and of

Hydrology and water resources of small islands: a practical guide

A

to

IHP-111,

4.6

Prepared by

A.

E.

contributions from

A.

L.

Edited by

A.

Unesco

5.

30.

31.

32.

33.

34.

35.

36.

37.

38.

39.

40.

41.

42.

43.

44.

45.

46.

47.

48.

49.

50.

Discharge of selected rivers of the world. (Multilingual English/French/Spanish/Russian.) Volume 111 (Part IV): Mean monthly and extreme discharges (1976-1979). 1985.

Aquifer contamination and protection. 1980. (Also published in French and Arabic.) Methods of computation of the water balance of large lakes and reservoirs. (English only.) Vol. I: Methodology. 1981.

Vol.11: Case studies. 1985.

Application of results from representative and experimental basins. 1982. (English only.) Groundwater in hard rocks. 1985. (Also published in French and Spanish.)

Groundwater models. Vol. I: Concepts, problems and methods of analysis with examples of their application. 1982.

Sedimentation problems in river basins. 1982. (Also published in French.) Methods of computation of low stream flow. 1982. (English only.)

Proceedings of the Leningrad Symposium o n specific aspects of hydrological computations for water projects. 1981. (Russian only.)

Methods of hydrological computations for water projects. 1982. (Also published in French.) Hydrological aspects of drought. 1985. (Also published in French.)

Guidebook to studies of land subsidence due to groundwater withdrawal. 1985. (English only.) Guide to the hydrology of carbonate rocks. 1984. (English only.)

Water and energy: demand and effects. 1985. (English only.) Manual on drainage in urbanized areas. (English only.) Vol. I: Planning and design of drainage systems. 1987.

Vol.11: Data collection and analysis for drainage design. 1987.

The process of water resources project planning: a systems approach. 1987. (Also published in Spanish.)

Groundwater problems in coastal areas. 1987. (English only.)

The role of water in socio-economic development. 1987. (English only.)

Communication strategies for heightening awareness of water. 1987. (English only.)

Casebook of methods for computing hydrological parameters for water projects. 1987. (Also published in French.)

Hydrology and water resources of small islands: a practical guide. 1991. (English only.) Guidelines on the study of seawater intrusion into rivers. 1991. (English only.)

expression of any opinion whatsoever on the part of UNESCO concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries.

Published in 1991 by the United Nations Educational, Scientific and Cultural Organization 7. place de Fontenoy, 75700 Paris

Printed by:

Imprimerie de la Manutention, Mayenne ISBN 92-3-102753-0

0 UNESCO 1991 Printed in France

Preface

Although the total amount of water on Earth is generally assumed to have remained virtually constant during recorded history, periods of flood and drought have challenged the intellect of m a n to have the capacity to control the water resources available to him. Currently, the rapid growth of population, together with the extension of irrigated agriculture and industrial development, are stressing the quantity and quality aspects of the natural system. Because of the increasing problems, m a n has begun to realize that he can no longer follow a 'use and discard' philosophy

-

either with water resources or any other natural resource. A s a result, the need for a consistent policy of rational management of water resources has become evident.

Rational water management, however, should be founded upon a thorough understanding of water availability and movement. Thus, as a contribution to the solution of the world's water problems, , in 1965 Unesco began the first world-wide programme of studies of the hydrological cycle

-

The knowledge of the world's water resources had substantially improved. Hydrology became widely recognised as an independent professional option and facilities for the training of hydrologists had been developed.

Conscious of the need to expand upon the efforts initiated during the International Hydrological Decade and, following the recommendations of M e m b e r States, Unesco, in 1975, launched a n e w long-term intergovernmental programme, the International Hydrological Programme (IHP), to follow the Decade.

Although the IHP is basically a scientific and educational programme, Unesco has been aware from the beginning of a need to direct its activities toward the practical solutions of the world's very real water resource problems. Accordingly, and in line with the recommendations of the 1977 United Nations Water Conference, the objectives of the International Hydrological Programme have been gradually expanded in order to cover not only hydrological processes considered in interrelationship with the environment and human activities, but also the scientific aspects of multi-purpose utilization and conservation of water resources to meet the needs of economic and social development. Thus, while maintaining IHP's scientific concept, the objectives have shifted perceptibly towards a multi-disciplinary approach to the assessment, planning, and rational management of water resources.

As part of Unesco's contribution to the objectives of the IHP, two publication series are issued: 'Studies and Reports in Hydrology' and 'Technical Papers in Hydrology'. In addition to these publications and in order to expedite exchange of information, some works are issued in the form of Technical Documents.

The purpose of the continuing series 'Studies and Reports in Hydrology' to which this volume belongs, is to present data collected and the main results of hydrological studies, as well as to provide information on hydrological research techniques. T h e proceedings of symposia are also sometimes included. It is hoped that these volumes will furnish material of both practical and theoretical interest to water resource scientists and also to those involved in water resources assessments and the planning for rational water resources management.

Contents

Preface Introduction

i

vi

-

-

-

- -

Origin and implementation of the Project Objectives and outline of the Guide

List of authors Acknowledge m e n ts

List of abbreviations and symbols List

units

1. CHARACTERISTICS OF SMALL ISLANDS

~ ~~~

1.1

1.2 Inventories of small islands 13

1.4 Problems of small islands

CONDITIONS FOR FRESHWATER OCCURRENCE IN SMALL ISLANDS 2.1 Climate

Definitions and distinction between small and very small islands Types of water occurrence on small islands

2.

2.1.1 Island-climate relationships 2.1.2 Climatic zones

2.1.3 Global and regional climate 2.1.4 Local climate

2.1.5 Rainfall and evaporation for selected islands 2.2 Geology and geomorphology

2.2.1 2.2.2

C o m m o n geological situations in small islands Conditions that affect water resources

23 Soil and vegetation cover 2.3.1 Soil and water resources

2.3.2 Vegetation cover and water resources 3. HYDROLOGY OF SMALL ISLANDS

3.1 Hydrometeorology 3.1.1 Precipitation

3.1.2 Temperature and humidity 3.1.3 Solar radiation

3.1.4 Wind

3.2 Hydrological processes 3.2.1 Precipitation 3.2.2 Surface retention 3.2.3 Infiltration

3.2.4 Soil water redistribution 3.2.5 Evaporation and transpiration 3.2.6 Surface runoff

3.2.7 Groundwater recharge and discharge

vi vi vii viii ix

X

xi xii xiii 1

10 10 10 10 13 19 21 24 24 41 42 42 42 44 44 44 44 45 45 45 45 46 48 49 50 50 51

3.3 Water balance 3.3.1 Fundamentals 3.3.2

3.3.3

3.3.4 Application to small islands Estimation of water balance ternis

Reference zones and time intervals Uses and types of water balance 3.4

3.4.1 3.4.2 3.4.3 3.4.4 3.4.5 3.4.6 3.4.7 3.4.8 3.4.9 3.4.10 3.4.11 3.4.12 3.4.13 3.4.14

Precipitation Evapotranspiration Surface runoff

Other water flow terms

Groundwater recharge from precipitation Groundwater inputs from other sources Groundwater output terms

Storage terms

Water balance calculations Specific methods for small islands Sample water balance results

Checking groundwater recharge calculations Major influences on island water balance Rainwater catchments

3.5 Groundwater hydrology 3.5.1

3.5.2 3.5.3 3.5.4 3.5.5 3.5.6 3.5.7 3.5.8 3.5.9 3.5.10

Basics of freshwater-saltwater relationships Groundwater occurrence

Groundwater and geology Groundwater flow conditions Hydrogeological parameters Vertical salinity distributions

Calculation of freshwater-saltwater relationships Groundwater chemistry and groundwater flow Environmental isotope hydrology

Groundwater quality

4. WATER RESOURCES ASSESSMENT AND MONITORING IN SMALL ISLANDS 4.1 General principles

4.2 Hydrometeorology and surface water resources 4.2.1 Network design

4.2.2 4.2.3 4.2.4 4.2.5 4.2.6

Observation, measurement and recording of data Collection and transmission of data

Processing, archiving and presentation of data Methods of hydrological data analysis Data availability and further information 4.3 Groundwater resources

4.3.1 4.3.2 4.3;3 4.3.4 4.3.5 4.3.6 4.3.7 4.3.8 4.3.9 4.3.10

General remarks

Preliminary data gathering and interpretation Geological and geophysical surveys

Remote sensing Exploratory drilling Well logging

Observation networks and sampling procedures Field equipment for water quantity and quality Simplified methods for surveying

Hydrogeological parameter evaluation

51 51 52 52 53 54 54 54 60 63 64 73 73 76 76 78 79 81 81 82 88 88 95 96 99 107 107 108 114 122 130 132 132 133 133 136 141 141 142 144 145 145 145 146 150 153 154 156 161 163 163

4.5

4.4.1 General considerations

4.4.2 Effects of salt water penetration 4.4.3 Saltwater upconing

4.4.4 Calculation of interface changes 4.4.5 Exploitable groundwater resources 4.4.6 Groundwater modelling

4.4.7 Groundwater quality issues

4.4.8 Groundwater pollution and contamination 4.4.9

Other sources 4.5.1 Desalination 4.5.2 Importation 4.5.3 Wastewater reuse 4.5.4 Weather modification

Ecological and environmental aspects of groundwater

5. WATER RESOURCES DEVELOPMENT A N D MANAGEMENT IN SMALL ISLANDS 5.1 Water uses

5.1.1 Water supply 5.1.2 Agriculture 5.1.3 Industry 5.1.4 Hydro-power 5.1.5 Other uses

5.1.6 Comments and comparisons 5.2 Water resources development

5.2.1 5.2.2 5.2.3 5.2.4 5.2.5 5.2.6 5.2.7 5.2.8 5.2.9 5.2.10 5.2.11 5.2.12 5.2.13 5.2.14 5.2.15 5.2.16 5.2.17 5.2.18 5.2.19

Overview

Planning and design Rainwater

Surface water Groundwater Desalination Importation Wastewater reuse

0 t her non- conv enti onal methods Non-potable water systems Water storage

Water treatment Water distribution Energy sources P u m p s

Project implementation Water for agriculture Water for hydro-power Cost comparisons 5.3 Water resources management

5.3.1 5.3.2 5.3.3 5.3.4 5.3.5 5.3.6 5.3.7 5.3.8 5.3.9 5.3.10 5.3.11

Overview Major issues

National water policy and plans Water legislation

Institutional arrangements

Detailed planning and management aspects Data collection and interpretation

Appropriate techniques and technology Adaptation of guidelines

Operation and maintenance

D e m a n d management and pricing policy

165 166 168 171 173 174 184 186 190 192 192 199 200 200 20 1 201 201 204 204 204 205 205 208 208 209 209 213 217 226 232 234 236 238 239 240 243 246 247 247 247 249 249 252 252 253 254 254 257 259 261 262 263 264 267

5.3.12 Water quality surveillance for public health 5.3.13 Community involvement and education 5.3.14 Counter-disaster planning and management 5.3.15 Long term climatic change

6. TRAINING A N D RESEARCH 6.1 Training

6.2 Research requirements 6.2.1 Surface water hydrology 6.2.2 Groundwater hydrology 6.2.3 Other aspects

6.3 Technical cooperation 7. REFERENCES

7.1 General studies and reports 7.2

73 Proceedings of international events 7.4 General references

Regional studies, reports and bibliographies

8. CASE STUDIES

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

Anguilla

Antigua and Barbuda Bermuda

Canary Islands Cayman Islands Hawaiian Islands Majuro atoll Mauritius Montserrat Scottish Islands Seychelles Silba

Zhoushan Islands APPENDICES

Appendix 1 A p p e n d i x 2

Characteristics of selected small islands

Outline of a possible syllabus for a course on small island groundwater hydrology for graduate students

269 270 271 272 273 273 274 274 275 275 276 277 277 277 278 279 318 320 328 333 339 356 362 368 375 380 383 389 394 401 406 406 434

Origin and implementation of the Project

At its Sixth Session, the Intergovernmental Council of the International Hydrological Programme

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The plan of IHP-111 identifies eighteen themes which have been grouped under four main sections. Section 1 deals with hydrological processes and parameters for water projects and includes T h e m e 4

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At its Twelfth Session (22-25 October 1984), the IHP Bureau appointed the rapporteurs for IHP-111 Project 4.6 and at its Thirteenth Session (2-4 December 1985) confirmed Mr. A . Diaz Arenas as Principal Rapporteur and Mr. J. Febrillet Huertas as CO-rapporteur. The International Association of Hydrogeologists (IAH), the Water Research Branch of the Division of Natural Resources and Energy, Department of Technical Co-operation for Development of the United Nations (UNDTCD), the Council of Europe and several IHP National Committees expressed their interest in co-operating with this project. Messrs. E. Custodio (IAH), R. Dijon (UNDTCD), L.

Simler (Council of Europe) and A. Falkland, T. Chapman and T. Daniel1 (Australia) were nominated as Correspondents.

Mr. Nelson da Franca. Programme Specialist of the Division of Water Sciences of Unesco w a s responsible within the IHP Secretariat for the Project.

During 24-28 February 1986, the rapporteurs met at Unesco Headquarters in Paris to prepare the report:

'Hydrology and Water Balance of Small Islands: A Review of Existing Knowledge', using the information received from several IHP national committees and international organisations interested in the hydrology of small islands.

This report, prepared by Messrs. A. Diaz Arenas and J. Febrillet Huertas w a s published in September 1986 by Unesco in the series Technical Documents in Hydrology.

During its Seventh Session (Paris, 18-23 June 1986), the Intergovernmental Council approved a second phase for the project: preparation by a group of rapporteurs, appointed by the IHP Bureau of a guide on hydrology and water resources of small islands.

At its Fourteenth Session (21 June 1986). the IHP Bureau appointed the rapporteurs to prepare the Guide.

Mr. A. Diaz Arenas (Cuba) w a s appointed as Principal Rapporteur and Messrs. A. Falkland (Australia), E. Custodio (IAH) and L. Simler (Council of Europe) as CO-rapporteurs. The Principal Rapporteur met the IHP Secretariat, at Unesco Headquarters on 23 June 1986 to prepare the preliminary draft table of contents of the Guide, a preliminary distribution of chapters or sub-chapters to be written by the rapporteurs and a preliminary work schedule.

A meeting of the rapporteurs with IHP Secretariat w a s held at Unesco Headquarters in Paris from 22 to 26 November 1987, to analyse the contributions received, to revise the first draft of the Guide, to identify parts that needed editing or revision and to decide on the form and time schedule of the publication. During the meeting Mr.

A. Falkland w a s designated as editor of the Guide.

Relations with other projects

Unesco has promoted and supported three other projects dealing specifically with the hydrology of small islands: a regional project in Southeast Asia and the Pacific coordinated by R O S T S E A , the Hydrogeological Atlas of the Caribbean Islands coordinated by ROSTLAC, and a M A B project.

The Division of Water Sciences, through the Regional Office of Science and Technology for Southeast Asia (ROSTSEA) and in response to the needs of Southeast Asia and Pacific countries, launched in 1985 a programme involving research studies on water resources problems of small islands (water balance of small islands project).

Indonesia and the Philippines, two countries consisting of more than 20,000 islands, were selected for the programme. In Indonesia, the study w a s conducted by the Centre of Environmental Studies of the Hasanuddin University in Ujung Pandang, Sulawesi. The selected islands, Barang Caddi (34 ha) and Barang L o m p o (42 ha), are flat islands of coral origin, with populations of approximately four and one thousand inhabitants, respectively. In the Philippines, the study started in Marinduque Island, which is representative of high islands of volcanic origin. Its area is 960 square kilometres with approximately 180,000 inhabitants. The research w a s carried out by the National Hydraulic Research Centre of the University of Philippines.

The Division of Water Sciences, through the Regional office for Science and Technology for Latin America and the Caribbean (ROSTLAC) and in response to the needs of the Caribbean countries, launched in 1986 a project to prepare hydrogeological maps of 100 islands of the region, in order to compile a Hydrogeological Atlas of the Caribbean Islands.

The Division of Ecological Sciences is responsible for Project 7, Rational Management of Island Ecosystems, within the framework of the M a n and Biosphere Programme (MAB). T o date, the project has realized a multi-disciplinary survey on a group of Mediterranean small islands: Skiathos, Skopelos, Chalki (Greece); Gozo (Malta); Kerkennah (Tunisia); Stromboli, Basiluzzo, Panarea, Vulcano, Lipari, Salina, Filicudi and Alicudi (Italy).

Besides these projects, there are two projects in the framework of the IHP-111, which are related to small islands: Project 4.9

-

-

This IHP-111 Project has contacted several other projects, namely, the United Nations Department of Technical Cooperation for Development (UNDTCD) on small island water resources assessment, development and management projects in the Caribbean and Pacific region, the European Network of Scientific and Technical Cooperation on the Mediterranean Islands of the Council of Europe, and the project of the United Nations Environment Programme (UNEP) on the preparation of a Guidebook for Water Resources Management in Small Mediterranean Islands and Isolated Coastal Areas.

Relevant international scientific events

Since the mid-1960's the theme 'hydrology of small islands' has become a subject of international interest.

A number of scientific meetings have been organised for the identification of problems and the exchange of experience under this theme. Major international events specific to the hydrology of small islands in the last 16 years are:

Seminar on Water Management in Small Island States, organised by the Cyprus Joint Technical Council and the Commonwealth Engineers Council. Nicosia, Cyprus, October 1989.

Interregional Seminar on Water Resources Management Techniques for Small Islands, sponsored by the United Nations Department of Technical Co-operation for Development in co-operation with the Commonwealth Science Council and the United Nations Development Programme. Suva, Fiji, June 1989.

Second Workshop on the H~dro~eological Atlas of the Caribbean Islands, organised by Unesco/Regional Office for Science and Technology for Latin America and the Caribbean. Caracas, Venezuela, 6-9 September 1988.

South-east Asia and the Pacific Regional Workshop on Hydrology and Water Balance of Small Islands, organized by the Chinese National Committee for the International Hydrological Programme and supported by Unesco. Nanjing, China, March 1988.

First Workshop on the Hydrowological Atlas of the Caribbean Islands, organised by Unesco/Regional Office for Science and Technology for Latin America and the Caribbean. Santo Domingo, Dominican Republic, 7-10 October 1986.

Seminar on Water and Sanitation in Small Mediterranean Islands and Isolated Coastal Areas, organised by the United Nations Environment Programme and the World Health Organisation. Gobierno de la Comunidad Autonoma de Ias Islas Baleares, Palma de Mallorca, Spain, September 1986.

Interregional Seminar on Development and Management of Island Groundwater Resources, sponsored by the United Nations. Hamilton, Bermuda, December 1985.

European Workshop on Hydrological Management in Mediterranean Islands, organised by the Council of Europe in co-operation with the Ministry of Agriculture and Natural Resources of Cyprus. Nicosia, Cyprus, October 1984.

Workshop on Water Resources of Small Islands, sponsored by the Commonwealth Science Council and a number of other organisations including Unesco. Suva, Fiji, July 1984.

Meeting on Water Resources Development in the South Pacific, organized by ESCAP. Suva, Fiji, March 1983.

Seminar on Water Resources Assessment, Development and Management in Small Oceanic Islands of the Caribbean and West Atlantic, supported by the United Nations Department of Technical Co-operation for Development, the Commonwealth Science Council and the Government of Barbados. Bridgetown, Barbados, October 1980.

Seminar on Selected Water Problems in Islands and Coastal Areas, with special regard to desalination and groundwater,.organized by the United Nations Economic Commission for Europe. San Anton, Malta, June 1978.

Seminar on Hydrolorn of Volcanic Rocks, organised by Unesco and the General Directorate of Public Works of Spain. Arrecife de Lanzarote, Canary Islands, Spain, 1974.

follows:

Eleventh Salt-Water Intrusion Meeting, organized by the Polytechnical University of Gdansk, Sopot, Gdansk, Small States Conference on Sea Level Rise, MalB, Republic of Maldives, November 1989.

Fourth International Conference on Rain Water Cistern Systems, sponsored by the International Development Research Centre, Canadian International Development Agency and Phillipine Water Works Association.

Manila, Phillipines, August 1989.

International Colloquium on the Development of Hydrological and Water Management Strategies in the H u m i d Tropics, organised by UNESCO with cooperation from other organisations. Townsville, Australia, July 1989.

Second Workshop on the Hydrogeological Atlas of the Caribbean Islands, organized by Unesco and the Ministerio del Ambiente y de 10s Recursos Naturales Renovables (MARNR). Caracas, Venezuela, September 1988.

Sixth International Coral Reef Symposium. Townsville, Australia, August 1988.

Tenth Salt-Water Intrusion Meeting, organized by the National Science Foundation and the Ministry of the Flemish Community. Ghent, Belgium, M a y 1988.

Simposio Internacional sobre Recursos Hidriulicos. Canarias Ama-2000, organized by Consejeria de Obras Publicas, Gobierno de Canarias. Santa Cruz de Tenerife, Canary Islands, Spain, March 1987.

Third International Conference on Rain Water Cistern Systems, sponsored by the International Development Research Centre. Khon Kaen, Thailand, January 1987.

First Workshop on the Hydrogeo1og;ical Atlas of the Caribbean Islands, organized by Unesco and the Instituto Nacional de Recursos Hidriulicos (INDRHI). Santo Domingo, Dominican Republic, October 1986.

Third Caribbean Islands Water Resources Conmess, sponsored by the U.S. Geological Survey and other organisations. St. Thomas, U.S. Virgin Islands, July 1986.

Ninth Sal t-Water Intrusion Meeting, organized by the Delft University of Technology. Delft, The Netherlands, M a y 1986.

Fifth International Symposium on Groundwater, organized by the European Mediterranean Commission for Water Planning and the International Association of Hydrogeologists. Taormina, Sicily, November 1985.

International Seminar on Non-Conventional Water Use in Developing Countries, sponsored by the United Nations.

Willemstad, Curapo, Netherlands Antilles, April, 1985.

Second International Conference on Rain Water Cistern Systems, sponsored by the Caribbean Research Institute and the Virgin Islands Branch of the American Society of Civil Engineers. St. Thomas, U.S. Virgin Islands, June 1984.

International Conference on Groundwater and M a n , organized by the Australian Government Department of Resources and Energy. Sydney, Australia, December 1983.

Eighth Salt-Water Intrusion Meeting, organized by the University of Bari, Italy, May 1983.

International Conference on Rain Water Cistern Systems, sponsored by the U.S Department of Interior and the Fourth International Conference of the European Mediterranean Commission for Water Planning. Marseille, France,

Poland, M a y 1990.

University of Hawaii. Hawaii, U.S.A., June 1982.

M a y 1982.

Objectives and outline of the Guide

This Guide has been prepared to assist technicians, hydrologists, engineers and managers in the identification, assessment, development, management and protection of water resources on small islands. It is not intended as a textbook or manual of practice, but rather as a guide to the selection of methods and practices, appropriate to the special conditions of small islands. Efforts were made to provide as many useful and accessible references as were available, so that readers can further research items of particular interest to them.

The Guide is presented in eight chapters:

Chapter 1 presents definitions and distinctions between small and very small islands, some details and Chapter 2 deals with the conditions for water occurrence in small islands including climate, geology, problems of small islands and the types of freshwater occurrence on small islands.

geomorphology, soils and vegetation.

Chapter 3 describes the hydrology of small islands including hydrometeorology, hydrological processes, water balance studies. Because of its importance to the majority of small islands, an additional section is provided on groundwater hydrology particularly related to freshwater-saltwater relationships.

Chapter 4 presents details of water resources assessment and measurement in small islands. The assessment and measurement of hydrometeorological parameters, surface water resources and groundwater resources are considered. Assessment of alternative approaches for developing or enhancing freshwater resources such as desalination, importation, wastewater reuse and weather modification are also considered.

Chapter 5 outlines the uses of water on small islands and presents details of, and guidelines for, water resources development and management in small islands.

Chapter 6 considers training, education, research and technical cooperation aspects of small island water resources.

Chapter 7 contains reference material and Chapter 8 presents thirteen case studies of small islands from various parts of the world.

Contributors to the Guide

This Guide has been compiled by a group of rapporteurs and specialists in water resources. T h e group was The authors of and contributors to the chapters were:

assisted by Mr Nelson da Franca of the Division of Water Sciences, Unesco.

Introduction:

Chapter 1:

Chapter 2:

Chapter 3:

Chapter 4:

Chapter 5: A. Falkland Chapter 6:

N. da Franca, A . Falkland A. Diaz Arenas, A. Falkland

E. Custodio (2.2: 2.3), A. Diaz Arenas (2.1), A. Falkland (2.1) E. Custodio (3.3, 3.4, 3.9, A. Diu Arenas (3.1,3.2),

A. Falkland (3.1,3.2, 3.4)

E. Custodio (4.3, 4.4), A. Falkland (4.1,4.2, 4.3,4.5), L.Simler (4.5)

E. Custodio, A. Falkland The case studies in Chapter 8 were prepared by:

Anguilla:

Antigua

&

Bermuda:

Canary Islands:

C a y m a n Islands:

Hawaiian Islands:

Majuro:

Mauritius:

Mont serrat : Scottish Islands:

Seychelles:

Silba:

Zhoushan Islands:

E.P. Wright and J.A. Barker

P. Barragne-Bigot and V. Yearwood.

M.P. R o w e

E. Custodio, J. JimCnez, J. Nuiiez, L. Puga and J. Braojos K.S. N g and R.G.B. Beswick

F.L. Peterson S.S. Anthony H. Ramrekha J. Koirala N.S. Robins R. Cetoupe

0. Bonacci and J. Margeta Z. Qiansheng and M. Zhixin

List of authors

Anthony, Stephen S.; Hydrologist, United States Geological Survey, Water Resources Division, Honolulu, Hawaii, Barker, John A.; Hydrogeologist, Hydrogeology Group, British Geological Survey, Wallingford, U.K.

Barragne-Bigot, Phillippe; Hydrogeologist, United Nations Department of Technical Cooperation for Development, Beswick, Richard G.B.; Director, T h e Water Authority, Grand Cayman, Cayman Islands, British West Indies.

Bonacci, Ognjen; Professor for Hydrology, Civil Engineering Institute, Faculty of Civil Engineering Sciences, Split, Braojos, Juan Jost; Tech. Civil Engineer, Water Resources Service, Tenerife, Canary Islands, Spain.

Cetoupe, Ronald; Engineer, Water and Sewerage Division, Public Utilities Corporation, Victoria, Mahe, Seychelles.

Custodio, Emilio; Dr. Ind. Eng., C I H S D I T , Polytechnic University of Catalonia, Barcelona, Spain.

Diaz Arenas, AndrCs; Professor Ing., Instituto de Hidroeconomia, La Habana, Cuba.

Falkland, Tony; Civil Engineer/Hydrologist, ACX Electricity and Water, Canberra, Australia.

Jimtnez, Jost; Civil Engineer, Water Resources Service, Las Palmas, Canary Islands, Spain.

Koirala, Jaya; United Nations associate expert Engineer, Montserrat Water Authority, Plymouth, Montserrat.

Ma Zhixin; Senior Hydrological Engineer, General Hydrological Station of Zhejiang Province, Hangzhou, China.

Margeta, Jure; Water Resources and Environmental Engineering, Civil Engineering Institute, Faculty of Civil Ng, Kwok-Choi Samuel; Hydrogeologist, The Water Authority, Grand Cayman, C a y m a n Islands, British West Nuiiez, Jost Antonio; Geologist, Water Resources Service, Las Palmas, Gran Canaria, Canary Islands, Spain.

Peterson, Frank L.; Professor of Hydrogeology, Department of Geology and Geophysics, University of Hawaii, Puga, Luis; Economist, Water Resources Service, Tenerife, Canary Islands, Spain.

Ramrekha, H.; Hydrologist, Hydrology Section, Central Water Authority, Rose Hill, Mauritius.

Robins N.S., Hydrogeologist, Hydrogeology Group, British Geological Survey, Wallingford, U . K .

Rowe, Mark P.; Hydrogeologist, Public Works Operations and Engineering Division, Ministry of Works and Simler, Louis; Professor, Ing., Council of Europe, Strasbourg, France.

Wright, Edward P.; (formerly) Head of Overseas Hydrogeology, British Geological Survey, Wallingford, U.K.

Yeanvood, Veronica; Hydrologist, Antigua Public Utilities Authority, St. John's, Antigua.

Zhou Qiansheng; Senior Hydrological Engineer, General Hydrological Station of Zhejiang Province, Hangzhou, U.S.A.

Regional Small Islands Water Project, Bridgetown, Barbados.

Yugoslavia.

Engineering Sciences, Split, Yugoslavia.

Indies.

Honolulu, Hawaii, U.S.A.

Housing, Hamilton, Bermuda.

China.

Acknowledgements

The editor sincerely thanks the other authors for their valuable contributions to this Guide. In particular, the dedication and extensive contributions from Professor Emilio Custodio are gratefully acknowledged.

The following organisations are thanked for nominating authors for the Guide: the International Association of Hydrogeologists (E. Custodio) and the Council of Europe (L. Simler).

Professor Custodio's services were made possible by the International Course on Groundwater Hydrology, Barcelona, mainly through the Polytechnic University of Catalonia (College of Civil Engineering, Department of Ground Engineering) and the Water Board of Catalonia. The International Course enabled him to devote time to this project and also provided typing, draughting and administrative support for his contributions. Special mention is made of M s . Raquel Burgos w h o undertook the typing and Javier Custodio w h o drew the figures for his contributions.

ACT Electricity and Water, Canberra, Australia is thanked for enabling the editor to devote time to complete this Guide and for its provision of additional assistance from the Hydrology and Water Resources Section for the preparation of drawings and text. T h e former Australian Government Department of Housing and Construction and Department of Territories are also thanked for their support and encouragement in the early stages of this project.

Helpful comments on the draft for section 5 were provided by Dr. Peter H a d w e n (UNDTCD Chief Technical Advisor and Project Manager of the United Nations Water Project (RAS/87/009 based in Fiji) and these are gratefully acknowledged

Valuable advice and assistance with a number of the sections of the Guide were provided by Mick Fleming, Principal Research Scientist, Division of Water Resources, Commonwealth Scientific and Industrial Research (Australia), Tony W e b b , Lecturer, School of Civil Engineering, Australian Defence Force Academy, and Dr.

Fereudiyn Ghassemi of the Centre for Resource and Environmental Studies, Australian National University. Their efforts are also gratefully acknowledged.

The staff of the Hydrology and Water Resources Section of A m Electricity and Water are particularly thanked for their help with the preparation of drawings and text for the Guide. In particular, the dedicated assistance from Helen Jobson w h o spent many hours formatting the final document and Kathleen Hunt w h o prepared and corrected many of the drawings is most gratefully appreciated. Other members of the section w h o assisted in many ways were Barbara Pink, Stuart Rosborough, Jan Rogers, Ross Knee, Christopher Zoppou and N o r m Mueller; their contributions are also gratefully acknowledged. M a n y thanks are also due to the librarians Jane Taylor and Kay Richardson for their efficient and friendly service with reference material, and to Tracey Booker w h o assisted with the typing.

I also thank m y o w n family for support and encouragement during the preparation of this Guide.

Tony Falkland Editor

Canberra, Australia December 1990

List of abbreviations and symbols

AC AWRC BGH B.P.

ED ENS0 E, ESCAP ET, ET, ET, f FAO FC FSM IAHS IHP IRC ITCZ L M ME MSF P PE PVC R RO S SR T T D S U.K.

U.S.A.

U.S.S.R.

UNDP UNDTCD Unesco USGS

vc

WASH W H O W M O W P

c,

1

asbestos cement

Australian Water Resources Council

Badon Ghijben (Ghyben) Herzberg (often written as 'Badon Ghyben Herzberg' or simply 'Ghyben Herzberg')

before present

coefficient of variation electrodialysis

El Niiio Southern Oscillation potential evaporation

Economic and Social Commission for Asia and the Pacific actual evapotranspiration

potential transmission rate potential evaporation infiltration capacity

Food and Agriculture Organisation field capacity

Federated States of Micronesia rainfall intensity

International Association of Hydrological Sciences International Hydrological Programme (of Unesco)

International Reference Centre for Community Water Supply and Sanitation.

Inter-Tropical Convergence Zone length

mass

multi-effect distillation multi-stage flash distillation precipitation

polyethylene polyvinyl chloride recharge (to groundwater) reverse osmosis

soil moisture content surface runoff

time (also, temperature) total dissolved salts United Kingdom

United States of America

United Soviet Socialist Republics United Nations Development Programme

United Nations Department of Technical Cooperation for Development United Nations Educational, Scientific and Cultural Organisation United States Geological Survey

vapour compression

Water and Sanitation for Health Project World Health Organisation

World Meteorological Organisation wilting point

List of units

Bq.1-1 d-I g.cm-3 g.d-' ha kg.n~-~

km kW kWh.m-3 1.c-l.d-l 1.m-2

1s-1

1.s-'.km-2 m m-1 m.d-l m.y-1 m3.d-l m3.h-l m3.s-1 m3.s-1 .krn-2 m4.d-l mg.1-1 MJ.m-3 m m mm.d-l m m . month- mm.y-1 M W

ng.1-1

S-1

Tu Y-' pg.1-1 pS.cm-1

bequerels per litre (decays per second per litre) per day

grams per cubic centimetre grams per day

hectare

kilograms per cubic metre kilometre(s)

kilowatt

kilowatt hour per cubic metre litres per capita per day litres per square metre litres per second

litres per second per square kilometre metre(s)

per metre metres per day metres per year cubic metres per day cubic metres per hour cubic metres per second

cubic metres per second per square kilometre equivalent hydraulic energy (rn3.d-' .m) milligrams per litre

megajoules per cubic metre millimetre(s)

millimetres per day millimetres per month millimetres per year megawatt

nanograms per litre per second

tritium units per year

micrograms per litre

microsiemens (micromhos) per centimetre

1. Characteristics of small islands

1.1

small

small

Small islands have particular hydrological and water resource assessment, development and management problems which distinguish them from medium and large islands.

Although many distinguishing features of small islands can be identified, the need to identify a limiting area for a small island became obvious at the beginning of Project 4.6 of IHP-111. Hydrologists from countries at different latitudes and with a range of water resources problems and skills agreed that the hydrology of small islands was dictated by specific hydrological features. Although many limiting areas for small islands were proposed, it was not possible to reach a consensus. After discussions with many specialists, inter-governmental agencies and international scientists' associations with experience in the hydrology of islands, it was decided that the term 'small island' should apply to islands with areas less than approximately 1,000 km2 and to larger, elongated, islands where the maximum width of the island does not exceed 10 km (Diu Arenas and Febrillet Huertas, 1986).

Subsequently, at a meeting of the rapporteurs for this Guide in November 1987, it was decided to expand the maximum area to 2,000 km2. This change was made to include a number of islands with areas between 1,000 and 2,000 km2, which were considered to be similar in features and problems to those already incorporated into the small island category.

Thus, for the purposes of this Guide, the definition of a 'small island' is one where the area is not greater than 2,000 h2 or where the width is not greater than 10 km.

The limiting area for a small island chosen for a workshop on water resources of small islands, mainly in the Pacific Ocean (CSC, 1984), was 5,000 km2. This area was chosen to preclude islands with actual or potential surface water storage schemes of significant size.

In a recent Unesco publication, Wright (1989a) states that no precise delimiting area can be made but that an appropriate upper limit for a small island is in the range 5,000-10,000 km2.

The above discussion emphasises the need for some degree of flexibility in applying the definition of smallness to islands. There is obviously a range of areas, and other characteristics, which together make an island small in the sense that methods, techniques and approaches to hydrology and water resources issues cannot be directly applied from continental situations. In special cases, islands larger than 2,000 km2 could be considered small from a hydrological viewpoint.

The use of area as the only criterion for defining a small island differs from an approach taken by some specialists, w h o emphasise demographic aspects. However, from a hydrological perspective and for the purposes of this Guide, it is considered that area is the major determining factor in classifying islands as small or not.

Very small islands

A further distinction is made between small and very small islands. Very small islands have very limited options for the development of freshwater resources as a consequence of their size. These islands include the small sand cays of the Caribbean Sea and the coral atolls of the Pacific and Indian Oceans, where surface water does not exist in an exploitable form and fresh groundwater resources are limited. On these islands, conventional options for freshwater supplies are limited to groundwater development and rainwater harvesting.

Further inclusions in the very small island category are some of the small volcanic and raised coral islands where groundwater is very limited, and geological conditions are not favourable to surface water storages. On these islands, the only conventional freshwater option is rainwater harvesting which can only satisfy relatively small water requirements. In extreme cases, non-conventional freshwater options such as desalination and importation may be the only secure way of meeting freshwater demand.

Although it is not intended to apply a rigid definition, a very small island would generally have an area of not greater than 100 km2 or a width not greater than 3 km (Dijon, 1984). These physical limits generally mean that very limited surface or groundwater resources willbe present. In very small islands, approaches to the assessment, development and management of water resources is normally required on an island specific basis, whereas there may be some scope for a slightly more generalised approach with groups or archipelagos of larger-size small islands. In all cases, approaches to hydrology and water resources developed for continental conditions cannot be applied without first assessing the special hydrological features present in such islands.

In subsequent sections of this Guide, the term 'small island' is used for both small and very small islands and the term 'very small island' is used to distinguish this particular category of small islands from the others.

1.2

of

There are many small and very small islands in the world and they are found in all oceans, most of the seas and in m a n y large lakes and rivers (see Figure 1.1). It would be an almost impossible task to provide a comprehensive list of small islands. Because of the magnitude of such a task and its questionable value, it has never been accomplished. It w a s not attempted as part of this project either, as it is not greatly important to the study of the hydrology and water resources of small islands.

National bodies, however, have generated lists of their o w n islands. For example, the U.S. Geological Survey have compiled a list of over 18,000 islands which c o m e under the jurisdiction of the U.S.A. (Matalas, 1987).

It is noted by Matalas that the total number of these islands varies according to the rise and fall of sea level.

On a global scale, lists of islands, including about 200 small islands, are provided by Showers (1979; 1989) with s o m e details of location, physiography and population. A s the number of small islands in the Pacific Ocean alone is more than 30,000 (Dijon, 1984), the representation of small islands in this list is very low, highlighting the difficulty faced in compiling a comprehensive list of small islands.

Other publications provide data about selected small islands. For instance, Bryan (1953) provides a list of atolls in the world. About 400 atolls are listed by Bryan, the majority being in the Pacific Ocean (about 285) with 15 in Indonesia, 73 in the Indian Ocean, 26 in the Caribbean Sea and one in the Atlantic ocean. Douglas and Douglas (1989) provide details of m a n y of the small and large islands of the Pacific.

As part of this Project, a number of national organisations were contacted to supply basic geographic, topographic, demographic and climatic information about small islands in their countries. The information supplied plus additional data from other references is listed in the table in Appendix 1. The table is provided to give some idea of the diversity of small islands in the world. A total of 375 islands covering many oceans and seas are included.

M a p s of part of the Pacific Ocean and all of the Caribbean Sea are shown in Figures 1.2 and 1.3, respectively. These m a p s were selected to show the location of some of the largest groups of small populated islands in the world. A s previously mentioned, there are over 30,000 islands in the Pacific Ocean. The number of populated islands is unknown but is in the order of 1,000. In the Caribbean Sea, there are several thousand small islands, of which about 100 are inhabited.

Other large numbers of small islands occur in:

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-

-

-

-

There are m a n y small islands in lakes, rivers and inland seas. Examples of very small islands in rivers where human activity is significant, are H o g g (57 h2), Wakenaam (45 km2, Leguan (40 km2) and Fort (5 km2) islands in the Essequibo River, Guyana.

The case studies and m a n y of the references provided in section 7 of this Guide contain further information on locations, physiography and other details of small islands.

-

1.3

of

Types of water

Water on small islands occurs as either fresh or saline water. W h e n the salt content is low, water can be considered as fresh. Freshwater if unpolluted from biological and chemical substances can be used for drinking and other potable purposes. Water with higher salinity, ranging from slightly brackish to seawater is also often found on small islands. Occasionally, hyper-saline water, with salinity greater than seawater, can occur. The type of water of most interest to m a n is freshwater and emphasis is thus placed on this type of water resource.

Types of freshwater

Freshwater on small islands can occur naturally or be artificially provided by various treatment and enhancement methods.

Characteristics of small islands

Characteristics of small islands

4

Characteristics of small islands

0 0 0 0 0 0

a

P

4.. P 3 Iyp

'i

0 m N 0 0 0 0 0

0)

x

5

Naturally occurrinz freshwater

All naturally occurring freshwater originates from precipitation. The types of natural freshwater occurrence can be conveniently described under three headings: precipitation water, surface water and groundwater.

Precipitation on small islands can occur as rain, snow, dew condensation and fog interception and other types (see section 3.1.1). Rainfall is by far the most prevalent form of precipitation. S n o w is generally limited to small islands of the temperate and cold zones, but can occur on high islands belonging to warm and hot zones, for instance the Canary Islands (refer to Canary Islands case study). D e w condensation and fog interception can be an important source of water for some islands in arid climatic zones. Rainwater is often collected from artificial surfaces and sometimes natural surfaces as a resource to be used for drinking and other purposes. Direct rainfall on the ground is used, as on continents, for generally small scale rain-fed ('dry-land') agricultural activities.

Surface water occurs on small high islands in the form of ephemeral and perennial streams, rivers and springs, and as freshwater lagoons, lakes and swamps. Perennial streams, rivers and springs occur mainly in high volcanic islands where the volcanic rocks are not too permeable. They m a y also occur in limestone islands. In many cases, there are m a n y small river catchments, making the assessment of surface water resources difficult and laborious. In most small islands there are no flowing streams.

Freshwater lagoons and small lakes are not c o m m o n but are found on some small islands. These can occur in the craters of extinct volcanoes or depressions in the topography. Most small island lakes and lagoons are brackish.

Groundwater occurs on small islands as two main types of aquifers: perched and basal. Perched aquifers commonly occur over horizontal confining layers or aquicludes, but m a y also be retained in compartments by series of vertical volcanic dikes. S o m e perched aquifers are underbrtesian pressure. Basal lenses can occur on both high and low islands in the form of coastal aquifers or freshwater lenses overlying sea water. S o m e of these fresh water lenses m a y be 20 or more metres thick; in small low lying coralline islands, freshwater lenses are generally m u c h thinner than this.

The transition zone thickness depends on both natural conditions (permeability of the geological formation, tidal and recharge fluctuations and other factors) and artificial influences (man-induced extraction). Neither the freshwater nor transition zones are of constant thickness; they vary spatially (across islands) and temporally (according to climatic influences on rainfall patterns).

Details of naturally occurring freshwater resources are considered throughout the subsequent sections of this Guide.

Other sources of freshwater

W h e n climatic, geological and other conditions are not favourable for the occurrence of sufficient freshwater, it m a y be necessary to use 'artificial' (or 'non-conventional') methods to provide small islands with their freshwater requirements.

There are a number of potential alternatives, some proven while others are doubtful, to naturally occurring freshwater sources.

Freshwater can be m a d e available by treating highly saline water (desalination) or used water (wastewater reuse).

Desalination reduces the salinity of seawater or brackish water to meet guidelines for drinking water.

Nowadays, there are a number of successful desalination systems based on distillation or membrane techniques.

Small islands are surrounded by an abundance of seawater which has the potential to supply very high demands for freshwater provided the desalination process is affordable and is within the capacity of the community to operate and maintain the equipment.

Similarly, treating wastewater is a technically feasible option for some purposes. Wastewater includes sewage effluent, industrial discharges and stormwater runoff. The necessity to protect the natural environment from wastewater pollution has led to m u c h improved purification techniques. The adoption of wastewater treatment and subsequent recycling as a means of supplying fresh (but non-potable) water is determined largely by economic factors. For potable purposes, its adoption is not only governed by economic but also by public health and aesthetic considerations and consequently is rarely used.

Another alternative to naturally occurring freshwater sources is importation which is conceptually a simpler solution, but must be viewed seriously for technical and economic viability. Where small islands are close to continents, submarine pipelines may be a practical solution. For more distant islands, transporting water in ships or barges is a possible option.

Freshwater enhancement techniques including artificial recharge and weather modification are further options.

Obtaining water from different sources and using it for more than one purpose can also be considered. M a n y small islands do operate 'conjunctive use' water supply schemes. Typically, these schemes include the naturally

Between the freshwater lens and the underlying seawater a saline water transition zone exists.

occurring freshwater sources of groundwater and rainwater but m a y also include desalination, wastewater reuse and/or importation.

Further details of these and other 'non-conventional' methods of providing freshwater are considered in sections 4.5 and 5.2.

Other types of water

As mentioned previously, not all water occurring on small islands is fresh. Water which is not fresh can also be considered as a resource and the direct use of brackish water, wastewater or seawater for certain uses is often a serious option on many small islands with limited freshwater resources. Typical uses are brackish water and/or wastewater for irrigation and seawater for sanitary flushing and cooling.

Non-potable water uses and applications are considered further in sections 5.1.5 and 5.2.10.

1.4 Problems of small islands

Outline

Small islands have special physical, demographic and economic features. Their very reduced areas, shortage of natural resources (arable land, freshwater, minerals and conventional energy sources), geological and orogenic complexity, isolation and widespread nature of their territories, exposure to natural disasters (typhoons, hurricanes, cyclones, earthquakes, volcanic eruptions and tsunamis) make the hydrological and water resources problems of these islands usually very serious. High demographic pressures and economic fragility are also encountered which add to the natural problems in many small island states.

Environmental conditions

Small islands are specialised and fragile land units, often exposed to severe environmental conditions such as the natural disasters listed above. The impact of the sea is greater than on continental land masses and large islands. There have been m a n y instances of small islands being damaged and even overtopped by storm generated waves. Expected sea level rise due to the 'greenhouse effect' (UNEP, 1985), the much-publicised global warming from increasing levels of carbon dioxide and other greenhouse gases (Boyle and Ardill, 1989), is of concern to many small island nations, particularly the very low lying coral atolls and cays (Pernetta and Hughes, 1989; McLean, 1989;

Woodroffe, 1989a and 1989b; and Mal& Declaration, 1989). Corrosive atmospheric conditions, owing to proximity to the sea, cause equipment and materials to rapidly deteriorate.

Water quantity

Problems of water quantity arise from difficult climatic conditions such as capricious rainfall, and from geological conditions such as extremely permeable rocks (of volcanic origin or karstified limestone) or impermeable rocks without significant storage potential, or topographical features not suited to the development of surface water resources. In extreme cases, there m a y be no permanent freshwater available.

Permanent rivers and springs occur only where rainfall is relatively high and well distributed over the year, and where favourable topographic and geological conditions occur. In most cases, natural flows of more than a few tenths of litres per second cannot be guaranteed. River basins (catchments) are often large in number but small in size and with limited regulation capacity. F e w high islands have topographical and hydrological conditions suited to the construction of significant water storages.

The turnover time of groundwater systems on most small islands tends to be short, generally a few years at most, but m a y be shorter than a year. Thus, freshwater lenses and perched aquifers m a y deplete in dry seasons to a stage where there is minimal or no freshwater resources even under natural (no pumping) conditions. T h e additional stress of pumping from such lenses can easily induce saltwater intrusion if care is not taken in the design and operation of extraction facilities. On some high limestone islands, the rocks are too permeable to enable a freshwater lens to form and there m a y be no fresh groundwater and no surface water.

Water quality

Sea water intrusion is a serious problem for the water quality of small and very small islands, especially where over- exploitation occurs due to increasing populations and tourist, industrial or agricultural developments. In these cases, the natural water balance can be dramatically and adversely altered. T h e tourist industry, n o w a major activity in many small islands, demands large volumes of water with high physical, chemical and bacteriological standards. Frequently, tourist accommodation has such a high water consumption that is can severely stress or even exceed the water resources capacity of an island. Then, the development of non-conventional water resources such as desalination of sea water or brackish groundwater, or water importation by tanks and barges m a y be required to satisfy the demand for water.

Inorganic and organic contamination of drinking water wells and rivers is a frequent phenomenon in many small islands. Water pollution caused by uncontrolled use of fertilizers, herbicides and pesticides is prevalent in some islands. Commonly, in addition to domestic wastewaters ('grey waters') other more aggressive wastewaters