Vietnamnetherlands mekong delta masterplan project me kong delta water resources assessment studies

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Vietnam-Netherlands Mekong Delta Masterplan project MEKONG DELTA WATER RESOURCES ASSESSMENT STUDIES January 2011 TABLE OF CONTENTS TABLE OF CONTENTS....................................................................................................... 2 LIST OF FIGURES................................................................................................................ 4 LIST OF TABLES ................................................................................................................. 5 ABBREVIATIONS AND ACRONYMS................................................................................ 5 CHAPTER 1. DESCRIPTION OF THE MEKONG DELTA OF VIETNAM.................. 6 1.1. ADMINISTRATIVE OVERVIEW ................................................................................ 6 1.2. TOPOGRAPHY .............................................................................................................. 6 1.3. RIVER SYSTEM ............................................................................................................ 7 CHAPTER 2. HYDROLOGY AND SURFACE WATER RESOURCES....................... 10 2.1. CLIMATE AND EXPECTED CLIMATE CHANGE ................................................. 10 2.1.1. Air temperature........................................................................................................... 10 2.1.2. Evaporation................................................................................................................. 10 2.1.3. Air humidity ............................................................................................................... 10 2.1.4. Wind........................................................................................................................... 10 2.1.5. Rain ............................................................................................................................ 12 2.1.6. Climate change ........................................................................................................... 13 2.2. HYDROLOGICAL REGIMES .................................................................................... 15 2.3. SURFACE WATER QUANTITY ................................................................................ 16 2.4. FLOODING................................................................................................................... 17 2.5. SALTWATER INTRUSION ........................................................................................ 19 2.6. IMPACTS OF UPSTREAM DEVELOPMENTS ........................................................ 21 2.6.1. Current upstream flow................................................................................................. 21 2.6.2. Further data on the upstream flow of the Mekong river................................................ 23 2.7. POSSIBLE MEASURES TO IMPROVE THE SITUATION ..................................... 24 2.7.1. Integrated water resources planning for MDV and Decision No.84/2006/Q -TTg....... 24 2.7.2. Adaptation measures for climate change and sea level rise........................................... 24 CHAPTER 3. WATER QUALITY ................................................................................... 26 CHAPTER 4. HYDROGEOLOGY AND GROUNDWATER RESOURCES .............. 36 4.1. GEOLOGY.................................................................................................................... 36 4.1.1 Tectonics and faulting ..................................................................................................... 36 4.1.2 Stratigraphy .................................................................................................................... 36 4.2. HYDROGEOLOGY AND GROUNDWATER RESOURCES.................................... 36 4.2.1. The aquifer system in the Mekong Delta ........................................................................ 36 4.2.2. Groundwater quality ...................................................................................................... 39 2 4.2.3. Groundwater reserves..................................................................................................... 42 4.2.4. Present Groundwater utilization ..................................................................................... 43 CHAPTER 5. WATER DEMAND AND WATER BALANCE ....................................... 45 5.1. METHODOLOGY........................................................................................................ 45 5.2. RURAL, URBAN AND INDUSTRIAL WATER DEMAND....................................... 45 5.3. WATER FOR NAVIGATION...................................................................................... 48 5.4. PRESENT WATER BALANCE................................................................................... 48 CHAPTER 6. ISSUES TO BE SOLVED .......................................................................... 51 CHAPTER 7. _Toc283708758CONCLUSIVE REMARKS ........................................... 53 References .......................................................................................................................... 55 APPENDIX 1;Water demand assessment source materials............................................. 56 APPENDIX 2; Description of aquifer systems in the Mekong Delta .............................. 59 3 LIST OF FIGURES Figure 1: Administrative map of the Mekong Delta of Vietnam...................................6 Figure 2: Topographic map of the Mekong Delta.........................................................7 Figure 3: Map of existing river/canal system of the Mekong Delta in Vietnam ............9 Figure 4: Map of hydro-meteorological stations.........................................................11 Figure 5: Spatial distribution of annual rainfall ..........................................................12 Figure 6: Flooding extent in the Mekong Delta with sea level rise 75cm....................15 Figure 7: Variation of flood levels 2000 at Tân Châu and Châu c..........................17 Figure 8: Spatial distribution of maximum flood flow and total volume of the flood 2000 (using VRSAP model simulation) ...................................................18 Figure 9: Salinity intrusion isolines in some dry years ...............................................20 Figure 10: Water works development plan for Mekong Delta ....................................25 Figure 11: Water quality monitoring network ............................................................27 Figure 12: pH in 2008 at some stations ......................................................................30 Figure 13: pH in different water resources in 2002-2008 ...........................................30 Figure 14: EC in fields and main streams in 2008 ......................................................31 Figure 15: Salinity at M Tho, 2002-2008 ................................................................31 Figure 16: TSS in fields and rivers in 2008 (right) and TSS in rivers in 2002- 2008 (left) ..........................................................................................................32 Figure 17: T-N in river courses and canals in (data for 2008).....................................32 Figure 18: NH4+ and NO2&3- (data for 2002-2008) ....................................................33 Figure 19: T-N in 2002-2008 & T-P in 2002-2008....................................................33 Figure 20: BOD5 and COD (data for 2008). ..............................................................34 Figure 21: BOD5 in 2002-2008 & COD in 2002-2008 .............................................34 Figure 22. Cross –section III-III ................................................................................37 Figure 23: Map of 120 sub-irrigation areas ................................................................47 4 LIST OF TABLES Table 1: Rainfall frequency all the Mekong Delta................................................. 13 Table 2. The change of climate and sea level in southern Vietnam up to 2100 by an average emission scenario (B2)............................................................................. 14 Table 3. Level of monthly temperature change (°C) in southern Vietnam up to 2100 by an average emission scenario (B2) ........................................................................ 14 Table 4. Average monthly rainfall change (%) in southern Vietnam up to 2100 by an average emission scenario (B2)............................................................................. 14 Table 5: Total flow volumes in the Mekong Delta at Tan Chau and Chau Doc stations (unit: Million m3). ................................................................................................ 16 Table 6. The Mekong river flow at some locations................................................ 22 Table 7. Comparision of recent flow with previous flow at Pakse (unit: m3/s) ...... 22 Table 8. Comparision of recent flow with previous flow at Kratie (unit: m3/s) ..... 22 Table 9. List of hydropower projects upstream Mekong basin .............................. 23 Table 10: List the standard method of testing water quality .................................. 28 Table 11: Coliform in 2008 (MPN/100 ml)........................................................... 35 Table 14. Results of calculation of static reserves (m3/day)................................... 42 Table 15. Results of calculation of dynamic reserves (m3/day).............................. 42 Table 16: Groundwater utilization in the Mekong Delta........................................ 44 Table 17: Monthly water flow demand ................................................................. 46 Table 18: Monthly water volume demand ............................................................. 46 Table 19: Water requirements for navigation/waterway transportation................. 48 Table 20: Present water balance............................................................................ 49 ABBREVIATIONS AND ACRONYMS POR Plain of Reeds LXQ Long Xuyen Quadrant CMP Ca Mau Peninsula MD Mekong Delta of Vietnam TCVN Vietnamese Standard 5 CHAPTER 1. DESCRIPTION OF THE MEKONG DELTA OF VIETNAM 1.1 ADMINISTRATIVE OVERVIEW The Mekong Delta of Vietnam (MD) is formed by the lower part of the Mekong river delta, and includes 13 cities and provinces of Long An, Tien Giang, Dong Thap, Vinh Long, Tra Vinh, Can Tho, Hau Giang, Soc Trang, Ben Tre, An Giang, Kien Giang, Bac Lieu and Ca Mau. The total natural area comprises approximately 3.96 million hectares (excluding Duc Hoa District Long An Province and Phu Quoc island province Kien Giang), accounting for 79% of the whole MD and forming 5% of the Mekong River basin. The Mekong Delta of Vietnam is surrounded by: (a) Vietnam-Cambodia border in the North; (b) Pacific ocean / South China Sea to the East (the so-called East sea), (c) Gulf of Thailand in the West (the so-called West sea), and (c) Vam Co Dong River and Ho Chi Minh City in the North-West (Figure 1). Figure 1: Administrative map of the Mekong Delta of Vietnam 1.2 TOPOGRAPHY The Mekong Delta of Vietnam consists of flat terrain, mostly of average height of 0.7 to 1.2 m, except for some high hills in the northern delta province of An Giang (Map 1/25000, Ministry of Water Resources, 1984 and also in the 2003 Digital Terrain Model of the Mekong delta). Along the Cambodian border, the terrain is highest, from 2.0 to 4.0 m above sea level, then lower to the central plains, from 1.0 to 1.5 m high, and only 0.3 to 0.7 m in the tidal and coastal areas. 6 Figure 2: Topographic elevation map of the Mekong Delta 1.3 RIVER SYSTEM The Mekong Delta river system comprises a relatively dense network of river courses and canals, including the natural river systems and canals: - The main natural river branches and canals in the Mekong delta are formed by the two systems of the Tien River and Hau River (respectively the lower branches of the Mekong and Bassac rivers). These rivers flow to the sea in estuaries via nine river mouths as Tieu, Dai, Ba Lai, Ham Luong, Co Chien , Cung Hau, Dinh An, Ba Thac and Tranh De (river mouths in the territory of Ba Thac in Soc Trang province have been covered in the 1970’s) and a short river Vam Nao river linking the Tien and Hau 7 main branches. Vam Co River (including the Vam Co Dong-Vam Co Tay) runs parallel to the east of the Tien River, Cai Lon-Cai Be River, My Thanh, Ganh Hao, Ong Doc, Bay Hap rivers flow to the West and East Sea. The Tien and Hau rivers transfer the largest amounts of water with a total annual flow of 325.41 billion m3 observed at station Tan Chau (on the Tien river) and 82.43 billion m3 in station Chau Doc (on the Hau river); the flow rate on the Tien River / Hau River is 80/20. Both the Tien river and Hau river are wide and deep, with the average width of about 1000-1500 m and an average depth of 10-20 m (and locations where the depth is over 40 m). However, near the mouth, the river widens and the riverbed is raised by siltation. Within the two rivers courses many elongate islands have formed. River processes cause shore erosion and sedimentation and complex flow patterns cause instability of river banks. Vam Co River system consists of two branches (Vam Co Dong and Vam Co Tay), that originate in Cambodia, and flow east through the Mekong Delta. - The Cai Lon-Cai Be are tidal rivers, derived from the center of the Ca Mau peninsula and flow to the sea through the Cai Lon river mouth. The estuary is very wide but not deep. - The system of manmade canals in the Mekong delta was constructed primarily during the past century, with the primary purpose to develop agriculture and transportation. Until now, the canal system has developed into a dense network with 3 levels of major, primary and secondary canals. The primary and secondary canal systems have a high density, with some 80-10 m / ha, and a total of 30,000-40,000 km of canals in all the Mekong Delta. Figure 3 below illustrates the density of the irrigation systems. 8 Figure 3: Map of existing river/canal system of the Mekong Delta in Vietnam 9 CHAPTER 2. HYDROLOGY AND SURFACE WATER RESOURCES 2.1 CLIMATE AND EXPECTED CLIMATE CHANGE The Mekong Delta, being located in a tropical monsoon region, is hot year-round and has a seasonal distribution of dry-wet months depending on the operation of the monsoon. The dry season usually coincides with the period of control of the NorthEast monsoon that lasts from November to April, and the weather is characterized by dry heat and little rain. The wet season coincides with the period of control of the South-West monsoon that lasts from May to October, and the climate is characterized by hot, humid, and (high) rainfall. Specific features are as follows: 2.1.1 Air temperature The average temperature in January varies from about 27-28oC. May is the month with the lowest temperature (an average of 25.5oC) and the hottest month is IV (28oC). There is a relative equal distribution of temperatures across the delta region. 2.1.2 Evaporation The evaporation regime also changes little over time and space. In terms of time in the year evaporation is highest in the months III, IV and V. The highest amounts for these months vary around 180-220 mm. As soon as the rain starts in months VIII to IX, lower evaporation is reached, from 100-150 mm. 2.1.3 Air humidity Relative humidity reaches high values in May and decreases towards the dry season. Average humidity in months VIII, IX and X ranges from 84-89%, while in months II and III it ranges from 67-81%. 2.1.4 Wind The winds in the north-east of the Mekong delta are prevalent during the dry season, from months XII-IV and in the south-west prevalent during the rainy season, (months V-X). Average wind speeds are about 2.0 m/s. General, closer to the sea, wind speeds often increase in months I, II and III. Wind speeds in low-pressure periods and storms can reach 15-18 m/s (with a storm number of 5 in 1997). Sunshine hours are on average 6 hours per day (approximately 2,000-2,500 hours per year). Months II, III have the highest number of sunshine hours, with 8-9 hours a day, while months VIII, IX have less hours of sunshine, with an average 4.6 to 5.3 hours per day. 10 Figure 4: Map of hydro-meteorological stations. Monitoring data are available on monthly temperature T, humidity H, wind speed W, Solar radiation S, water evaporation E. Calculation of potential evaporation (ETo) is on the basis of these meteorological data. 11 2.1.5 Rain The Mekong Delta has an average rainfall of approximately 1800 mm, but with an uneven distribution both in space and time. The western region has the most rainfall with annual average from 2000-2400 mm, while the east has from 1600-1800 mm rainfall on average. The central plains stretching from Long Xuyen, Chau Doc-Can Tho to Tra Vinh - Cao Lanh - Go Cong have the lowest rainfall, with averages of 1200-1600 mm. The amount of rain is very unevenly distributed over the year. Approximately 90% of annual rainfall is concentrated in the rainy months. Rainfall in the dry season accounts for only 10%, with months I, II, III having almost no rain (often triggering severe droughts). In the rainy season occasionally, there is continuous rain, which may last for 3-5 days, with a relatively large amount of rain, causing flooding and an increase in water levels. Figure 5: Spatial distribution of annual rainfall 12 Rainfall data: Data on monthly rainfall are available for 23 years in 42 stations distributed evenly throughout the region. Calculated statistical mean ( ) and standard deviation ( ) of monthly rainfall time series are often formulated in a probability distribution Log-normal R = ex, z, where R is the rainfall, x= +z , the z value by 0.8416, or zero or +0.8416 corresponding value R with a frequency of 80% (in low water) or 50% (medium water) or 20% (much water). There are no data gaps in rainfall time series, except for a few month in 2-3 years of U Minh and Ganh Hao stations. By calculating the average arithmetic average of rainfall stations in the whole Mekong Delta region reached approximately 1660 mm, in a wet year the frequency of 20% of annual rainfall reaches 1955 mm (up 17% compared to average), in dry years it only reaches 1410 mm (down 15%). Table 1: Rainfall frequency all the Mekong Delta Unit: mm Month Annual Freq. I II III IV V VI VII VIII IX X XI XII total 80% 2.5 2.5 3.9 12.5 94.5 140.0 146.1 150.8 171.9 198.0 51.4 8.2 1410.8 50% 7.1 7.9 13.5 36.0 152.4 196.6 220.3 214.4 239.3 278.6 104.4 24.3 1659.3 20% 26.0 29.1 51.6 111.3 251.0 278.1 336.6 306.7 334.9 394.6 222.9 77.2 1955.6 2.1.6 Climate change Climate change is one of the biggest challenges for mankind in the 21st century. Climate change will seriously affect the production, life and the environment throughout the world. Temperature increases, sea level rises causing flooding, salt water intrusion and detrimental effects on agriculture. All this creates a substantial risk for the industrial and socio-economic system in the future. In Vietnam during the last 50 years, average annual temperatures have risen about 0.5 to 0.7°C, sea level rise was about 20 cm. Climate change has caused disasters, particularly typhoons, floods and more intensive droughts. The Ministry of Natural Resources and Environment (MONRE) studied possible climate change and sea level rise scenarios for Vietnam. Under these scenarios, the climate in all regions of Vietnam will undergo changes. In the late 21st century, average temperatures in Vietnam could have increased by about 2.3°C; total annual rainfall and wet season rainfall may have increased while dry season rainfall will have decreased. A sea level rise is anticipated of about 75cm. Scenarios of global greenhouse gas emission levels were selected to calculate the alternative climate change scenarios (A2, B2=average) for Vietnam. The figures represent the calculated changes compared to the figures over the period 1980-1999. 13 Table 2. The change of climate and sea level in southern Vietnam up to 2100 by an average emission scenario (B2) Milestones in the 21st century Parameters 2020 2030 2040 2050 2060 2070 2080 2090 2100 Temperature (oC) increase 0.4 0.6 0.8 1.0 1.3 1.6 1.8 1.9 2.0 Rainfall increase (%) 0.3 0.4 0.6 0.7 0.8 0.9 1.0 1.0 1.0 Sea level rise (cm) 12 17 23 30 37 46 54 64 75 Table 3. Level of monthly temperature change (°C) in southern Vietnam up to 2100 by an average emission scenario (B2) Months Milestone in the 21st century 2020 2030 2040 2050 2060 2070 2080 2090 2100 XII-II 0.3 0.5 0.6 0.8 1.0 1.3 1.5 1.5 1.7 III-V 0.4 0.6 0.8 0.9 1.2 1.4 1.7 1.8 1.9 VI-VIII 0.5 0.7 0.9 1.2 1.5 1.8 2.0 2.1 2.1 IX-XI 0.5 0.6 0.9 1.2 1.4 1.8 1.9 2.1 2.3 Table 4. Average monthly rainfall change (%) in southern Vietnam up to 2100 by an average emission scenario (B2) Months Milestones in the 21st century 2020 2030 2040 2050 2060 2070 2080 2090 2100 XII-II -3.0 -4.4 -6.2 -8.1 -8.7 -11.4 -12.8 -14.2 -15.4 III-V -2.8 -4.1 -5.8 -7.5 -9.1 -10.6 -12.0 -13.2 -14.3 VI-VIII 0.3 0.5 0.6 0.9 1.1 1.2 1.4 1.5 1.6 IX-XI 2.6 3.8 5.3 6.8 8.3 9.6 10.9 11.9 13.0 Based on scenarios of sea level rise, inundation maps have been constructed, the first step is for the area of the Mekong delta based on the topographic map scale 1/2000 and 1/5000 and the digital elevation model with a 5x5m resolution for the whole region. Average current sea levels were calculated based on the measured tidal data at Vung Tau (period 1979-2007). An inundation area map of the Mekong delta for 75 cm of sea level rise is shown in the figure below. 14 Figure 6: Flooding extent in the Mekong Delta with a sea level rise 75cm 2.2 HYDROLOGICAL REGIMES Hydrological regimes in the Mekong Delta are affected directly by the river flow, the tidal regime of the East Sea (South China Sea) and for some parts of the delta by the tidal regime in the Gulf of Thailand (West Sea). The East Sea has a semi-diurnal and irregular sea-tide regime, while the West Sea is diurnal. Based on the influence of these diverse tidal patterns and cycli, the Mekong Delta can be divided into three different regions hydrologically. These are: (a) the northern plains, including sections of the province of An Giang and Dong Thap, an area about 300,000 ha), where the impact of the river floods is dominant; (b) an area with combined river flood-tidal impacts; this region is bound by the Cai Lon river-Xeo Chit channel Lai Hieu canal - Mang Thit river and Ben Tre-Cho Gao canals with an area of about 1.6 million ha), and (c) the coastal delta regions with direct influence of the primary tides; this includes the entire coastal region of the East Sea, with an area of about 2.0 million ha). Seasonal flooding in the Mekong Delta usually begins in months VI-VII and ends in months XI- XII, with an average peak flow entering the delta of around 28,000-30,000 m3/s. This is followed by a seasonal average dry flow of about 3,000-5,000 m3/s. Both high and low flood regimes prevail for about 6 months. 15 Total average annual river flow in the Mekong Delta in the upstream parts of the Tien and Hau rivers is ~408 billion m3 (according to data measured from 2000 to 2008 at Tan Chau and Chau Doc stations). With flooding in the flood season the border region accounts for 14-18% of the total flood into the territory of Vietnam (estimated 57 billion m3) and surface water by rain on the plains accounts for 11% of the whole water volume (estimated 45 billion m3). 2.3 SURFACE WATER QUANTITY Firstly, stations in main tributaries (stations on the river) or primary river/canals (tributary channels) are identified. Statistical analysis can be performed on the monthly flow data of stations to obtain delta-wide hydrological characteristics. In the Mekong Delta there are only five stations along the main river courses i.e. on the Tien river (Tan Chau and My Thuan stations), the Hau river (station Chau Doc and Can Tho) and on the Vam Nao river (Vam Nao station). Vam Nao river is the short connection between the Tien River and Hau River in An Giang province. Data time series for this area are short; stations My Thuan and Can Tho have data continuously from 2003 to 2006, station Tan Chau, Chau Doc and Vam Nao have data continuously from 2000 to 2007. Stations Tan Chau and Chau Doc are most important for measuring the flow of the entire basin, viz. the upstream Mekong River, to the Mekong Delta. Station Vam Nao quantifies flow from the Tien river to the Hau river. Stations My Thuan and Can Tho quantify flow of the two rivers Tien and Hau river after flowing through the Plain of Reeds area (POR) and Long Xiuyen Quadrant (LXQ), and after distribution of water through main irrigation channels in the POR and LXQ regions. Stations My Thuan and Can Tho are also influenced by sea tide. The Mekong Delta has some local hydrological stations that measure the water level in the regular channels and other stations measuring water in coastal estuaries. Estuarine coastal stations and local stations in the coastal provinces measure salinity. However, all the local stations and the estuarine stations do not consistently measure the flow. Therefore, within the delta sub-regions are considered as secondary basins; these are: Plain of Reeds (POR), Long Xuyen Quadrant (LXQ), the Middle Plains, between the courses of the Tien and Hau rivers, and the Ca Mau Peninsula (CMP). Since comprehensive quantification of local flow data for these sub-areas is not possible, a quantitative analysis is performed by means of a river hydraulic model simulation. Table 5: Total flow volumes in the Mekong Delta at Tan Chau and Chau Doc stations (unit: Million m3). Freq. Month Annual 16 average I II III IV V VI VII VIII IX X XI XII P80% (dry) 17329 9577 6918 5989 8056 16257 32953 55687 62951 63397 42290 28527 366162 P50% (AVR) 20536 11844 8390 6924 10328 22699 42177 62745 68040 66297 47917 33353 405207 P20% (Wet) 24338 14648 10174 8006 13243 31693 53984 70697 73540 69331 54293 38996 448415 2.4 FLOODING Every year, the overflowing of the Mekong River floods a large area in the northern part of the Mekong Delta. Floods may reach an area of about 1.2 to 1.4 million ha in a regular flood and even 1.7 to 1, 9 million ha in a major flood, with a depth from 0.5 to 4.0 m and the time from 3-6 months. Flooding in the Mekong Delta can be divided into three periods. Early flood season (months VII-VIII): the main rivers flood quickly and the rivers flowing into canals to fields distribute the floodwaters. During this flood much silt is brought that forms the main source for rice field nutrients in the flood season. Figure 7: Variation of flood levels 2000 at Tân Châu and Châu c 17 Figure 8: Spatial distribution of maximum flood flow and the total volume of the flood in the year 2000 (using VRSAP model simulation). 18 A second flooding period is when floodwaters reach high levels (exceeding 4.0 m at Tan Chau, and Chau Doc exceeding 3.8 m). These floodwaters enter the delta from two directions a) perpendicular from the main river courses and b) from the VietnamCambodia border region directly. The border flows spill over after flooding and silt deposition in the most flooded areas of Cambodia, and subsequently overflow into the POR and LXQ The third period is the period when the flood is in recession, usually starting by the end of October when the flow spilling from Cambodia has decreased, down the Mekong delta flood water recedes gradually until December According to the classification of the National Center for Meteo-Hydrological Forecasting, the water levels at Tan Chau for a low flood are: 4.0 m (warning level I), 4.0 to 4.5 m; for an average flood (warning level II) and for a major flood >4.5 m (warning level III), with a corresponding flood frequency of 13.2%, 46.2% and the average large flood 40.6%. Statistics show that in 60 years, on average every 2 years there is a flood alarm for warning level III (at Tan Chau the water level is over 4.5 m). 2.5 SALTWATER INTRUSION Saltwater intrusion in the Mekong Delta is a complex process, depending on the magnitude of the floods, the ability to supply fresh water from upstream during the dry season, summer-autumn paddy production status and timing of the rainy season. In general, with late monsoon rains and with water volumes from upstream to just below 70% of the average, the salt will intrude very far inland, like for example in 1977, 1993, 1998 and 2004-2005. Every year, the highest salinities occur late in the dry season, usually in April, sometimes in early May. With salinity 1g/liter, the length of the salinity intrusion is 40 - 50km upstream, the less likely on the Mekong River branches and higher on the Vam Co River. With the start of the flood season, flood waters from upstream push the salt back to the estuaries. In the mid-flood season (Month IX, X), salt 1g/liter is usually located in the estuaries only. In the Plain of Reeds salt intrusion progresses through the Tien River and Vam Co Tay River. Vam Co Tay has no resources upstream, and salt water may intrude far. Salinity also depends on the Tien River flows from the upstream Mekong River. Years of often large flood flows may push the salt intrusion outwards. In contrast, after a small flood the salt intrusion may reaches far upstream the rivers and canals. In the Ca Mau Peninsula, the salt water intrusion is extremely serious and most complex in the Mekong Delta. The land area is bordered on two side by the East Sea and West Sea respectively. Two different tidal regimes affect the river flow in the canal system and restrict the transfer of fresh water from the Hau river towards the deeper interior fields. The Long Xuyen Quadrant area is directly affected by salt water from the West Sea. The main canals in the Long Xuyen Quadrant from Tri Ton to Cai San take water from the Hau river and drain this out to sea. The West sea tide has only a small amplitude. 19 Most of the west coast channels have salinity control gates, but Vam Rang and Ha Giang channels are still open enabling saltwater intrusion. Figure 9: Salinity intrusion isolines in some dry years 20
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