o w f l o w c h a r a c t e r i s t i c s o f

ungauged catchments

. . .

R e l a t i o n s h i p w i t h catchment c h a r a c t e r i s t i c s

. . .

I n t e r n a l d u r a t i o n and frequency l i n k a g e s

. . .

Low f l o w e s t i m a t i o n f o r ungauged catchments i n t h e Sahel zone

. . .

Concluding remarks

. . .

5

.

THE RECENT DROUGHT I N TROPICAL AREAS

. . .

5.1 5.2 5.2.1 5.2.2 5.3 5.4 5.4.1 5.4.2 5.5 5.5.1 5.5.1.1 5.5.1.2 5.5.1.3 5.5.1.4 5.5.1.5 5.5.2 5.5.2.1 5.5.2.2 5.6 5.6.1 5.6.2 5.6.3 5.6.4 5.6.5 5.6.6 5.6.7 5.7 5.8 5.9

. . .

General c h a r a c t e r o f t h i s d r o u g h t

. . .

R e g i o n a l coverage

. . .

C h a r a c t e r i s t i c s o f t r o p i c a l d r o u g h t s g e n e r a l l y and o f t h e A v a i l a b l e d a t a

. . .

A n a l y s i s o f t h e d a t a

. . .

N u m e r i c a l i n d i c e s

. . .

Some d e t a i l s o f t h e a n a l y s i s

. . .

R e s u l t s

. . .

P r e s e n t a t i o n o f r e s u l t s

. . .

D i s c u s s i o n o f r e s u l t s

. . .

Year by y e a r account o f t h e d r o u g h t

. . .

Comparisons o u t s i d e t h e t r o p i c a l zqne

. . .

Summarising s t a t e m e n t

. . .

Maximum and minimum y e a r l y d i s c h a r g e s during t h e drought Maximum y e a r l y d i s c h a r g e

. . .

Annual minimum d i s c h a r g e s

. . .

Comparison w i t h e a r l i e r droughts

. . .

A f r i c a n S a h e l

. . . ’. . . .

I n d i a

. . .

C e n t r a l America and Mexico

. . .

South America

. . .

T r o p i c a l South A f r i c a

. . .

A u s t r a l i a and Oceania

. . .

More e x t e n s i v e h i s t o r i c a l researches

. . .

Second p e r i o d o f t h e r e c e n t d r o u g h t i n t r o p i c a l areas

. .

C o n c l u s i o n

. . .

Average annual d i s c h a r g e s during t h e drought

. . .

. . . . . . . . .

r e c e n t drought

. .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6

.

TEMPERATE ZONE DROUGHT

. . .

6.1 6.1.1 6.1.2 6.1.3 6.1.3.1 6.1.3.2 6.1.3.3 6.2 6.2.1 6.2.2 6.2.2.1 6.2.2.2 6.2.2.3 6.2.2.4

. . .

Region31 coverage

. . .

C h a r a c t e r i s t i c s o f temperate zone d r o u g h t

. . . .

C o n t r a s t between S a h e l and temperate zone d r o u g h t C l i m a t i c c o n t r a s t s

. . .

V e g e t a t i v e and s o c i e t a l c o n t r a s t s

. . .

H y d r o l o g i c a l c o n t r a s t s

. . .

Droughts o f t h e r e c e n t p a s t

. . .

Western Europe

. . .

P a s t d r o u g h t s i n N o r t h America

. . .

General

. . .

West m a r i t i m e r e g i o n

. . .

N o r t h e r n p r a i r i e s

. . .

The n o r t h e a s t r e g i o n

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

74 74 75 77 77 77 77 79 79 80 82 85 85 85 85 87 87 88 88 88 89 89 89 89 89 92 92 93 93 94 95 95 96 96 96 98 98 98 100 1 0 1 115 117 117 117 117 119 119 119 120 121 121 122 122 122 123 126

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6.4 6.4.1 6.4.2 6.4.3 6.4.4 6.4.5 6.4.6 6.4.7 6.5 6.5.1 6.5.2 6.5.3 6.6 7

.

7.1 7.2 7.3 7.3.1 7.3.2 7.3.3 7.3.4 7.3.5 7.4 7.5 7.6 7.7 8

.

8 . 1 8.1.1 8.1.2 8.1.3 8.1.4 8.1.5 8.1.6 8.2 8.3

D e t a i l s o f d r o u g h t i n western Europe d u r i n g t h e 1970s

. . .

¹²⁷

General

. . .

¹²⁷

The 1971-74 and 1976 droughts i n Czechoslovakia

. . .

¹²⁸

The d r o u g h t i n France from December 1975 t o J u l y 1976 129 The 1976 d r o u g h t i n t h e F e d e r a l Republic o f Germany ( B a v a r i a )

. . .

¹³⁰

The 1976 d r o u g h t i n Belgium and i t s consequences

. . .

¹²⁷

. . .

The 1976 d r o u g h t i n t h e Netherlands

. . .

¹³¹

Drought i n t h e 1970s i n t h e U n i t e d Kingdom

. . .

¹³¹

Droughts i n t h e temperate zone o f t h e U n i t e d S t a t e s d u r i n g t h e 1970s

. . . .

Areas a f f e c t e d d u r i n g t h e 1976-77 d r o u g h t

. . .

¹³⁴

Chronology o f t h e d r o u g h t i n t h e n o r t h e r n p r a i r i e r e g i o n

. . .

¹³⁶

134 Chronology o f t h e drought i n western USA

. . .

¹³⁴

. . .

¹³⁷

PROSPECTS FOR THE LIMITATION OF THE CONSEQUENCES OF HYDROLOGICAL DROUGHT

. .

¹⁴¹

General

. . .

¹⁴¹

Surface water management

. . .

¹⁴¹

Groundwater management

. . .

¹⁴²

The need t o a v o i d o v e r e x p l o i t a t i o n

. . .

¹⁴²

Groundwater source development

. . .

¹⁴³

P o s s i b i l i t i e s f o r augmenting a q u i f e r recharge and y i e l d

. . .

¹⁴³

Groundwater q u a l i t y problems

. . .

¹⁴³

General c o n c l u d i n g remarks

. . .

¹⁴⁴

A r t i f i c i a l enhancement o f p r e c i p i t a t i o n

. . .

¹⁴⁴

consequences

. . .

¹⁴⁵

. . .

¹⁴⁵

Reduction o f e v a p o r a t i o n

. . .

¹⁴⁴

Land management. l o g i s t i c a l and s o c i a l measures f o r m i t i g a t i n g d r o u g h t RECOMMENDATIONS

. . .

¹⁴⁷

General recommendations f o r r e s e a r c h

. . .

¹⁴⁷

. . .

¹⁴⁷

Research i n t o d r o u g h t i n d i c e s

. . .

¹⁴⁷

Drought mechanisms

. . .

¹⁴⁸

Drought consequences

. . .

¹⁴⁸

Suggestions f o r i n t e r n a t i o n a l c o - o p e r a t i o n

. . .

¹⁴⁹

Reference t o Chapter 8

. . .

¹⁴⁹

Droughts i n t i m e and space

. . .

¹⁴⁷

Drought surveys

. . .

¹⁴⁸

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Foreword

A s e r i e s o f s t u d i e s on droughts were undertaken d u r i n g t h e I n t e r n a t i o n a l H y d r o l o g i c a l Decade launched b y Unesco i n 1964.

From 1968 t o 1973, d u r i n g t h e severe d r o u g h t which a f f e c t e d t h e Sahel and o t h e r t r o p i c a l r e g i o n s , t h e i n t e r e s t e d governments and i n t e r n a t i o n a l O r g a n i z a t i o n s d i d t h e i r b e s t t o amelio- r a t e t h e d i r e c t consequences o f t h i s c a t a s t r o p h e , t o s t u d y t h e c o n d i t i o n s and causes o f t h e drought, and t o recommend measures which could, i n t h e f u t u r e , m i t i g a t e t h e e f f e c t s o f such droughts.

Panel o f E x p e r t s f o r t h e I n t e r n a t i o n a l H y d r o l o g i c a l Decade requested t h a t a r e p o r t b e p r e p a r e d on t h i s s u b j e c t by t h e WMO r a p p o r t e u r on l o w f l o w s and r e l a t e d aspects o f droughts. T h i s r e p o r t , w r i t t e n i n c o - o p e r a t i o n w i t h t h e I n t e r n a t i o n a l A s s o c i a t i o n o f H y d r o l o g i c a l Sciences, was p r e - sented b y t h a t A s s o c i a t i o n t o t h e I n t e r n a t i o n a l conference on t h e R e s u l t s o f t h e I n t e r n a t i o n a l H y d r o l o g i c a l Decade and on F u t u r e Programmes i n Hydrology convened j o i n t l y by Unesco and WMO a t Unesco Headquarters i n P a r i s (2-13 September 1974).

The I n t e r g o v e r n m e n t a l C o u n c i l o f t h e I n t e r n a t i o n a l H y d r o l o g i c a l Programme o f Unesco decided a t i t s f i r s t s e s s i o n ( P a r i s , 9-17 A p r i l 1975) t o a p p o i n t a r a p p o r t e u r w i t h t h e t a s k o f p r e p a r i n g a s t a t e - o f - t h e - a r t r e p o r t on h y d r o l o g i c a l aspects o f d r o u g h t s (IHP p r o j e c t 3.5). The r a p p o r t e u r was t o work i n c l o s e c o - o p e r a t i o n w i t h WMO and IAHS, and t a k e i n t o account t h e work done b y o t h e r i n t e r n a t i o n a l o r g a n i z a t i o n s concerning d r o u g h t s ( i n p a r t i c u l a r FAO and UNEP) and t o make p r o p o s a l s f o r f u t u r e a c t i o n .

A t i t s f i f t h s e s s i o n i n Ottawa (July 19761, t h e Commission f o r Hydrology o f WMO a l s o appointed a r a p p o r t e u r t o s t u d y and a p p l y methods o f i n d e x i n g c o n t i n e n t a l s c a l e droughts and p o s s i b l y e x t e n d i n g t h i s t o encompass t h e 1975-1976 European drought.

decided t o recommend t h e e s t a b l i s h m e n t o f a j o i n t Unesco/WMO p a n e l t o p r e p a r e a s t a t e - o f - t h e a r t r e p o r t on h y d r o l o g i c a l aspects o f droughts.

The j o i n t p a n e l composed o f t h e Unesco and WMO r a p p o r t e u r s , D r . J.A. R o d i e r (France) and M r . M.A. Beran ( U n i t e d Kingdom), met i n P a r i s on 8 November 1977 and r e v i s e d t h e d r a f t o u t l i n e which had been p r e s e n t e d b y D r . Rodier t o t h e IHP I n t e r g o v e r n m e n t a l C o u n c i l a t i t s June 1977 s e s s i o n and d i s t r i b u t e d t h e t a s k between Unesco and WMO f o r t h e p r e p a r a t i o n o f t h e r e p o r t .

The f o l l o w i n g e x p e r t s have a l s o c o n t r i b u t e d t o t h e p r e p a r a t i o n o f t h i s r e p o r t : D r . L. D o r i z e (France) f o r s e c t i o n s 2.4, 3 . 1 and 3.2; P r o f e s s o r J. F l o h n ( F e d e r a l R e p u b l i c o f Germany) f o r s e c t i o n 2.3; P r o f e s s o r J. Namias (USA) f o r s e c t i o n s 3 . 1 and 3.3; M r . L. S e r r a

(France) f o r s e c t i o n s 1.2 and 4.1.

As f a r as t h e assessment o f r a i n f a l l and r u n o f f i s concerned, t h e WMO E x e c u t i v e Committee's

During i t s second session, t h e I n t e r g o v e r n m e n t a l C o u n c i l o f t h e IHP (20-27 June 1977)

M.A. Beran J.A. Rodier

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1. Introduction

1.1 GENERAL

Drought i s g e n e r a l l y viewed as a s u s t a i n e d and r e g i o n a l l y e x t e n s i v e occurrence o f below average n a t u r a l water a v a i l a b i l i t y , e i t h e r i n t h e form o f p r e c i p i t a t i o n , r i v e r r u n o f f o r groundwater.

Drought should n o t be confused w i t h a r i d i t y which a p p l i e s t o those p e r s i s t e n t l y dry r e g i o n s where, even i n normal circumstances, water i s i n s h o r t supply. The consequences o f d r o u g h t s a r e f e l t most k e e n l y i n areas which a r e i n any case a r i d . However i t i s manifested, d r o u g h t

a d v e r s e l y a f f e c t s t h e economy by reducing, o r e v e n e l i m i n a t i n g , a g r i c u l t u r a l p r o d u c t i o n , h e r d s o f c a t t l e , energy g e n e r a t i o n , and domestic and i n d u s t r i a l water supply. Developing c o u n t r i e s a r e p a r t i c u l a r l y prone t o these adverse e f f e c t s on two counts: i f d i r e c t l y a f f e c t e d b y a d r o u g h t t h e d i f f i c u l t economic s i t u a t i o n i n t h e d e v e l o p i n g c o u n t r y h i n d e r s i t s a b i l i t y t o t a k e s w i f t a c t i o n t o reduce t h e d i s a s t r o u s consequences; and i f d r o u g h t s t r i k e s a c e r e a l s u p p l y i n g

developed c o u n t r y t h e supply o f t h e commodity t o t h e d e v e l o p i n g c o u n t r y i s reduced and i t s p r i c e i s increased.

Drought may be so severe t h a t famine may ensue, and i n some cases t h e s i t u a t i o n may be such t h a t , d e s p i t e i n t e r n a t i o n a l c o o p e r a t i o n , i t c o u l d cause t h e d e a t h o f m i l l i o n s o f human beings.

As has been i m p l i e d , t h e e s s e n t i a l f e a t u r e o f d r o u g h t i s t h a t i t i s t i e d t o t h e i d e a o f a d e f i c i t i n t h e supply o f m o i s t u r e f o r some s p e c i f i c purpose. Abnormal l o w f l o w s i n r i v e r s are, o f course, g e n e r a l l y experienced d u r i n g any d r o u g h t but t h e i r s t u d y i s r a t h e r d i f f e r e n t . Low f l o w s t u d i e s w i l l be concerned w i t h t h e s t a t i s t i c a l t r e a t m e n t , and t h e u n d e r s t a n d i n g o f t h e p h y s i c a l development, o f f l o w s a t a p o i n t a l o n g a r i v e r i n t h e s h o y t term. By c o n t r a s t t h e s t u d y o f drought concerns t h e d e s c r i p t i o n o f r a i n f a l l , r i v e r f l o w , s o i l m o i s t u r e and groundwater over a season, y e a r o r s e v e r a l y e a r s and a l s o o f t h e s p a t i a l e x t e n t o f t h e phenomenon.

O f t h e numerous aspects o f drought, t h e p r e s e n t r e p o r t c o n c e n t r a t e s on t h e s i n g l e aspect o f h y d r o l o g i c a l drought, i . e . t h e d e f i c i t i n t h e r u n o f f o f r i v e r s , w i t h some a t t e n t i o n t o t h e d e f i c i t i n p r e c i p i t a t i o n and t h e d e f i c i t i n groundwater.

1.2 HYDROLOGICAL DROUGHT

1 . 2 . 1 Various aspects o f d r o u g h t

Drought by d e f i n i t i o n c o n s i s t s o f a s u s t a i n e d p e r i o d o f d e f i c i t perhaps l a s t i n g a few months o r even many years. C o n d i t i o n s w i t h i n a d r o u g h t may v a r y c o n s i d e r a b l y i n space and t i m e i n accordance w i t h t h e s p a t i o - t e m p o r a l i r r e g u l a r i t y o f t h e r a i n f a l l d i s t r i b u t i o n and w i t h t h e h e t e r o g e n e i t y o f , t h e h y d r o l o g i c a l response o f t h e r i v e r b a s i n s t h a t a r e a f f e c t e d . The

c h a r a c t e r o f d r o u g h t may be d i f f e r e n t f o r t h e d i f f e r e n t c l i m a t o l o g i c a l and h y d r o l o g i c a l regimes t h a t a r e found i n t h e w o r l d . A s e x p l a i n e d below i t a l s o d i f f e r s v e r y much a c c o r d i n g t o t h e use t o which t h e water i s put.

where a shortage o f r a i n d e p t h and d u r a t i o n during t h e r a i n y season need n o t much a f f e c t t h e p a s t u r e so l o n g as g e r m i n a t i o n and growth i s p e r m i t t e d , however g r a i n p r o d u c t i o n may be v e r y much reduced. So such a d r o u g h t may have a more marked e f f e c t on t h e c e r e a l grower t h a n on t h e p a s t o r a l i s t .

I n t r o p i c a l areas i f t h e y e a r l y d i s t r i b u t i o n s o f p r e c i p i t a t i o n i s such t h a t t h e r e a r e a s m a l l number o f r u n o f f - p r o d u c i n g heavy storms a t t h e end o f t h e r a i n y season i n a d d i t i o n t o a number o f s l i g h t r a i n f a l l s t h r o u g h o u t t h a t season, t h e r e may be a d e f i c i t i n annual r a i n f a l l but t h e t o t a l y e a r l y r u n o f f may be normal. Thus a l t h o u g h t h e r e i s no h y d r o l o g i c a l d r o u g h t i n t h i s circumstance n e v e r t h e l e s s c r o p y i e l d w i l l be low.

A n example o f t h i s v a r i a t i o n w i t h water use i s found i n t h e more a r i d p a r t o f t h e Sahel

A water d e f i c i t d u r i n g t h e c r i t i c a l p e r i o d f o r a g r i c u l t u r e i s c o n s i d e r e d as a d r o u g h t b y

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farmers, but i f t h e p r e c e d i n g w i n t e r and s p r i n g had been v e r y h u m i d r e s e r v o i r s may w e l l be f u l l and, as f a r as h y d r o - e l e c t r i c p l a n t managers a r e concerned, t h e r e i s no drought.

I n years when t h e t o t a l r a i n f a l l i s normal but o f low i n t e n s i t y , and e s p e c i a l l y when

a s s o c i a t e d w i t h h i g h winds, t h e recharge t o a q u i f e r s w i l l be inadequate even i f r i v e r r u n o f f i s normal. There w i l l t h e n be d r o u g h t f o r t h e u s e r s o f t h e a q u i f e r .

The most severe droughts, such as t h e r e c e n t case i n t h e Sahel, s u f f e r on a l l counts: low r a i n f a l l , low r i v e r f l o w and storages, and d e p l e t e d a q u i f e r s .

O f t h e s o r t s o f drought which have been mentioned above those which concern a g r i c u l t u r e a r e discussed i n t e c h n i c a l n o t e No. 1 3 8 "Drought and a g r i c u l t u r e " produced by a WMO working group.

A s s t a t e d above h y d r o l o g i c a l drought i s considered t o b e a d e f i c i t o f r u n o f f below normal c o n d i t i o n s , o r e l s e a d e p l e t i o n o f a q u i f e r l e v e l s even though, t h r o u g h o v e r - e x p l o i t a t i o n , t h e water s u p p l y f u r n i s h e d b y t h e a q u i f e r s may remain t h e same as b e f o r e t h e drought.

1.2.2 The d e f i n i t i o n o f d r o u g h t

I t i s t e m p t i n g t o search f o r simple statements such as a r e found i n d i c t i o n a r i e s which

encapsulate t h e i d e a o f a drought. T h i s i s h a r d l y p o s s i b l e because h y d r o l o g i c a l o b s e r v a t i o n s t h a t a r e made - r a i n depth o r r i v e r f l o w r e l a t i v e t o t h e i r average - a r e f a r from c o n s t a n t i n t i m e and a r e n o t qynchronous one w i t h another.

e q u a l l y and s i m u l t a n e o u s l y a f f e c t a l l t h e p o i n t s o f t h e globe, n o t even a c o n t i n e n t . r e t a i n q u a l i t a t i v e c o n n o t a t i o n s .

d e f i n e p r e c i s e l y what one wants t o l e a r n about a drought and t o which p a r t i c u l a r p o i n t s and c h a r a c t e r i s t i c s one must pay most a t t e n t i o n . We t h e r e f o r e r e g a r d d r o u g h t n o t as a d e f i n a b l e e n t i t y i n i t s e l f but as a "prime mover" which has a t t r i b u t e s o r consequences. I t i s these consequences, i n p a r t i c u l a r t h e h y d r o l o g i c a l consequences, t h a t we focus upon and d e f i n e i n t h i s r e p o r t .

v i c t i m s as e x c e p t i o n a l droughts because t h e i r d e s t r u c t i o n i s c o n f i n e d t o t h e v a l l e y bottoms w h i l e d r o u g h t s c h a r a c t e r i s t i c a l l y s t r i k e a t immense areas a t t h e same time.

and human s o c i e t y : t h i s can l e a d t o s e v e r a l d i f f e r e n t c h a r a c t e r i s a t i o n s as a l r e a d y g i v e n i n t h e p r e v i o u s subsection. I n any case i t i s e v i d e n t t h a t t h e n o t i o n o f drought i s r e l a t i v e , but i t s c h i e f c h a r a c t e r i s t i c i s a decrease o f water a v a i l a b i l i t y i n a p a r t i c u l a r p e r i o d and over a p a r t i c u l a r area r a t h e r t h a n a g e n e r a l decrease i n water a v a i l a b i l i t y . I t i s t h u s t h i s abnormal d i s t r i b u t i o n , which must b e considered t y p i c a l o f droughts.

r i v e r f l o w , o f groundwater, o r e l s e i s h e l d . f o r subsequent r e l e a s e a f t e r m e l t i n g as snow o r i c e . Because o f t h e d i f f e r e n t i n b u i l t d e l a y s i n t h e p a r t s o f t h e l a n d phase o f t h e h y d r o l o g i c a l c y c l e t h e d i f f e r e n t m a n i f e s t a t i o n s o f t h e d r o u g h t ( i . e . l a c k o r absence o f r a i n f a l l ) a r e n o t s i m u l t a n e o u s l y f e l t . Thus i t i s conceivable t h a t a c e r t a i n month can b e considered d r y b y a c l i m a t o l o g i s t and an a g r i c u l t u r i s t who a r e p r i m a r i l y i n t e r e s t e d i n r a i n f a l l , but normal, o r even above average, b y t h e h y d r o l o g i s t who i s more concerned w i t h f l o w s . That i s why i t i s necessary t o c l o s e l y d e f i n e , n o t o n l y t h e f a c t o r considered ( r a i n o r f l o w ) , but a l s o t h e d u r a t i o n and p e r i o d s t u d i e d . The d e f i n i t i o n o f a base p e r i o d i s e s s e n t i a l when d r o u g h t i s t o be d e s c r i b e d i n terms o f ' p l u v i o s i t y ' o r ' h y d r a u l i c i t y ' . These two terms which a r e i n standard French usage

( p l u v i o s i t é and h y d r a u l i c i t é ) have been r e t a i n e d as a convenient shorthand f o r r a i n f a l l and r u n o f f t o t a l s i n some p e r i o d expressed as a r a t i o o r a percentage o f t h e l o n g t e r m average v a l u e over t h e same p e r i o d .

y e a r c o u l d i n c l u d e an abnormally d r y and an abnormally wet p e r i o d .

By analogy w i t h t h e t r e a t m e n t of f l o o d s

-

a l s o an abnormal d i s t r i b u t i o n o f f l o w - one tends t o r e g a r d t h e m i n i m u m i n s t a n t a n e o u s o r d a i l y f l o w reached d u r i n g t h e year as a c h a r a c t e r i s t i c o r measure o f t h e d r o u g h t s e v e r i t y o f t h a t year.

d i f f e r e n t . The f l o o d , i n p a r t i c u l a r i t s peak value, i s a t r a n s i t o r y phenomenon due t o m u l t i p l e and random causes such as t h e i n t e n s i t y and t h e d u r a t i o n o f t h e r a i n f a l l , t h e p e r m e a b i l i t y o f t h e ground e t c . The l o w f l o w i s a phenomenon which e v o l v e s much more s l o w l y and which i s v e r y i n t i m a t e l y t i e d t o s t r u c t u r e s o f g r e a t i n e r t i a , such as t h e t o t a l volume o f s t o r e d sources and t h e summer e v a p o t r a n s p i r a t i o n on t h e catchment. O n p h y s i c a l grounds i t i s d i f f i c u l t t o c l a i m t h a t t h e m i n i m u m f l o w i n a year i s t o t a l l y independent from those o f p r e v i o u s years even i f t h e r i v e r a t t h e p o i n t o f measurement had run d r y . However, i n p r a c t i c e one f i n d s t h a t t h e d a i l y f l o w sequence p r e s e n t s a p i c t u r e which does g i v e a m i n i m u m a p p a r e n t l y independent o f i t s neighbours. T h i s i s n o r m a l l y due t o t h e s u p e r p o s i t i o n o f man-induced e f f e c t s , e.g. o p e r a t i o n o f dams, f l o o d gates, pumped d i v e r s i o n s , on t h e r e l a t i v e l y smooth n a t u r a l hydrograph.

v a r i a t i o n s ; an extreme l a c k o f water can occur completely s e p a r a t e l y from a d r o u g h t i n t h e commonly understood sense o f t h e word. Besides t h i s requirement f o r a d j u s t i n g t h e f l o w s f o r

The drought o f one y e a r o r season does n o t C o n t r a r y t o f l o o d s , which can be measured and q u a n t i f i e d , d r o u g h t v e r y o f t e n seems t o

I n t h i s r e s p e c t

-

maybe even more here

-

i t i s e s s e n t i a l t o

No f l o o d , even t h e most c a t a s t r o p h i c and memorable, has been r e s p o n s i b l e f o r as many

One can s t u d y d r o u g h t on i t s own, o r d r o u g h t and i t s consequences on a g r i c u l t u r e , economy

Water which f a l l s i n t h e form of r a i n f a l l o f course reappears i n due t i m e i n t h e form o f

A normal annual p l u v i o s i t y ( c l o s e t o u n i t y ) can h i d e t h e f a c t t h a t t h e

I n f a c t t h e two phenomena a r e fundamentally

Any f l o w , but p a r t i c u l a r l y t h e m i n i m u m f l o w s , can t h u s d i s p l a y n o t a b l e a r t i f i c i a l

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a r t i f i c i a l v a r i a t i o n one must n o t f o r g e t another i m p o r t a n t d i f f i c u l t y concerning r e c o r d e d f l o w s : t h e measurement o f l o w f l o w i s o f t e n d e l i c a t e , which e x p l a i n s f r e q u e n t gaps and jumps i n t h e p u b l i s h e d d a t a , e s p e c i a l l y f o r t h e p a s t decades. I n many cases, t h e accuracy o f t h e

measurement i s o f t e n mediocre, e s p e c i a l l y over a s m a l l t i m e scale.

needed t o d e f i n e them:

What a r e t h e main n u m e r i c a l c h a r a c t e r i s t i c s t h a t a t t a c h t o l o w f l o w s ? Three numbers a r e a. t h e m i n i m u m f l o w v a l u e averaged o v e r n c o n s e c u t i v e days (n = 5, 10, 20,

30 o r more d a y s ) ;

b. t h e d a t e s o f t h e i r occurrence;

c. t h e frequency a t t r i b u t a b l e t o t h e phenomenon.

T h i s l a s t mentioned number w i l l be assigned e i t h e r by u s i n g t h e l o n g t e r m d a t a o f t h e s i t e A number o f ways o f d e s c r i b i n g frequency a r e i n common o r by analogy w i t h a comparable r e c o r d .

use :

a. a simple r a n k i n g , i . e . t h e i t h l o w e s t i n N y e a r s o f r e c o r d ;

b. an e m p i r i c a l p r o b a b i l i t y o f non exceedence based upon sample v a l u e s , i . e . i / N ;

c. a more thorough approach based upon a s t a t i s t i c a l a n a l y s i s o f a l l t h e annual minima c o n s i d e r i n g a c o r r e c t c h o i c e o f s t a t i s t i c a l d i s t r i b u t i o n and p l o t t i n g p o s i t i o n ;

d. a l t e r n a t i v e methods o f d e s c r i b i n g frequency express t h e p r o b a b i l i t y i n percentage terms of e l s e as r e t u r n p e r i o d b e i n g t h e average r e c u r r e n c e i n t e r v a l between non exceedances o f t h e s e l e c t e d l o w f l o w .

A n i d e a l r e p r e s e n t a t i o n o f l o w f l o w s e v e r i t y o c c u r r i n g i n a d r o u g h t would c o n s i s t o f a s e t o f l o w f l o w maps each one b e i n g f o r a g i v e n v a l u e o f n ( t h e d u r a t i o n o v e r which f l o w s a r e averaged) and a g i v e n frequency ( f o r example t h e median c o n d i t i o n

-

n o t exceeded by one h a l f o f l o w f l o w s , t h e q u i n q u e n n i a l and t h e d e c e n n i a l l o w f l o w s - n o t exceeded b y one f i f t h and one t e n t h o f n-day annual minima). I t must b e a p p r e c i a t e d though t h a t t h e p r o d u c t i o n o f such a s e t o f maps r e q u i r e s a dense gauging s t a t i o n network over t h e b a s i n s , and o b s e r v a t i o n s e r i e s

e x t e n d i n g bàck over s e v e r a l decades. U n f o r t u n a t e l y t h i s i d e a l i s seldom r e a l i s e d . S t a r t and end o f droughts

Droughts d i f f e r from o t h e r m e t e o r o l o g i c a l phenomena i n t h e i r temporal aspect. I t i s d i f f i c u l t t o t e l l a t what d a t e a d r o u g h t s t a r t e d , what d a t e i t ended, and t h u s how l o n g i t l a s t e d ; what i s c e r t a i n i s t h a t t h i s d u r a t i o n can be r e l a t i v e l y l o n g by comparison w i t h o t h e r m e t e o r o l o g i c a l events.

The s t a r t o f a drought, which can o n l y be determined t o w i t h i n a b r a c k e t o f 1 t o 2 months, depends v e r y much on ones own p o i n t o f v i e w which i s n o t n e c e s s a r i l y t h e same f o r t h e

a g r i c u l t u r i s t as f o r t h e h y d r o e l e c t r i c i a n . A d r o u g h t , i n t h e most g e n e r a l sense, does n o t s t a r t immediately a t t h e t e r m i n a t i o n o f t h e l a s t u s e f u l r a i n f a l l . For h y d r o l o g i c a l d r o u g h t , t h e commencement o f t h e phenomenon may be much delayed because o f t h e damping e f f e c t o f underground r e s e r v e s which c o n t i n u e t o s u p p o r t t h e f l o w s , a t l e a s t f o r a w h i l e a f t e r t h e c e s s a t i o n o f r a i n f a l l .

r a i n f a l l s a t u r a t e s t h e ground, r a i s e s t h e f l o w i n r i v e r s , and r e b u i l d s underground r e s e r v e s . Numerical c r i t e r i a deduced from t h e e m p i r i c a l s t u d y o f a c e r t a i n number o f cases have been proposed t o d e f i n e t h e end o f a p r o l o n g e d p e r i o d o f drought.

o r c h a r a c t e r i s t i c s ; examples a r e given above but many more c h a r a c t e r i s t i c s and methods o f q u a n t i f y i n g them a r e g i v e n t h r o u g h o u t t h i s r e p o r t . P a r t i c u l a r b r o a d c l a s s e s o f d r o u g h t a r e g i v e n i n t h e n e x t subsection.

1 . 2 . 3 V a r i o u s aspects o f h y d r o l o g i c a l d r o u g h t s

S i x types o f d r o u g h t may b e d i s t i n g u i s h e d based upon v a r i a t i o n s i n t h e d u r a t i o n , season o f year, o r s e v e r i t y :

1. A three-week t o three-month r u n o f f d e f i c i t during t h e p e r i o d o f g e r m i n a t i o n and p l a n t growth. T h i s c o u l d be c a t a s t r o p h i c f o r f a r m i n g t h a t i s dependent upon i r r i g a t i o n drawn d i r e c t l y from t h e r i v e r w i t h o u t t h e s u p p o r t o f

r e s e r v o i r s .

2. A minimum d i s c h a r g e s i g n i f i c a n t l y l o w e r o r more p r o l o n g e d t h a n t h e normal

The end o f d r o u g h t i s more v i s i b l e and t h u s e a s i e r t o determine, p a r t i c u l a r l y when abundant

To summarise, i t i s n o t d r o u g h t i t s e l f which i s s t r i c t l y d e f i n e d but i m p o r t a n t a t t r i b u t e s

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3 . 4 .

5 .

6 .

minimum but n o t n e c e s s a r i l y advanced much i n i t s p o s i t i o n r e l a t i v e t o t h e growing season. Because t h e g e r m i n a t i o n p e r i o d i s n o t a f f e c t e d t h i s t y p e o f d r o u g h t i s o f l e s s consequence t o a g r i c u l t u r e .

A s i g n i f i c a n t d e f i c i t i n t h e t o t a l annual r u n o f f . T h i s a f f e c t s hydropower p r o d u c t i o n and i r r i g a t i o n from l a r g e r e s e r v o i r s .

A below normal annual h i g h water l e v e l o f t h e r i v e r . T h i s may i n t r o d u c e t h e need f o r pumping f o r i r r i g a t i o n . T h i s t y p e o f drought i s r e l a t e d t o Type 3

-

d e f i c i t i n annual r u n o f f .

Drought e x t e n d i n g over s e v e r a l c o n s e c u t i v e years as w i t h t h e "Secas" o f N o r t h e a s t B r a z i l . Discharge remains below a l o w t h r e s h o l d o r t h e r i v e r s d r y up e n t i r e l y and remain d r y f o r a v e r y l o n g time.

A s i g n i f i c a n t n a t u r a l d e p l e t i o n o f a q u i f e r s . T h i s i s d i f f i c u l t t o q u a n t i f y because o b s e r v a t i o n o f t h e t r u e l e v e l o f t h e a q u i f e r i s d i s t u r b e d by t h e o v e r - u t i l i z a t i o n o f groundwater d u r i n g t h e d r o u g h t .

The second, t h i r d , f o u r t h and f i f t h c a t e g o r i e s o f d r o u g h t a r e c o n c e n t r a t e d on i n t h i s r e p o r t . The f i r s t c a t e g o r y was considered, t o some e x t e n t , i n t h e WMO r e p o r t "Drought and A g r i c u l t u r e " , w h i l e t h e s i x t h category, d i f f i c u l t t o assess, i s c l o s e l y t i e d t o t h e o t h e r s .

I t should a l s o be n o t e d i n t h i s c o n t e x t t h a t i f t h e r e i s a s u b s t a n t i a l l a g between n a t u r a l a q u i f e r recharge and t h e l e v e l i n t h e w e l l s , t h e n s h o r t d u r a t i o n d r o u g h t s may be damped o u t and reduced i n importance.

A n e f f e c t i v e s t u d y o f d r o u g h t t y p e 5 i n t r o p i c a l c o u n t r i e s r e q u i r e s t h e l a r g e s t and s m a l l e s t as w e l l as t h e average y e a r l y f l o w s t o be considered. The r e t u r n p e r i o d assessment should c o n s i d e r t h e d e f i c i t s i n i n d i v i d u a l y e a r s and o v e r a l l .

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2. Characteristics of hydrological drought

2 . 1 GENERAL

S i x d i f f e r e n t t y p e s o f h y d r o l o g i c a l drought were d e s c r i b e d i n S e c t i o n 1.2.3. Each one was expressed i n terms o f a d i f f e r e n t v a r i a b l e , e.g. t o t a l volume o f r u n o f f , m i n i m u m d i s c h a r g e a t an i n s t a n t and over a p e r i o d , maximum r i v e r l e v e l and m i n i m u m a q u i f e r l e v e l , but n e v e r t h e l e s s each p r e s e n t s common f e a t u r e s which w i l l be discussed i n t h i s c h a p t e r .

which t e n d t o m a i n t a i n a drought once s t a r t e d

-

t h i s ' p e r s i s t e n c e ' phenomenon i s d i s c u s s e d i n S e c t i o n 2.2. W i t h i n a l l s e r i o u s droughts t h e r e a r e l o c a l i t i e s which a r e p a r t i c u l a r l y h a r d - h i t and o t h e r s which a r e r e l a t i v e l y spared. A l s o one f i n d s d i f f e r e n c e s i n s e v e r i t y i n d i f f e r e n t p o r t i o n s o f t h e drought. Examples o f space and t i m e h e t e r o g e n e i t y a r e d e s c r i b e d i n S e c t i o n 2.3.

Longer t e r m phenomena g i v i n g r i s e t o i n c r e a s e s i n t h e l i k e l i h o o d o f d r o u g h t have b e e n p G S t U h t e d i n r e c e n t years and these a r e discussed i n S e c t i o n 2.4.

I n p a r t i c u l a r t h e r e a r e processes w i t h i n t h e h y d r o l o g i c a l c y c l e a c t i n g over a l l t i m e s c a l e s

2.2 THE PERSISTENCE PHENOMENON

2.2.1 I n t r o d u c t i o n and summary

Having r e s e r v e d t h e t e r m ' d r o u g h t ' f o r events which a f f e c t man's a c t i v i t i e s , i t i s t h e r e f o r e a t r u i s m t o s t a t e t h a t droughts p e r s i s t

-

if t h e y d i d n o t t h e i r impact on o u r a c t i v i t i e s would be minimal. However many have c l a i m e d t h a t t h e r e i s a tendency f o r d r o u g h t c o n d i t i o n s t o p e r s i s t over l o n g e r p e r i o d s than can be e x p l a i n e d b y chance alone. T h i s s e c t i o n d e s c r i b e s sources o f i n t e r a n n u a l p e r s i s t e n c e ( 2 . 2 . 2 ) and weighs t h e evidence f o r and a g a i n s t t h e non-randomness o f drought r e c u r r e n c e ( 2 . 2 . 3 ) . P a r t i c u l a r r e f e r e n c e i s made t o l a r g e s c a l e s t u d i e s o f r e g i o n a l , e s p e c i a l l y Sahel, d a t a t o t r y t o i s o l a t e t h e p a r t i c u l a r v a r i a b l e s which d i s p l a y p e r s i s t e n c e most markedly ( 2 . 2 . 4 ) . W i t h i n - t h e - y e a r p e r s i s t e n c e i s l e s s c o n t e n t i o u s - a l t h o u g h n o t a l l mechanisms a r e w e l l understood (2.2.5)

.

A c o n s i d e r a b l e body o f evidence p o i n t s t o annual r a i n f a l l and r u n o f f b e i n g independent random q u a n t i t i e s . A g a i n s t t h i s t h e r e e x i s t s t h e u n i v e r s a l l y r e c o g n i s e d phenomenon o f l o n g r u n s o f below (and t o a l e s s e r e x t e n t , above) average years such as d u r i n g t h e r e c e n t and c o n t i n u i n g Sahel drought. F o l l o w i n g sub-sections p r e s e n t t h e evidence f o r and a g a i n s t i n t e r a n n u a l

p e r s i s t e n c e and a t t e m p t t o r e c o n c i l e some o f t h e opposing evidence. I t seems l i k e l y t h a t h y d r o l o g i c a l memory i s most marked i n below average p e r i o d s i . e . t h e p r o b a b i l i t y o f ' d r y '

f o l l o w i n g ' d r y ' i s p r o p o r t i o n a t e l y g r e a t e r t h a n o t h e r c o n t i n g e n c i e s . I t a l s o appears t h a t t h e phenomenon i s more v i s i b l e i n l a r g e d a t a assemblages t h a n i n i n d i v i d u a l s t a t i o n r e c o r d s . 2.2.2 Sources o f p e r s i s t e n c e

H y d r o l o g i c a l d r o u g h t c o n s i d e r e d as an aspect o f t h e t o t a l h y d r o l o g i c a l c y c l e i s bound t o e x h i b i t some degree o f p e r s i s t e n c e because o f t h e l a r g e i n e r t i a o f some processes w i t h i n t h e c y c l e . The p r i m e example i s t h e r e l e a s e from a q u i f e r s t o r a g e which c o n t r i b u t e s t o , and i n t h e dry season may t o t a l l y account f o r , r i v e r discharge. The l o n g r e s i d e n c e and response t i m e s o f such sources b u i l d a smoothness i n t o h y d r o l o g i c a l response. H y d r o l o g i s t s w i l l o f course need no r e m i n d i n g t h a t l o c a l e f f e c t s so f a r d i s t u r b t h i s u n d e r l y i n g smoothness as sometimes t o l o s e i t a l t o g e t h e r i n random f l u c t u a t i o n s .

h y d r o l o g i c a l c y c l e , i t i s necessary t o r e a l i s e t h a t analogous f o r c e s o p e r a t e i n t h e atmospheric phase. There i s a c o n s i d e r a b l e m e t e o r o l o g i c a l l i t e r a t u r e d e s c r i b i n g those c l i m a t e anomalies

(departures from n o r m a l ) , o f t e n a s s o c i a t e d w i t h drought, t h e c l a s s i c s among which a r e t h e s o u t h e r l y s h i f t o f t h e i n t e r t r o p i c a l convergence zone which i s a s s o c i a t e d w i t h i n t e r t r o p i c a l

Although a q u i f e r s t o r a g e i s t h e major source o f i n e r t i a i n t h e l a n d phase o f t h e

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drought, and t h e ' b l o c k i n g ' tendency a s s o c i a t e d w i t h temperate zone drought. These c l i m a t e anomalies a r e themselves s u s t a i n e d b y medium and l o n g t e r m departures, some o f which a r e w e l l e s t a b l i s h e d such as u n c h a r a c t e r i s t i c a l l y low o r h i g h sea s u r f a c e temperatures. The sea-surface temperature o f t e n r e f l e c t s t h e h e a t c o n t e n t o f t h e water i n t h e upper l e v e l s o f t h e ocean. The temperature anomaly may a t times p e n e t r a t e t o depths o f a f e w hundred metres t o p r o v i d e l o n g - l a s t i n g " r e s e r v o i r s " o f h e a t - o r c o l d - analogous t o a water supply r e s e r v o i r . T h i s h e a t and i t s g r a d i e n t s , i n turn, can be exchanged w i t h t h e atmosphere t o h e l p govern t h e p o s i t i o n s o f t r o u g h s and r i d g e s i n t h e atmospheric f l o w p a t t e r n s (Namias, 1975).

packs. There a r e o t h e r s , s l i g h t l y more s p e c u l a t i v e , concerning feedback mechanisms (see S e c t i o n 2.2.5.2) and v a r i a t i o n s i n incoming s o l a r energy.

Another w e l l e s t a b l i s h e d c a u s a t i v e f a c t o r concerns t h e p o l a r o r equatorward s h i f t i n i c e

2.2.3 The evidence f o r and a g a i n s t i n t e r a n n u a l p e r s i s t e n c e

Attempts t o q u a n t i f y and t e s t f o r p e r s i s t e n c e f a l l i n t o two main groups. The f i r s t group uses sequences o f annual r a i n f a l l o r r i v e r f l o w d a t a t o compute t h e f i r s t o r d e r s e r i a l c o r r e l a t i o n c o e f f i c i e n t , P1.

p o p u l a t i o n whose t r u e p l v a l u e i s zero, i . e . s t o c h a s t i c a l l y independent. Table 2 . 1 l i s t s some r e s u l t s from t h e main exponents o f t h i s approach.

T h i s i s t e s t e d a g a i n s t t h e n u l l h y p o t h e s i s t h a t t h e sample d e r i v e s from a

Author Data Conclusions_about

average P l Y e v j e v i c h (1964) 1 4 0 r u n o f f r e c o r d s worldwide O. 18

II 446 r u n o f f r e c o r d s i n western USA

o.

2 0

II 1 1 4 1 r a i n f a l l r e c o r d s i n western

USA Not d i f f e r e n t

Hardison (1966) 180 r u n o f f r e c o r d s i n USA 0.17

Brunet-Moret (1975) 179 r a i n f a l l r e c o r d s i n t r o p i c a l

N. A f r i c a Between O and

Sonuga (1977) 1 4 r a i n f a l l r e c o r d s i n N. N i g e r i a Not d i f f e r e n t

t o p=o

O. 1 7 t o p=o Table 2 . 1 Experience w i t h f i r s t o r d e r s e r i a l c o r r e l a t i o n c o e f f i c i e n t s from

annual r a i n f a l l and r u n o f f d a t a

I n e v e r y case t h e c o n c l u s i o n i s b r o a d l y t h a t p e r s i s t e n c e as measured by t h i s i n d e x i s a r e l a t i v e l y minor f e a t u r e .

Although much used t h e e s t i m a t i o n o f P l , e i t h e r b y t h e standard f o r m u l a o f WMO (1966a) p60 o r t h e r o b u s t method o f Brunet-Moret (1975), i s n o t w i t h o u t problems o f computation and

i n t e r p r e t a t i o n . Trend due t o n a t u r a l causes o r d a t a inhomogeneityA such as m i g h t r e s u l t from a change i n s i t e o r o b s e r v a t i o n a l p r a c t i c e w i l l t e n d t o i n f l a t e t h e P l v a l u e a r t i f i c i a l l y ( P o t t e r , 1979).

s m a l l samples. F o r Y e v j e v i c h ' s samples t h e b i a s amounts t o o n l y 0.03; and was e m p i r i c a l l y recognised. B u t , i f t h e p o p u l a t i o n f s c c i s 0.3, t h e e s t i m a t e d f s c c o b t a i n e d from samples o f s i z e 25 would group around an average o f 0.21 ( W a l l i s and O'Connell, 1972), a c o n s i d e r a b l e b i a s .

Brunet-Moret's (1975) study was unusual, f i r s t l y i n a d o p t i n g a d i f f e r e n t method o f The presence o f s e r i a l c o r r e l a t i o n can i n t r o d u c e a downward b i a s i n t h e e s t i m a t e from

e s t i m a t i n g P l as t h e c o e f f i c i e n t , A, o f t h e f i r s t o r d e r Markov x i + l =

AX^ +

^{~ i + l}

where x i , x i + l a r e t h e r a i n f a l l t o t a l s i n years i and i+l and secondly i n i n v e s t i g a t i n g t h e causes o f v a r i a t i o n i n A. A t h a t p e r s i s t e n c e i s l e a s t f o r w e t t e r s i t e s , b e i n g 0.10 t o 0.17 annual average r a i n f a l l f o r which p e r s i s t e n c e was most marked) r a i n f a l l s t a t i o n s .

zi+l i s a random term;

o r a u t o r e g r e s s i v e equation:

t r e n d l i n e was drawn which shows f o r v a l u e s below 5 0 0 mm ( t h e d r o p p i n g t o near zero f o r 1 5 0 0 mm The second school o f p e r s i s t e n c e a n a l y s t s uses t h e s t a t i s t i c s o f runs, i . e . t h e l e n g t h o f successive years o f below ( o r above) average c o n d i t i o n s . Tests a r e d e s c r i b e d i n Brooks and C a r r u t h e r s (1953, p310 e t s e q ) , WMO (196633) and C l a r k e (1973). Such t e s t s a r e p a r t i c u l a r l y a t t r a c t i v e as t h e y conform d i r e c t l y w i t h t h e s u b j e c t i v e i m p r e s s i o n about droughts. The t e s t s above use random sequences t o base t h e n u l l h y p o t h e s i s but S a l d a r r i a g a e t a1.(1970), M i l l a n

(1972a, 1972b), Sen (1976), and G o t t s c h a l k (1977) t a b u l a t e p r o p e r t i e s o f r u n s from Markov

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processes w i t h v a r i o u s l e v e l s o f s e r i a l c o r r e l a t i o n . Table 2 . 2 summarises t h e r e s u l t s o f some run a n a l y s i s , some o f which a l l o w f o r s e r i a l c o r r e l a t i o n .

Author Data Conclusion

M i l l a n (197233) Jenkinson (1973) B u n t i n g e t a l (1976) Sonuga (1977)

Walker e t a l (1977) G o t t s c h a l k (1977) Kraus (1977)

Chervin e t a l (1981)

Colorado r i v e r r u n o f f (USA) Sahel r a i n f a l l b e t w e e n 1 2 ' and

1 4 ON

West A f r i c a n Sahel r a i n f a l l 1 4 N N i g e r i a n r a i n f a l l s t a t i o n s Sahel r a i n f a l l i n 16' t o 1835'N

band

Swedish r u n o f f

S i m i l a r r e g i o n t o Bunting, and I n d i a

Sahel and Soudano-Sahel

Long run found Appeared random Appeared random Appeared random Long r u n s found Long r u n s found Long r u n s found Very l o n g r u n s o f 1 5 y e a r s found - p r e s e n t d r o u g h t s t i l l c o n t i n u i n g t o 1980 Table 2.2 R u n a n a l y s i s r e s u l t s

Sonuga (1977) a l s o employed ' H u r s t h ' o r ' r e s c a l e d r a n g e ' a n a l y s i s which has been

prominent i n r e c e n t h y d r o l o g i c a l l i t e r a t u r e and whose r e l e v a n c e t o d r o u g h t h y d r o l o g y l i e s i n i t s c l o s e a s s o c i a t i o n w i t h r e s e r v o i r d e s i g n c a l c u l a t i o n s . W a l l i s e t a l (1973) p r o v i d e s t a b l e s f o r t e s t i n g t h e ' H u r s t h' s t a t i s t i c a g a i n s t an independent random Normal process. C l a r k e (1973) p r e s e n t s an a l g o r i t h m f o r computing H u r s t h from a d a t a sequence. However, t h e v a l u e s found f e l l w i t h i n t h e bounds t h a t a r e expected f o r an independent s e r i e s , see a l s o P o t t e r (1979) f o r a s i m i l a r finding from 49 U n i t e d S t a t e s r i v e r f l o w s t a t i o n s .

2.2.4 Can t h e c o n f l i c t i n g evidence be r e c o n c i l e d ?

There i s more evidence o f p e r s i s t e n c e i n r u n s a n a l y s i s t h a n i n s e r i a l c o r r e l a t i o n a n a l y s i s . A d e t a i l e d s c r u t i n y o f t h e v a r i o u s s t u d i e s i n t o Sahel r a i n f a l l g i v e s t h e necessary c l u e s t o r e s o l v i n g t h i s paradox.

d i f f e r e n c e from a random sequence. Walker e t a l . ( 1 9 7 7 ) extended t h e a n a l y s i s t o more s t a t i o n s but c o n f i n e d t h e i r s t u d y t o a d r i e r n o r t h e r n group i n t h e 16' t o 1835'N band. A l s o i n s t e a d o f t r e a t i n g each raingauge s e p a r a t e l y t h e y formed an i n d e x o f t h e average o f t h e percentage o f each t o i t s 1945-1974 normal (average p l u v i o s i t y ) . Two r u n s o f e x t r a o r d i n a r y l e n g t h appeared, one wet, from 1 9 5 0 t o 1959, and t h e o t h e r dry, from 1965 t o 1974. The a u t h o r s make t h e i m p o r t a n t p o i n t t h a t t h e t e s t i s n o t c o n c l u s i v e because t h e a n a l y s i s would n o t have been c a r r i e d o u t i f t h e drought had n o t occurred, i n o t h e r words a measure o f p r e - s e l e c t i o n o f t h e d a t a means t h a t t h e y w e r e n o t v i e w i n g a randomly s e l e c t e d segment. N e v e r t h e l e s s , t h e appearance o f t h e d a t a c o n t r a s t s g r e a t l y w i t h t h e e a r l i e r f i n d i n g s o f l i t t l e o r no p e r s i s t e n c e .

i t i s t r u l y t h e r e s u l t o f c o n f i n i n g t h e search t o t h e more n o r t h e r l y d a t a s e t (2.2.4.31, o r whether i t i s du? t o t h e use o f an i n d e x based upon w i d e l y s c a t t e r e d raingauges and so i s l e s s s u b j e c t t o l o c a l and temporary r e l i e v i n g storms. These p o i n t s a r e addressed i n t h e f o l l o w i n g subsections.

2.2.4.1

A n i n i t i a l q u e s t i o n t o be answered i s whether t h e e x t r a o r d i n a r y l o n g r u n s which a d m i t t e d l y c o u l d n o t reasonably a r i s e from a random and independent process c o u l d a f t e r a l l be expected from a process w i t h a moderate v a l u e o f P l . G o t t s c h a l k (1977) has s t u d i e d run l e n g t h s below p a r t i c u l a r t h r e s h o l d s f o r Markov processes. G r a p h i c a l and t a b u l a r m a t e r i a l i s p r e s e n t e d which

suggests t h a t simple p e r s i s t e n c e models would g i v e v e r y l o w p r o b a b i l i t i e s indeed (<< 0.01) f o r r u n s o f more t h a n f i v e years below t h e median w i t h s e r i a l c o r r e l a t i o n s c o n s i d e r a b l y i n excess o f those t h a t a r e observed i n p r a c t i c e . The o t h e r a u t h o r s l i s t e d above Table 2.2 p r e s e n t s i m i l a r conclusions.

Jenkinson (1973) analysed Sahel r a i n f a l l r e c o r d s between 1 2 ' and 14'N and found l i t t l e

The q u e s t i o n s must be asked whether t h i s i s a chance occurrence (2.2.4.1, 2 . 2 . 4 . 2 ) ; whether

Can processes w i t h l o w P1 d i s p l a y l o n g r u n s ?

Studies and reports in hydrology