Water Management in the Mekong Delta:
Changes, Conflicts and Opportunities
by
Ian White
Centre for Resource and Environmental Studies National Institute for the Environment
Institute of Advance Studies The Australian National University Canberra ACT 0200 Australia
_____________________________________________________________
IHP-VI Technical Documents in Hydrology No. 61 UNESCO, Paris, 2002
legal status of any country, territory, city or of its authorities, or concerning the delimitation of its frontiers or boundaries.
SC-2002/WS/47
Acknowledgments ...4
Summary...5
1. The Mekong...8
1.1 River of Change...8
1.2 This Study...11
1.2.1 Purpose...11
1.2.2 Study methods...11
1.3 Geography of the Mekong River Basin ...12
1.4 The Lower Basin ...14
1.4.1 Lower Basin Climate...14
1.5 Fisheries Resources of the Mekong...15
1.5.1 Wild capture fisheries ...15
1.5.2 Future demands and threats to wild capture fisheries ...16
1.5.3 Aquaculture...16
1.5.4 Constraints to aquaculture...17
1.6 Social, Cultural and Economic Features of the Basin ...18
1.7 Institutional Arrangements for Mekong Basin Resource Management...20
1.7.1 The Mekong River Commission ...21
1.8 Basin Development and Cooperation...23
2. The Mekong Delta ...25
2.1 The Delta at Large ...25
2.2 Vietnam’s Lower Delta ...25
2.3 Cambodia’s Upper Delta ...28
2.4 Hydrology and Climate of the Delta...30
2.4.1 Floods and seawater intrusion...31
2.4.2 Tidal influences...32
2.4.3 Seawater intrusion floodgates ...33
2.5 Surface Water Quality ...35
2.6 Groundwater in the Delta ...36
2.7 Soils of the Delta ...38
2.8 Acid Sulfate Soils...39
2.8.1 Oxidation of acid sulfate soils...40
2.8.2 Release of toxic metals ...40
2.8.3 Discharge of acidity into surface waters ...41
2.8.4 Impacts of acidity on estuarine ecosystems...41
2.8.5 Links between soils, hydrology and atmospheric emissions ...42
2.9 Saline Soils...43
2.10 Water and Land Constraints ...43
2.11 Integrated Management and Conflict Resolution...44
2.11.1 The use of multi-agent systems in natural resource management...44
3. Responses to Water and Land Issues of the Delta ...47
3.1 Completed Projects of the Mekong Secretariat ...47
3.1.1 Salinity intrusion forecasting ...47
3.1.2 Water balance study ...48
3.1.3 Water quality monitoring ...49
3.1.4 Management of acid sulfate soils...50
3.2 Work Plan of the Mekong River Commission Secretariat ...51
3.3 The Mekong Delta Master Plan...53
3.4 Saline Intrusion Floodgates ...54
3.5 Sedimentation and Hydrology of the Great Lake ...56
3.6 Perspectives for Australian Development Cooperation ...56
3.7 Australian Centre for International Agricultural Research Projects...57
3.8 The Farmers Response ...58
3.9 Summary...58
4. Opportunities for Integrated Research ...60
4.1 Management and Impacts of Saline Intrusion Floodgates in the lower Mekong Delta.61 4.1.1 Background ...61
4.1.2 Overall Objectives...62
4.1.3 Specific Objectives ...62
4.1.4 Expected Outcomes...62
4.1.5 Beneficiaries...62
4.2 Sedimentation and its Impacts on Cambodia’s Great Lake...62
4.2.1 Background ...62
4.2.2 Overall Objectives...63
4.2.3 Specific Objectives ...63
4.2.4 Expected Outcomes...64
4.2.5 Beneficiaries...64
4.3 Dry-Season Groundwater Supplies in the Mekong Delta...64
4.3.1 Background ...64
4.3.2 Overall Objectives...65
4.3.3 Specific Objectives ...65
4.3.4 Expected Outcomes...65
4.3.5 Beneficiaries...65
References...66
Terms of Reference
The terms of reference for this report are to prepare a state-of-the art monograph on the Mekong Delta which addresses:
(i) a succinct summary of past and present hydrology and water resource management activities, including those under the auspices of the Mekong River Commission and of other specialised agencies.
(ii) a critique of existing and past projects in terms of their success rate of implementation;
(iii) recommendations for future inter-disciplinary and inter-agency projects in the broad field of land-use (water management) which require an integrated approach at the subregional level on water management issues. The recommended programme should be able to attain achievable results within 3 years, take into account any limitations on-site infrastructure and incorporate the socio-cultural aspects of water management (i.e.
community water management) in the proposal.
(iv) to submit one copy of the manuscript as well as a typewritten mission report to UNESCO.
Acknowledgments
This work was sponsored by the United Nations Scientific and Cultural Organization, UNESCO, and supported by the Australian Centre for International Agricultural Research, ACIAR and Centre for Resource and Environmental Studies Australian National University.
The author wishes to thank Dr Mike Bonell, UNESCO-IHP, Dr Fereidoun Ghassemi, CRES, ANU, Hugh Milner and Andy Marr, SMEC, Brian Cummins, Cummins and Associates, Dr Philip Ford, CSIRO Land and Water, Dr Ian Willett, ACIAR, Associate Professor Mike Melville, University of NSW, Dr Phillip Gibbs, NSW Fisheries, Dr Philip Hirsch, University of Sydney, Professor Vo-Tong Xuan¸ Dr Le Quang Minh, Professor Vo Quang Minh, Nguyen Anh Tuan, Dr Troung thi Nga and Dr Nguyen Huu Chiem , Cân Tho University, Vietnam, Dr Pascal Perrez, CIRAD, France, Ms Erica Donner, CRES ANU, Dr Truong and Mr Nguyen Than Tin Sub, Institute Water Planning, Vietnam, Professor Nguyen An Nien and Professor Dong, Southern Institute of Water Resources Research, Vietnam, Dr To Phuc Tuong, IRRI, the Philippines, and Dr Sok, Hydrology, Mekong River Commission for assistance and many helpful discussions. Mr Vincent Leogardo, UNESCO IHP, is thanked for his generous help in preparing this report.
Summary
The Mekong River is one of the few great, largely unregulated rivers of the world. Its Delta is both agriculturally and aquatically highly productive and a major contributor to the region’s food production and export earnings. Water and land issues of the Delta must be considered as integral with those of the Mekong Basin as a whole. A majority of the Mekong Basin’s 60 million, ethnically diverse peoples rely on the River’s aquatic resources and rice production for their subsistence. For many, 40 to 60% of their protein intake is from fish from the Mekong. The prodigious fish resources rely on the annual fllooding of the Mekong.
The marked seasonal ebbs and flows of the River also impose severe constraints on its riparian communities, with vast wet season floods and dry season water shortages that allow seawater intrusions into the Delta.
These annual hardships are superimposed on nearly 60 years of devastating external and internal conflicts in the region. The six riparian countries making up the Basin have generally low external and per capita earnings. Harnessing the Mekong for hydropower generation and irrigation supplies, through cascades of main-stream and tributary dams, coupled with harvesting the Basin’s forests are ways of stimulating growth, increasing per capita income and regulating seasonal flows. These developments, however, are potentially in conflict with the subsistence needs and the livelihood security of the region’s poorest people. River regulation and diversions pose dilemmas, since they may decrease substantially the Mekong’s prodigious aquatic productivity. Up-catchment forestry also threatens water quality, productivity and dam capacity through potential increased sediment loads.
The Mekong River Commission’s task is to plan the sustainable development, use, conservation and management of the River’s water and related resources in mutually beneficial manner and to channel resources into its work program. Understanding the hydrology of the Basin and impacts of its regulation are central themes in its work plan, financed mainly by international multilateral or bilateral organisations. Major financing organisations have been criticised by non-government organisations as too narrowly focussed on infrastructure development and reliant on top-down approaches which ignore the needs of people. A key issue, identified by both proponents and critics of regulation structures is the paucity of reliable data on climate, hydrology, sediment yields, capture fisheries, social, economic and cultural aspects upon which to base sound decisions. In some important areas, such as water quality monitoring, the magnitude and complexity of water quality concerns are increasing at a rate that exceeds the capacities of riparian countries.
The low-lying Mekong Delta faces unique water land issues because of its sedimentary composition and geomorphology. The issues of Vietnam’s lower Delta differ from those of Cambodia’s upper Delta, which is dominated by the Delta’s natural flow regulator, Tonle
Sap and Cambodia’s Great Lake. In Vietnam’s lower Delta, the major land and water resource problems are: acute flooding in the wet season, with flood depths of more than 4 m in the northern Delta; acid sulfate soils constraints on crop productivity in over 40% of the lower Delta and associated, severe, acidic drainage waters with major implications for aquatic productivity; seawater intrusion in the dry season in the lower Delta, limiting rice production to one crop per year in saline intrusion areas; impacts of seawater intrusion floodgates on acidification; loss of coastal mangroves and impacts on coastal protection and fisheries.
In the Cambodia’s upper part of the Delta different issues need to be addressed. There is a surprising dearth of information on sediment fluxes and on the quantitative relation between flooding and the breeding/ feeding/ life cycle of fish despite their importance to riparian communities. The principle water and land concerns in the Cambodian section of the Mekong Delta are: impacts of upstream flow regulation on the water supply for flooding for rice production and fish production in the Great Lake; impacts of forest clearing on sedimentation and aquatic production in the Great Lake; impacts of downstream river regulation on flooding, rice and fish production. Recent estimates of sedimentation rates in the Great Lake are at least 8 times higher than those of the past 5,000 y. In both the upper and lower Delta, the availability and quality of domestic water supplies is a major issue. The control of downstream flooding and of saline intrusion in the lower Delta could be potentially in conflict with the need to reduce flooding in the upper Delta.
In the past, projects relevant to the specific needs of the Delta have tended to be narrowly focussed. The highest priority has gone to planning and design for hydropower and irrigation diversion. Main projects completed by the Mekong River Commission Secretariat with direct relevance to the Delta are: the Saline Intrusion Studies; the Water Balance Studies; the Management of Acid Sulfate Soils Project; and the Water Quality Monitoring Programme.
Even in these, the central thrust has been the impacts of upstream regulation, diversions to increase crop production and changes in landuse on the quantity of water in the Delta.
Broader issues such as the influence of saline intrusion on fish production and the importance of recent sedimentation to aquatic and terrestrial productivity have not been examined. The Secretariat’s present Work Plan still has a concentration of effort on infrastructure development but there are broader-based projects planned and underway.
The Secretariat’s water balance, salinity intrusion and acid sulfate soil management projects formed the basis of the Mekong Delta Master Plan, intended to underpin sustainable growth in the lower Delta. A central thrust of this plan is increased rice and aquaculture production.
A key outcome of this plan was a proposal to increase rice production in the region west of the Bassac by supplying a longer irrigation season and lowering or preventing salt-water intrusion into the region. This proposal, the Desalination of the Ca Mau Peninsula, has recently been completed. It is suggested, by analogy with the Australian situation that the impacts of this project on local fish production and waterway acidification may be severe with significant consequences for local communities. The impacts of already installed sluice gates near Soc Tranh were summed up succinctly by one farmer “floodgates have given us a road (track) and electricity. But no crops and no fish!”
A seminal, French benchmark study of sedimentation in Cambodia’s Great Lake provides an opportunity of assessing the impacts of changes in upcatchment and surrounding landuse on sediment and nutrient dynamics and fish production in the Lake. A comprehensive report to
the Australian government, Perspectives for Australian Development Cooperation, identified key areas for assistance in the Delta because it receives less priority development assistance and because of the concentration of poor there. Salinity and acid sulfate soils required assistance. It recommended an integrated approach to acid sulfate soils because they involved cross-sectoral land and water issues and one which was both precautionary and curative. The report also pointed out the uncertainty of dry season domestic water supplies in the Delta and the problems with acidity and salinity. Groundwater was seen as an increasingly important resource in these areas. Watershed and catchment planning were also identified as opportunities for assistance. The farmer’s response to the adverse conditions they face daily has been innovative and courageous.
Three proposed integrated projects for the Mekong Delta were developed out of the above analysis: Management and Impacts of Saline Intrusion Floodgates in the lower Mekong Delta; Recent Sedimentation and its Impacts on Cambodia’s Great Lake; and Dry-Season Groundwater Supplies in the Mekong Delta. Brief backgrounds, overall objectives, specific objectives, expected outcomes and beneficiaries are given for these projects.
1. The Mekong
1.1 River of Change
The Mekong (Thai: Mother of Waters) River has the twenty-first largest drainage basin and is the twelfth longest river with the eight largest annual discharge and second most diverse riverine fishery in the world. It is one of the world’s great, largely unregulated rivers. The water, land and biological resources of the Mekong Basin sustain an ethnically diverse and growing population in six countries; China, Burma, Lao PDR, Thailand, Cambodia and Vietnam (Fig 1.1). The Mekong Basin’s resources provide both great benefits and hardships for its peoples. The river is biologically highly productive and is a major source of protein.
Its wet season floods nurture vast rice crops. However, wet season flooding is severe with over 50% of the Mekong Delta (1.9 Mha) annual inundated. The floods of 1961, 1978, 1991, 1996 and 2000 caused major devastation and all except the 1996 flood had return intervals of greater than 1 in 50 years. Paradoxically, water shortages arise in the dry season, particularly in southwestern region of the Mekong Delta. These water shortages lead to seawater intrusion in streams in the lower Delta. Seawater intrusion, severe acid sulfate and saline soils and upstream deforestation impose social and economic constraints and uncertainties and limit agricultural production of staples such as rice and fish (Be, 1994;
Minh, 1995). The seasonal extremes, however, are necessary to sustain the Basin’s exceptional aquatic productivity (Roberts, 1993a) on which its riparian communities depend for most of their protein.
Since the 1930’s the Basin has been ravaged by wars of liberation and inter and intra country conflicts. These have had massive, long-term social, economic and cultural impacts on the peoples of the lower Basin and depleted populations, resources and institutional capacity, especially in resource management. International organisations are seeking to assist the region’s peoples by promoting development and growth in the Basin, mostly through large infrastructure construction projects, principally hydropower, flood mitigation and irrigation supply dams. The Mekong is seen by many as one of the great “undeveloped resources” of Southeast Asia. Less than 5% of both the Basin’s annual flow and its catchment are regulated at present. There are plans for a cascade of up to 9 mainstream “run-of-river” hydropower schemes in the Mekong together with as many as 50 tributary dams (Rothert, 1995). These plans have been criticised as fundamentally flawed (White, 1997) because of the dearth of information on climate, hydrology, ( Institute of Hydrology, 1982; 1984; 1988a, 1988b) and ecology, as well as a paucity of social, economic and cultural data and knowledge of the aspirations of riparian communities likely to be affected by river regulation (Greater Mekong Task Force, 1996).
The Basin is undergoing accelerating, political, cultural, economic, and water and land use changes. These changes have the potential to both benefit individual countries and to disadvantage their downstream or upstream neighbours as well as their own riparian communities. River regulation and changed landuse have major implications for the sustainability of many rural communities along the River (Derasary, 1996).
Fig. 1.1 The lower Mekong River Basin flowing from China to the South China Sea (Mekong River Commission, 1999)
Clearing of upstream forests has reportedly changed rainfall-runoff relations, resulting in larger, more frequent floods (Hirsch and Cheong, 1996) and an increase in dry season flows, primarily as a result of reservoir construction on tributaries (Institute of Hydrology, 1988a).
The frequency of major 1 in 50 year floods over the past 40 years is a of major concern, especially in the Delta. These factors, coupled with the putative impacts of global climate change, which suggest an increase in frequency of extreme events but with overall lower mean river flows for the Mekong (Lettenmaier, 2000) have led to increasing calls for regulation of Mekong flows.
Most major projects in the Basin or proposed projects have been criticised as narrowly focussed, involving a single or a small number of infrastructure-dependent outcomes (White, 1997). This is particular so in developments for flood mitigation, irrigation supply, hydropower generation or seawater intrusion mitigation. Such schemes have been faulted for their perceived lack of appreciation of the broad range and complexity of issues that need to be considered and the gamut of deleterious impacts that may ensue (White, 1963; Fraser- Darling, 1970; Challinor, 1973; Roberts, 1993b; 1995; Sluiter, 1993; McCully, 1996; Hirsch and Cheong, 1996).
1.2 This Study
The issues involved in the equitable management and sharing of the whole Basin’s resources while retaining local sovereignty and protecting local interests are complex. It is clear that the availability and sharing of knowledge on the prevailing hydrology, climate, ecology, economics, sociology and cultures and listening to and addressing the aspirations of its peoples are fundamental to the development, use and management of this vitally important Basin. The issues in the Mekong Delta mirror those of the Basin as a whole but also present some unique problems because of the Delta’s geomorphology.
1.2.1 Purpose
The purpose of this study is to overview broad issues in the Mekong Delta, to examine the success and failures of hydrologic developments in the Delta and to identify gaps in current hydrologic knowledge which require a broader, integrated approach to their solution in order to use and manage water and land resources of the Delta sustainably and equitably. It is undertaken at a time when major infrastructure developments are under way, and when important studies are being carried out such as the review of water quality monitoring (Ongley et al., 1997) and the Mekong River Commission-Murray-Darling Basin Commission/SMEC Mekong River Utilisation Program (H. Milner, private communication, Nov. 1997) to develop Basin-wide water use and management rules. Like the River itself, the situation is fluid and constantly changing.
1.2.2 Study methods
In this study, relevant, accessible publications and documents were reviewed, discussions were conducted, particularly with current study teams, and a field trip to the lower Delta was carried out in May 1997 with the assistance of the Australian Centre for International
Agricultural Research (White et al., 1997b). Because the Delta is an integral part of the Mekong and will bare the brunt of any large-scale changes in upstream hydrology, it is necessary to first consider background and status of the Mekong Basin as a whole before focussing on the Delta.
1.3 Geography of the Mekong River Basin
The Mekong River rises 5,000 m above sea level, where it is fed primarily from snow-melt in the Tanghla Mountains on the Tibetan plateau. It descends through steep, narrow gorges in south-western China, where it is called the Lancang (Turbulent) River, passes through the
‘Golden Triangle’ junction of Burma, Laos and Thailand, at an elevation of about 500m, crosses the highlands of Laos. It then forms a 900 km boundary between North-East Thailand and Laos, before descending the Khone Falls in southern Laos and 120 km of rapids in northern Cambodia (see Fig 1.1). After its confluence with the Tonle Sap River at Phnom Penh at the ‘Quatre Bras’, the Mekong splits into the 220 km long Bassac River and the 240 km Mekong, which runs almost parallel to the Bassac. These flow into the Mekong Delta through the 9 tributaries of the Cuu Long, the “nine dragons”, and out into the South China Sea at the end of its 4,200 km long journey (Pantulu, 1986; Sluiter, 1993; Mekong Secretariat, 1994; Hisrch and Cheong, 1996). The maximum width of the Mekong in the Delta during non flood periods is close to 1.2 km at Vam Nao. A summary of biophysical and landuse data of the Basin is given in Table 1.1.
TABLE 1.1 Biophysical and landuse data for the Mekong River Basin (Hirsch and Cheong, 1996).
Burma Cambodia Lao PDR Thailand Vietnam Yunnan Total Drainage Area
(103km2)
24 155 202 184 65 165 795
Basin Area (%)
2 20 25 23 8 21 100
Annual Runoff (%)
2 18 35 18 11 16 100
Forest Cover (%)
47 (whole country)
49-62 47 26 27 - -
Rate of Deforestation
(%)
6 (whole country)
3 2 1.5 3.2
(Central Highlands)
- -
Arable Land (103km2)
95.7 29.1 8.9 190 56.9 933 -
Irrigated Land (%)
- 15 20 20
(northeast)
40-50 (Delta)
- -
Hydropower Potential
(MW)
300 2200 13200 1000 2000 13000 31500
Hydropower Potential
(%)
1 7 42 3 6 41 100
The 795,000 km2 Mekong Basin covers a wide range of bioclimatic zones. Annual River from the discharge from the Basin is 475 km3, or a remarkable 600 mm on a whole basin areal average. The variation of annual runoff with drainage area down the Basin is shown in Fig. 1.2 (Pantulu, 1986). The changes in runoff down the basin reflect the impact of tributaries and the orographically-driven rainfall variation. The minimum between Kratie and Phnom Penh represents natural regulation by Cambodia’s Great Lake fed and discharged through Tonle Sap. Mean runoffs are misleading since the monsoonal climate results in an, on average, 15-fold variation between low (April or May) and high (September or October) flow. This flow variation imposes the combined annual hardships of wet seasons floods, and water shortages and saline intrusion in the dry season on populations in the Mekong Delta and leads to an inherent resource uncertainty in agricultural production, particularly in staples such as rice, and water-supply related health problems.
0 100 200 300 400 500
0 200000 400000 600000 800000
Drainage Area (km2) Mean Runoff (km3 /y)
0 50 100 150 200
Sediment Load (Mt/y)
Chieng Saen Vientiane Mukdahan Pakse Kratie Phnom Penh South China Sea
Sediment Load Runoff
Fig 1.2 Variation of mean annual runoff (line) and sediment load (solid square points) with drainage area for the lower Mekong Basin. The dip between Krate and Phnom Penh illustrates the natural regulation of Cambodia’s Great Lake and the Tonle Sap (from Pantulu, 1986, Mekong Secretariat, 1982).
The sediment loads are relatively low (concentrations between about 0.2 to 0.8 kg/m3) compared to other major Asian Rivers. The organic content of the sediments is high, about 6 to 8% of total suspended solids (Mekong Secretariat, 1982). Annual sediment loads down the Basin are also plotted in Fig. 1.1. It can be seen that there is a mean net deposition of 35 Mt/y of sediment at Phnom Penh, presumably during the flooding of the Great Lake.
Table 1.1 also lists the enormous potential for generating electricity from the fall of the Mekong and its tributaries, particularly in Laos and Yunnan Province, China. Currently, the only mainstream run-of-river dam on the Mekong is the Manwan hydropower Dam in Yunnan Province (Kunming Hydroelectric Investigation, Design and Research Institute, 1993) . This hydropower potential has attracted strong interest because of its ability to provide power for industrialisation and much needed external earnings for the riparian countries of the Basin. In addition, hydropower dams can be used to provide irrigation water, safe, reliable domestic supplies and to promote interbasin transfers. Major plans for cascades of dams along the Mekong and its tributaries, however, have attracted mounting and concerted opposition, particularly from community-based, non-government organisations.
These organisations argue that the full environmental, ecological, economic, social and cultural costs of the hydropower cascades could exceed their benefits. For decades, river regulation has been the central, contentious issue in the management and use of the Mekong River Basin.
1.4 The Lower Basin
The lower Mekong Basin, downstream from China and the Burma-Laos -Thailand intersection, covers parts of Lao PDR, northeast Thailand, 86% of Cambodia and 20% 0f Vietnam. The lower Basin represents 77% of the total Basin area and more than 80% of the annual flow. Much of the available data relates to the lower Basin because of the composition of the previous Mekong Committee (the river’s former main institutional management and development authority), as well as those of the subsequent Interim Mekong Committee and now the Mekong River Commission (Lao PDR, Thailand, Cambodia and Vietnam). The lower Basin’s resources are of particular interest to the member nations.
1.4.1 Lower Basin Climate
The lower Basin is in the centre of the Asian tropical monsoon region with a summer-winter wind reversal due to differential heating of the extensive land and water masses. Its climate is governed mainly by seasonal monsoon winds. The southwest, wet season monsoon starts in mid March to mid-May and ends around mid-September to mid-October. The northwest dry season monsoon runs from mid-October to March. Rainfall in the wet season is typically afternoon or early evening, convective falls. In higher regions, rainfall is topographically driven. A short 7-14 day dry period frequently occurs in June or July due to high anticyclone circulation There are occasional tropical storms with large rainfalls in August and September (Pantulu, 1986; Mekong Secretariat, 1968; 1975). Mean annual rainfall of the lower Basin ranges from approximately 1,000 mm in northeast Thailand to more than 3,500 mm in the mountainous fringe of northeast Laos, where there is no clearly defined dry season.
Elsewhere in the lower Basin, little rain falls during the dry season. Relative humidities range from 50 to 98% and. mean solar radiation is estimated to be 1.12 MJ/m2/d. Estimated annual potential evaporation ranges from 1500 to 1800 mm. The seasonality of rainfall
excess and its consequent impact on river and tributary flow, largely govern the water, land and biological resources of the lower Basin., As it flows from the upper Basin to the lower Basin, at Chiang Soen, the Mekong has a less pronounced seasonality in flow because of the influence of upstream snowmelt.
1.5 Fisheries Resources of the Mekong
The Mekong is one of the most biologically diverse river systems in the world. Currently 1700 fish species have been recognised, although the list is by no means complete (Bao, et al., 2001; MRC, 2002a). There is also a corresponding diversity amongst other aquatic animals and insects. The annual flooding of the vast floodplains of the Mekong fuels this diversity, by when fish take advantage of the vast expanse of rich feeding grounds and the opportunities to breed, spawn and raise young. In the dry season, fish retreat to river channels and to permanent lakes and deep pools in the river. This annual flooding means that fish migration is the norm (Bao et al., 2001).
The Lower Mekong Basin has three major interconnected migration systems. The lower system lies downstream from the Khone Falls, and includes the Tonle Sap River and Great Lake system in Cambodia and the Mekong Delta. The middle system extends from above Khone Falls to the Loei River. In this system, floodplain habitats are connected with the large tributaries of the Mekong. The upper system runs upstream from the Loei River (MRC, 2002a). The complexity and interconnectedness of the migratory systems and the fundamental importance of the annual flooding are some of the main reasons behind growing opposition to regulating the main flows in the Mekong and to concerns over sediment loads from cleared areas. The fundamental importance of the Tonle Sap River and Great Lake system cannot be overstated.
Bao et al. (2001) highlighted the critical nature of habitat and flood patterns to the propensity of fish species to migrate, spawn and find dry-season refuges. Changes in flood patterns or water quality, blockage of important migration channels and destruction of dry season refuges could all adversely affect fish stocks that are crucial to the health, nutrition and livelihoods of some of the poorest people in the Lower Basin countries. Fish migrations therefore have many implications for regional development, planning and management. They recognised important fish stocks are shared between countries and concluded that joint management strategies are needed to ensure appropriate development.
Many fish species migrate trans-boundary during their life cycle. Several migratory stocks are shared, including the endangered Giant fish species. There are, however, no institutional arrangements at the regional level for joint management of trans-boundary fish resources. At the local level, there are long standing traditions of fisheries management being undertaken by communities in the Lower Mekong Basin. Local rules on fishing are often connected with spiritual beliefs. These help sustain local resource levels and to ensure equitable distribution (MRC, 2002a).
1.5.1 Wild capture fisheries
Fish is the major source of protein for people in the lower Mekong Basin. It is estimated that wild capture fisheries produce annually over 1.6 million tonnes (Bao et al., 2001; MRC, 2002a). The total value of the catches is about $US1.4B.The size of inland fisheries, however
has been grossly under-reported because of the subsistence nature of the sector (Mekong River Commission, 1999). During a field trip to the lower Delta, fisheries experts had ample data on aquaculture production but had no information and little curiosity about wild capture fisheries. In boat trips along the canals it was evident that wild capture fisheries effort was enormous with major netting structures at least every 50m.
Average fish consumption ranges from about 30 kg per capita in mountainous areas, to 70 kg around the Great Lake Tonle Sap area in Cambodia. During the dry lean seasons, fermented and dried fish are used in place of fresh fish and most households use fish sauce all year round. Most fish are consumed locally or traded fresh at village, district and provincial markets. There is also trade in fish within the Mekong Basin and its neighbouring catchments. Exports are limited, but increasing (MRC, 2002a).
1.5.2 Future demands and threats to wild capture fisheries
It has been predicted that there will be a 20 percent increase in fish demand in the Lower Mekong Basin over the next 10 years. Increased fishing may increase in overall catches in the short term. This however will be accompanied by a continued decrease in the larger slow- growing migratory species in the catches. To mitigate the decline in biodiversity will require coordination and integration of management interventions at all levels. Current analyses suggest there is no indication that future increases in fishing effort will lead to decreased catches or reduced diversity for the non-migratory fish species(MEC, 2002). However, this predicated on the major assumption that the integrity and spatial extent of the floodplains remain intact.
The major threats to sustaining capture fisheries include (MRC, 2002a):
• Destruction of spawning grounds or dry season refuges by habitat alterations
• Local changes in the quantity and quality of water available for sensitive habitats and the timing of hydrological events,
• Pollution from agriculture and urban development.
• Construction of dams, weirs or diversions which act as physical barriers to fish migrations.
• Increased sediment load due to deforestation.
1.5.3 Aquaculture
Aquaculture in the Lower Mekong Basin is diverse and includes the production and sale of fry and fingerlings and raising wild or artificially produced fingerlings in enclosed or semi- enclosed water bodies. Total production Basin is estimated to be 260,000 tonnes per year with a farm gate value of about US$ 270,000M. million. There are relatively few large-scale commercial farms in the Lower Mekong, although there are large catfish farms in the Bassac River and large integrated fish farms near towns and cities in Northeast Thailand. Most aquaculture production comes from small-scale operations run by rural households and this is becoming increasingly important throughout much of the Basin. Small-scale aquaculture contributes to food supply in areas where wild fish are deficient. It also provides opportunities for supplementary income and diversity. Except in Cambodia, fish ponds and rice fields are the most common means of producing fish throughout the Basin (MRC, 2002a).
The Mekong Delta has the largest aquaculture area (330,000 ha) and freshwater production is above 170,000 tonnes. An estimated 80,000 ha are presently under rice-fish culture, with a mean annual production of 370 kg/ha. There are more than 100 hatcheries in the area and the most commonly farmed species are catfish, barbs, carps, tilapia, gouramis and sand goby.
There are about 5,000 fish cages in the Delta that are mostly stocked with fry and juveniles from the wild.
In Cambodia, most of the aquaculture production comes from cages and pens. River catfish and snakeheads are the dominant species. Northeast Thailand is the second largest area in the Lower Mekong Basin for aquaculture production. There production has expanded significantly over the last decade and annual output is in the range of 65,000 tonnes. Cage culture of tilapia has recently expanded in reservoirs and in the Mekong River.
Governments see aquaculture as a high priority. They support investments in aquaculture and fund research, infrastructure, education and extension. As with many government enthusiasms, effort is focussed on narrow outcomes without consideration of broader and interconnected issues.. There is no separate legislation on aquaculture in any MRC-member country. However it is under review in all.
Trans-boundary issues such as genetic quality of broodstock have yet to be addressed. There are major environmental concerns about the more intensive forms of aquaculture. There include :
• the balance between exotic and indigenous species,
• culture of predator species,
• collection of juveniles form the wild,
• water pollution and
• the spread of fish disease (MRC, 2002a):.
The past 10 years has seen five-fold increase in aquaculture production. Continued expansion could contribute to meeting some of the needs for fish products in the Lower Mekong.
However, aquaculture sales are strongly influenced by market demand, particularly in the local market. The demand will depend on the number of consumers who can pay the price, often US$ 1.00 or more per kg.
Aquaculture growth in the lower Mekong needs an expansion in hatcheries and nursing capacity. Centralised large government hatcheries have not been successful. Development of local, small-scale hatcheries, trading networks, and on-farm breeding appear to offer more promise in supporting rural, small-scale aquaculture (MRC, 2002a).
1.5.4 Constraints to aquaculture
There are several constraints to the development of aquaculture. Many of these are institutional rather than technical. The capacity and resources of government institutions for participatory extension and research is limited. Capacity building is required to support development. The development of aquaculture to date has been a narrow sectoral approach.
It is now acknowledged that the promotion of aquaculture in the Mekong Basin should take food security and poverty alleviation as a starting point for interventions and there needs to
be an emphasis on building capacity in local institutions. Aquaculture needs to be integrated into fisheries projects and wider rural development strategies. Aquaculture, capture fisheries and reservoir management are parts of a holistic system. The past focus on policy and development efforts for aquaculture alone, while ignoring wild fisheries, could result in a dramatic loss of fundamentally important wild fisheries resources. This could severely affect food security for the entire Lower Mekong Basin, particularly for poor people (MRC, 2002a).
A recent analysis of finances and risks of selected aquaculture activities in the Basin concludes that pond and cage aquaculture has high potential in Lao PDR, Cambodia and Viet Nam, in terms of both commercial development and small-scale family enterprises directed at poverty alleviation (MRC, 2002b). It found from a financial perspective, aquaculture compares well with alternative traditional enterprises such as rice and fishing, and other new enterprises such as fruit and coffee production. While risk levels were necessarily somewhat higher than traditional activities, they were generally similar to, or lower, than other new enterprise types.
The Mekong River Commission has espoused an individual catchment approach to resource management in the Basin. This is designed to be a bottom-up planning and data gathering process, with assistance from people whose livelihoods depend on the resources of the catchments. It is modelled on catchment management throughout Australia’s Murray-Darling Basin and elsewhere. There, it is probably too early to determine whether the voices of those who depend on the catchment for their livelihood is truly heard.
1.6 Social, Cultural and Economic Features of the Basin
The lower Mekong Basin has an estimated population of 60 million. About 45-50 million of these inhabitants are farmers and fishers relying directly on the Mekong River and its associated land resource (Mekong River Commission, 1999). This population is ethnically very diverse. Only in Cambodia does one ethnic group, the Khmer, dominate the country’s basin area. In China, minorities exceed the Han Chinese. Lao PDR has 68 ethnic groups.
Vietnam’s Delta population is mainly Kinh but concentrations of Khmer, Chams and ethnic Chinese exist there. Economic indicators for the Mekong’s riparian countries are listed in Table 1.2.
TABLE 1.2. Economic indicators of Mekong River riparian countries (Kirsch and Cheong, 1996)
Burma Cambodia Lao PDR Thailand Vietnam Yunnan GDP
(billion US $) 11 2.03 1.46 140.3 17.4 4.51
GDP per capita
(US $) 250 206 335 2377 240 465
GDP Annual Growth Rate
(%)
6.4 4.9 8.0 8.5 8.8 11.8
GDP Agriculture
(US $) 47.1 44.8 57.4 11.1 32.3 21
Burma Cambodia Lao PDR Thailand Vietnam Yunnan GDP Industry
(US $) 14.4 19.6 17.9 42.1 25.3 54
GDP Industry
(US $) 38.5 35.6 24.8 46.8 42.4 25
Trade Balance
(billion US $) -0.724 -0.243 -0.205 -9.5 -0.9 -
Current Account Balance (billion US $)
-0.294 (1994/95)
- -0.104
(1992)
-8.4 (1994)
-1.1 (1994)
-
Foreign Debt
(billion US $) 5.5 (1993/4)
1.0 (1992)
1.92 (1992)
27.4 (1994)
19.6 (1994)
- Consumer Price
Increase (%, 1994)
35 26.1 6.7 5 9.9 21.7
The average level of income between countries in Table 1.2 varies by a factor of 10. This is somewhat misleading in regard to income derived from the Basin. The northeastern region of Thailand is the country’s poorest, while the Delta is Vietnam’s most prosperous region. In addition, in the Delta most income comes from local farm or aquatic production whereas a significant proportion of income in northeastern Thailand comes from off-farm remittances.
The figures in Table 1.2 take no account of the late 1997 financial crises in Asia and are indicative only of relative wealth of countries within the Basin. Financial crises have major implications for the development and management of the Basin. It is quite clear from Table 1.2 that most of the Basin’s inhabitants are subsistence farmers and fishers. In the lower Mekong Basin, fish is as important to riparian communities as rice (Sluiter, 1993) and makes up 40-60% of protein intake (Pantulu, 1986). The limited and dated information on fish consumption shows that the average annual per capita fish consumption in Cambodia was 25.4 kg which exceeds others in the lower Basin, with Vietnam 20.8 kg, northeast Thailand, 11.5 kg and Laos, 10.2 kg (University of Michigan, 1976). These figures, however, must be considered approximate.
In subsistence economies, there are clearly extremely limited internal resources to undertake the necessary planing, monitoring, implementation and management of Basin-wide projects.
There are two main external, loans-funded resource developments proposed for the Basin, hydropower and forestry. These are seen as important sources of national income and earners of foreign exchange, however both activities are potentially at odds with the subsistence needs and livelihood security interests of the region’s poorest people (Hirsch and Cheong, 1996), a issue emphasised by many non-government agencies working within the region. The impacts of both hydropower and forestry developments on the productivity and biodiversity of Mekong fisheries is a major concern (Roberts 1993b; 1995). Internal development projects also have not been problem free. The thrust for increased rice production from the Mekong Delta has seen farmers move into areas badly affected by salinity and acidity and has generated the need for salinity intrusion protection.
1.7 Institutional Arrangements for Mekong Basin Resource Management
Table 1.3 (modified from Hirsch and Cheong, 1996) provides a summary of regional, institutional evolution in the management of the Mekong Basin as an entity.
The Committee for Coordination of the Comprehensive Development of the Lower Mekong Basin, or Mekong Committee as it became known, was established through funding provided by the United Nation’s Economic Commission for Asia and the Far East, ECAFE, in order to catalyse development of the Basin and to increase per capita income of the riparian countries (ECAFE, 1957). The United Nations Development Programme, UNDP, has been a major and consistent supporter of the Committee since its inception. Some have considered the Committee as a type of Marshall Plan for mainland Southeast Asia (Jacobs, 1995). As a consequence, the Committee and its successors have both coordinated resource management in the Basin and channelled development assistance to approved projects. This dual role has been seen by some as a potential conflict of interest.
The US Army Corps of Engineers and the US Bureau of Reclamation have long been interested in large scale engineering works on the Mekong and its tributaries. They saw the annual flooding of millions of hectares of Mekong lowlands as the major impediment to modernizing the region’s agriculture (Gráiner Ryder in Sluiter, 1993). Their solution was to propose impoundment of water in large storage dams, from which controlled releases would feed all-year-round, export-crop production and would generate income-earning hydropower.
The Corps report (United Nations, 1958), together with the Basin Indicative Plan (Mekong Secretariat, 1970), which was a synthesis of earlier projects, formed the basis for planned Basin development. The Mekong Committee and its successors have been seen by some critics as being progeny of the Corps of Engineers, having a “one dimensional”
preoccupation with infrastructure construction, despite the existence of contemporary studies of the non-engineering aspects of Basin development (White, 1963).
TABLE 1.3. Evolution of institutional arrangements for the management of the Mekong Basin Year Institutional Development
1957 Formation of Mekong Committee 1970 Indicative Basin Plan
1971 Nam Ngum Dam Completed
1975 Cambodia withdraws from Mekong Commission 1978 Interim Mekong Committee established
1987 Revised Indicative Basin Plan
1992 ADB commences Greater Mekong Subregion Initiative
1994 Hanoi agreement on Cooperation for the Sustainable Development of the Mekong River Basin
Year Institutional Development
1995 “Run-of-River” mainstream hydropower dams proposed Mekong River Commission established
1999 Restructuring of the Secretariat to achieve its goals
The withdrawal of Cambodia under Pol Pot regime forced the Mekong Committee into abeyance in 1975. In order to fill the vacancy, Vietnam, Thailand and Laos formed the Interim Mekong Committee in 1978. This remained almost dormant until the mid 1980’s when a Revised Indicative Plan was developed and released (Interim Mekong Committee, 1988). Disagreements arose in the early 1990’s on the procedures under which one member country could veto plans of another and also on the conditions for the re-entry of Cambodia.
1.7.1 The Mekong River Commission
The Mekong River Commission came into being in 1995 after UNDP-sponsored meetings culminated in the signing of the draft of the Agreement on the Cooperation for the Sustainable Development of the Mekong River Basin, on 28 November 1994. The four lower Mekong riparian countries endorsed this draft Hanoi agreement. It was based on the principles of sovereign equality, territorial integrity and environmental protection to enable the four signatory countries to use the resources of the Mekong in a reasonable and equitable manner. The Agreement provided freedom of navigation throughout the mainstream Mekong to promote regional cooperation and development. Importantly, it allowed for adding new members to the Commission, but removed the right of individual country veto.
The four countries also adopted the concept of a Basin Development Plan to identify and prioritize joint and basin-wide projects for action. Geography, hydrology, environment, climate and the rights and interest of all riparian countries were to be accommodated in the Plan. It has been seen by some as significant that the UNDP press release on the Agreement failed to mention the rights and interests of riparian citizens of the Basin when it recognised the need to harness the “destructive power of the River during peak wet seasons.”
The mandate of the Mekong River Commission is:
To cooperate and promote in a constructive and mutually beneficial manner in the sustainable development, utilization, conservation and management of the Mekong River water and related resources for navigational and non- navigational purposes for social and economic development and well-being of all riparian States, consistent with the need to protect, preserve, enhance and manage the environmental and aquatic conditions and maintenance of the ecological balance exceptional to this river basin.
The Commission’s vision for the Basin is:
An economically prosperous, socially just and environmentally sound Mekong River Basin.
The mission of the Commission is:
To promote and coordinate sustainable management and development of water and related resources for the countries’ mutual benefit and the people’s well-being by implementing strategic programmes and activities and providing scientific information and policy advice (Mekong River Commission, 1999).
The Mekong River Commission consists of three permanent bodies. There are: the Council, at Ministerial and Cabinet level which makes policies, decisions, and resolves differences;
the Joint Committee at permanent secretary level to carry out policies; and the Secretariat, responsible for technical and administrative support for the Council and the day-to-day operations of the Commission. The priorities of the Council can be judged from the work programme of the Secretariat which concentrates on four major areas of work: policy and planning; environment and monitoring; resources development and management; and programme support (Mekong River Commission Secretariat,1995, Mekong River Commission, 1999). More recently the Commission has reorganised its work into three programmes (Mekong River Commission, http://www.mrcmekong.org/programme; 2002):
• The Core Programmes consisting of :
− the Basin Development Plan
− the Water Utilisation Programme
− the Environment Programme.
• Support Programmes which sustain the implementation of other MRC programmes through a Capacity Building Programme.
• The Sector Programmes focus on specific sectors and address regional issues that are significant to the management of the entire Mekong River Basin. There are 5 Sector Programmes:
− the Fisheries Programme.
− the Agriculture, Irrigation and Forestry Programme.
− the Water Resources and Hydrology Programme.
− the Navigation Programme.
− the Tourism Programme
The Secretariat’s Water Resources and Hydrology Programme, a key programme in the overall planning and management of the Basin, has four main components: monitoring; real- time forecasting; planning and design; and applications. Present and planned projects of the programme include improvement of the Basin-wide hydrometeorological network, groundwater investigations, flood forecasting and damage reduction, upgrading of salinity intrusion forecasting in the Mekong Delta, water balance of the lower Mekong Basin, Phase IV and Mekong morphology and sediment transport. The Programme is seeking funds for several of these projects.
The early 1990’s also saw sweeping changes in natural resource management within member countries. The most significant of these was the creation of ministries specifically concerned with the environment in each of the member countries. In order to meet these changing circumstances the Secretariat was restructured in 1999. Its operational structure is shown in Fig. 1.3 (Mekong River Commission, 1999).
Fig. 1.3 Operational structure of the Mekong River Commission Secretariat (Mekong River Commission, 1999)
1.8 Basin Development and Cooperation
One of the important rôles of the Mekong River Commission is to act as a channel for Basin- wide development assistance. The three most important multilateral or international agencies involved in large scale Mekong project financing and administration are the Asian Development Bank, the World Bank and the UNDP. Other UN agencies, such as UNESCO, UNEP and ESCAP also play important rôles in heritage listing, and providing training for resource assessment and management. A variety of bilateral agencies, from Australia, Canada, Denmark, the European Union, Germany, Japan, Sweden, the UK and the US, also provide important assistance.
The 1990’s saw a marked increase in the number of non-government organisations operating in the Mekong Delta. Their main rôles in natural resource management has been at the community level, in advocacy for community rights and environmental values and in community capacity building. Many of these organisations have been strident in their criticisms of projects planned or undertaken by the Asian Development Bank (Uramoto et al., 1997; Imhof; 1997a), the World Bank (Imhof, 1997b) the UNDP (Probe Alert, 1995) and Japan (Lammers, 1997) whom they accuse of “ignoring people and embracing top-down
Policy Decisions (CEO)
Planning Information Analysis/
Environmental monitoring
Implementation
development”. In relation to this, major donor countries to the Commission’s work, Denmark, Sweden and Australia, have also urged the Commission to address public participation and consultation in its projects.
It is clear from the above that the Mekong is undergoing rapid and far-reaching changes. Of the many pressing issues in this evolving area, a key issue, identified by both proponents and critics of structures to regulate flow on the Mekong is the paucity of reliable data on climate, hydrology, sediment yields, capture fisheries, social and cultural aspects upon which to base sound decisions. In some important areas, such as water quality monitoring, the magnitude and complexity of water quality concerns are increasing at a rate that exceeds the capacities of riparian countries to deal with the issue (Ongley et al., 1997)
2. The Mekong Delta
2.1 The Delta at Large
The Mekong Delta is a 49,520 km2 triangle of recent (<10,000 y BP), generally fertile, alluvial and marine deposits extending from Kratie in southeastern Cambodia through Phnom Penh and southern Vietnam to the south China Sea (see Fig. 2.1). The Delta is flat and low-lying with elevations between 0.5 and 3m above mean sealevel apart from a small area in the northwest with elevations over 100m. Vietnam’s portion covers 74% of the Delta, while Cambodia occupies the rest. Deltaic sediments vary in depth from over 500 m at the mouth of the Delta to 30 m at Kratie. Deposition in the Delta expands the coast of the Ca Mau Peninsula at a rate of up to 150 m/y, while coastal erosion occurs along the South China coast ( Pantulu, 1986). The Delta’s population of 16 million people make it the most densely populated part of the Basin. Nearly 85% of the population are rural.
Drainage and canal construction in the upper Delta for agriculture and transport was commenced during the Angkor empire, over a 1000 y ago (Van Zuylen, 1991). Major canal construction over much of the Delta, particularly for transport, commenced in earnest with the French colonisation of Indochina 120 years ago (Sluiter, 1993). Canal construction for irrigation and drainage has accelerated in 1910-30 and since the end of the Indochina War in 1975. The Delta has now over 10,000 km of major canals that have profoundly altered the Basin’s hydrology.
The Delta suffered severe damage during the Indochina War. Defoliants, bombing, land clearing and drainage destroyed wetlands and forests. About 1,300 km2 of melaleuca and 1,200 km2 of mangrove forests were lost. At the end of the war in 1975, considerable resettlement occurred throughout the Vietnamese portion of the Delta as the government sought to feed its people (Sluiter, 1993). About 8,000 km2 of marsh lands in the Plain of Reeds, in the Delta’s northeast were rendered unfit for fish and agricultural production through drainage of estuarine acid sulfate soils.
2.2 Vietnam’s Lower Delta
The Vietnamese portion of the Delta occupies 39,000 km2, of which 24,000 km2 are now used for agriculture and aquaculture and 4,000 km3 for forestry. Cultivation in the Delta is a relatively recent practice with floating rices being used prior to paddy rice during wet season flooding (Brocheux, 1995). Primary products from the Delta contribute over 30% to the Gross Domestic Product and the Delta is Vietnam’s rice bowl, producing 50% of the nation’s
rice (NEDECO, 1993) and contributing to Vietnam’s place as the second largest rice exporter in the world. This increase has been largely the result of the 1986 Congress of the Communist Party’s ‘doi moi’ (renovation) policy, allowing private enterprise in agriculture, trade and industry. This enabled farmers to lease land for up to 50 years.
Fig 2.1 . The Mekong Delta (NEDECO, 1993)
This liberalization of agriculture has seen a shift to higher yielding “green-revolution” rices and cultivation, which has been accompanied by a steep increase in the use of artificial fertilizers, herbicides and pesticides with subsequent impacts on water quality and concerns with effects on fish and shrimp (Be, 1994). Fish and shrimp aquaculture production are also important contributors to the Vietnamese economy and export earnings, particularly in brackish areas (NEDECO, 1993). Major concerns in this sector are the impacts of agricultural chemicals on the quality of shrimp and the transmission of diseases in drainage waters (Be, 1994). The 1 in 100 year late 1997 typhoon Linda destroyed most of the semi- intensive aquaculture ponds and remaining mangroves on the Delta’s Ca Mau Peninsula (see Fig. 2.1). It demonstrated the lower Delta’s extreme vulnerability to storm surge.
0 50 100 150 200 250 300
Jan Feb Mar April May Jun Jul Aug Sep Oct Nov Dec P or Ep (mm/month)
Rainfall
Penman Evaporation Mekong Delta
0 5 10 15 20 25 30 35
Jan Feb Mar April May Jun Jul Aug Sep Oct Nov Dec Q (103 m3 /s)
Phnom Penh
River Flow
Fig 2.2. Mean monthly rainfall and Penman Evaporation for the Mekong Delta and mean monthly river flow for the Mekong at Phnom Penh (from NEDECO, 1993).
The Vietnamese government has earmarked the Delta as a prime area for expanding the production of food, export commodities and consumer goods (Sluiter, 1993) and is planning for a 6.5% to 8% per annum growth in the region. The potential for expansion of agricultural land is only a further 2,000 km2 (NEDECO, 1993). It follows that the expected increases of up to 50% in rice production to 16 million tonnes by 2015 will have to come from improved varieties, increased chemical inputs and increased double and triple cropping. Double and triple cropping is limited mainly by the availability of fresh water from the Mekong in the dry season and flooding in the wet season (Minh, 1995). There are approximately 1,000 km2 of triple rice cropping, 10,000 km2 of double cropping and 1,300 km3 of single cropping per year. The demand for increased production has direct implications for freshwater availability and perhaps also for water quality and fisheries production. Currently about 80% of surface water abstracted is used for agriculture while only 5% is domestic consumption. Expanding demands for water have increased the use of groundwater, particularly for domestic supplies (Hirsch and Cheong, 1996, Ghassemi and Brennan, 2000).
2.3 Cambodia’s Upper Delta
Cambodia’s upper part of the Delta has about 2,000 km2 of irrigation. At least 30,000 km2 is flooded during the wet season or covered by permanent wetlands. Farmers here have actively encouraged flooding for over 100 years by digging ‘colmatage’ or irrigation canals through river levees. In addition to supplying irrigation water, canals also supply nutrient-rich sediment to fields. Extensive, and in human terms, devastating ‘Pol Pot’ irrigation canals constructed in Cambodia in 1975-1979 were failures due to poor hydraulic design and are in desperate need of refurbishment.
The hydrology of the upper Delta (Figs 2.2 and 2.3) is dominated by the Tonle Sap and Cambodia’s Great Lake (Figs 1.2 and 2.1). Flow in the low relief Tonle Sac reverses direction from filling the Great Lake during the peak of wet season to draining it during the dry season, making it a natural regulator of flows into the lower Delta. During the filling process the Great Lake quadruples in area to over 15,000 km2 and increases in depth in some places from 1 to 9 m with a maximum volume of 60 km3 (Fig. 2.4, Carbonnel and Guiscafre, 1963). Flooding of surrounding forests and fields exposes increased, rich soil- and forest-sources of nutrients to fish and provides protected spawning areas (Dennis, 1986) The Great Lake is an immense resource to Cambodia and is the heart of its agricultural production. Its fisheries are of major importance, but catches have declined over the past 50 years from of order 100,000 tonnes in the 1940’s to of order 30,000 tonnes to-day. In addition to its importance to Cambodia, Tonle Sap provides fish that migrate as far as Yunnan (Hirsch and Cheong, 1996).
Clearing of forests around the Great Lakes and the Quatre Bras started early this century and has accelerated. In the late 1960s there were an estimated 8,000 km2 of surrounding forests which had dwindled to 3,000 km2 in 1992 (Dennis and Woodsworth, 1992). Cutting of flooded forests around the Great Lake has been banned since 1987, however, the pressures for development of this fertile area for agriculture have not abated. There are varying estimates of the total amount of Cambodian forests remaining, from 30 to 60%. In 1992, Cambodia’s logging was more than 1.5 million m3. This is estimated to be about 7 times the sustainable yield (Economist Intelligence Unit, 1993). Deforestation is blamed for the decreasing fish catches. It has also been claimed to be responsible for increased
sedimentation in the Lake. It has been estimated that sedimentation has increased from 20 mm/y, in the 1960’s, to 40 mm/y, in 1990, (Sluiter, 1993). Invaluable, comprehensive bench mark studies of the Lake’s hydrology and sedimentation in the early 1960’s (Carbonnel and Guiscafre, 1963) gave a mean sedimentation rate of 0.3 mm/y over the past 5,000 y. The deposition of 35 Mt/y of sediment, evident in Fig. 2 at Phnom Penh (Mekong Secretariat, 1992), can also be used to estimate a mean sedimentation rate. If we assume that this is deposited annually over the 15,000 km2 of the flooded Great lake, and that the mean sediment density is 1 t/m3, then the deposition rate is a more creditable 2.3 mm/y, or an 8- fold increase over the previous 5,000 y average of Carbonnel and Guiscafre (1963).
Increased sedimentation is a major issue as it gradually reduces the capacity of the Great Lake to regulate dry season flows in the Delta.
0 10 20 30 40 50 60 70
Feb-62 Apr-62 Jun-62 Jul-62 Sep-62 Oct-62 Dec-62 Feb-63 Mar-63 May-63 Volume (km3 )
Cambodia's Great Lake
Fig 2.3. The seasonal variation of the volume of Cambodia’s Great Lake, from Carbonnel and Guiscafre (1963).
The Mekong Secretariat has proposed a dam on the Tonle Sap, at the entrance to the Great Lake, as a strategy to reduce flooding around the Lake in the wet season. It would also provide downstream irrigation in the dry, boosting agricultural production, particularly in Vietnam. Cambodia has strongly opposed and continues to oppose this dam (Sluiter, 1993).
Its impact on fisheries production and on the subsistence farmers and fishers around the Lake could be enormous and may prevent the current fish migration up the Mekong, from the sea.
In addition, if the proposed dam prevents flooding around the Lake in the wet season, it may therefore increase downstream flows, exacerbating flooding in Vietnam’s potion of the Delta.
2.4 Hydrology and Climate of the Delta
The climate in the Delta is tropical monsoon and is influenced by both the southwest and northeast monsoons. In general the dry season runs from December to April while the wet season spans May to November. Fig. 2.2 (redrawn from NEDECO, 1993) summarises the mean monthly rainfall and Penman evaporation of Vietnam’s lower Delta. Also shown is and the mean monthly stream flow for the Mekong at Phnom Penh. The pronounced seasonality of the rainfall and stream flow is obvious in Fig. 2.2. Average annual temperature in the Delta is close to 28°C. Mean monthly temperatures run from 25°C in January through to high of around 28.9°C in April. The mean monthly relative humidity varies from a low of around 74% in the dry to 83% in the wet season. The marked seasonality is also reflected in the volume of Cambodia’s Great Lake in the upper Delta. Fig. 2.3, using the invaluable data of (Carbonnel and Guiscafre, 1963) shows the variation in the volume of the Lake. The Lake is a natural flow regulator for the lower Mekong acting as a flood storage in the wet season until early October and a supply reservoir in the dry as the Lake drains from October on. The magnitude of its importance as a flow regulator for the Delta can be judged from Figs 2.3 and 1.2.
There is also a marked spatial variation in annual rainfall across the Delta which depends on the direction of the monsoon in the southwest and an orographic influence in the north (Fig 2.4, Minh 1995). The length of the rainfall season is also spatially dependent (Fig 2.5, Minh, 1995). It varies from 4 months in the north, to 7 months in the southwest and also reflects the direction of the monsoon.
Fig. 2.4 Distribution of annual rainfall across the Mekong Delta (Minh, 1995).
2.4.1 Floods and seawater intrusion
The marked seasonality in rainfall leads to both annual floods and water shortages in the Basin. In the wet season almost 50% of the Delta is flooded (1,900 km2). The maximum depth of inundation in the wet season is principally governed by the topography of the basin, the influence of the upstream flow from the Mekong and Bassac Rivers and tidal inundation in the south (Fig. 2.6, Minh, 1995). A smaller influence from the spatial variation of rainfall is also evident in Fig. 2.6. In the northern part of the Delta, in the Plain of Reeds, inundation depths can exceed 4 m. The original rice production systems in the Delta took advantage of wet season flooding by using floating rice crops.
Fig. 2.5. Distribution of length of rainfall season over the Mekong Delta (Minh, 1995).
In the dry season, flow in the Mekong is insufficient to prevent saline intrusion and extensive salinization of waterways occurs in the lower Delta. Fig. 2.7 (Minh, 1995) shows the extent of salinity intrusion at the beginning of the saline intrusion (BSI) and the end of the saline intrusion (ESI). The whole of the Ca Mau Peninsula in the Delta’s southwest, in Fig. 2.7, is salinized for 6 months during the dry as there is insufficient freshwater flow in the Mekong to displace saline intrusion from the southwestern sector of the Delta. Figures 2.6 and 2.7 exemplify two of the main hydrologic problems of the Delta, wet season floods and dry season saline intrusion.
The length of the wet season is an important factor in rice production, particularly for double and triple cropping. The frequency of occurrence of early season and mid season drought is
also critical. The delay of the onset of the wet season (early season drought), and the occurrence of mid season drought are key determinants of rice production. These droughts are also spatially distributed across the basin (Fig 2.8, Minh 1995). Proposed dams on upper Mekong, especially at the Tonle Sap, are designed to address this problem and provide water resource security during dry seasons and droughts.
Fig. 2.6. Mean annual depths of wet-season flooding across the Mekong Delta (Minh, 1995).
2.4.2 Tidal influences
Streams and canals in the Mekong Delta are influence by the tides of both the East and West Seas. In the East Sea the tide is semidiurnal but irregular and has a large tidal amplitude of 3 to 3.5m. The regime has a 15 day cycle average tidal level has a maximum in December and a minium in July. The tidal effects from the East Sea propagate over much of the Delta through the main and farm canal systems. Farmers use these tidal fluctuations to drain and flood their lands. Drainage of floodwaters can be impeded if wet season floods coincide with the spring tide.
Tides in the West sea are diurnal with a tidal range of about 0.8 to 1.2m. Canals in the Ca Mau Peninsula are influenced by both East and West Sea tides simultaneously. This can lead to a dead water zone which can prevent water movement from the Bassac River into the Ca Mau area (Ghassemi and Brennan, 2000).
2.4.3 Seawater intrusion floodgates
Studies of seawater intrusion (Mekong Secretariat, 1992a; 1992b;1993) developed a model to predict seawater intrusion into the lower Delta. Following these major studies, a proposal was developed to install saline intrusion floodgates on main canals along the Ca Mau Peninsula and South China Coast (NEDECO, 1993). The idea behind this scheme is to lengthen the growing season for rice from one to two crops in the saline intrusion affected areas. The discharges of the Mekong River are considered adequate to meet irrigation demand in the protected areas during the early periods of the dry and wet season, thus lengthening the growing season. In the dry season, Mekong flows are insufficient for irrigation in the protected areas (NEDECO. 1993).
Fig 2.7. Seawater intrusion into the Mekong Delta during the dry season. BSI and ESI are respectively the beginning and end of the seawater intrusion (Minh, 1995).
A series of 12 massive sluices or tidal floodgates have been installed on the major rivers and canals connected to the East and West Seas (Fig. 2.9) in an effort to prevent seawater intrusion into the Ca Mau Peninsula. The project, called the Quan Lo Phung Hiep Project cost over $US 12B and included the dredging of over 250 km of secondary canals. The project commenced in 1992 and was completed in 2001. The sluice gates are between 5 and 25m wide. They open automatically on the ebb and close on the spring tide. The objective of the project was to permit two rice crops per year to be grown in the irrigation area behind the sluices (Fig. 2.9) by decreasing the salinity ingress into the area behind the floodgates and increasing the flow of freshwater from the Bassac River during the dry season.
