Quang M. Nguyen
August 8, 2020
The Tonle Sap is a unique tributary of the Mekong river, connecting the largest freshwater lake in Southeast Asia of the same name with the Mekong river at Phnom Penh, the capital of Cambodia. Normally, water flows from the Tonle Sap lake to the Mekong river. But during the wet season, when water levels in the Mekong river are high enough, the Tonle Sap reverses its flow and allows water from the Mekong river to fill up the Tonle Sap lake, expanding its surface area from 2,500 km2 in June to about 16,000 km2 by the end of October or beginning of November . Areas surrounding the lake flooded with high water become spawning and breeding grounds for fish, producing an annual catch from 177,000 to 252,000 tons . The Tonle Sap lake acts as a natural reservoir for the Mekong system that regulates the flood flows downstream of Phnom Penh and provides supplement water for the Mekong Delta in Vietnam (MDV) dung the dry season.
Last year, a combination of climate change, El Niño and dams on the Mekong mainstream and tributaries caused the Tonle Sap to reverse its flow lately, in August instead of June, and lasted only 6 weeks instead of months . This year, the Tonle Sap may delay its flow reversal again because the water levels in the Mekong river at Stung Treng, Kratie, Kampong Cham and Neak Luong are still below their minimum levels recorded between 1960 and 2019, according to the Mekong River Commission (MRC) . These delays have caused some experts to fear that the Tonle Sap may not reverse its flow at all. This would not only reduce permanently the fish catch, but also threaten the entire ecosystem of the Tonle Sap  and the MDV.
The 1995 Mekong Agreement
The flow reversal in the Tonle Sap is an important hydrological event that was specified in Articles 5, 6 and 26 of the 1995 Mekong Agreement .
Article 5. Reasonable and Equitable Utilization. To utilize the waters of the Mekong River system in a reasonable and equitable manner in their respective territories, pursuant to all relevant factors and circumstances, the Rules for Water Utilization and Inter-basin Diversion provided for under Article 26 and the provisions of A and B below:
A. On tributaries of the Mekong River, including Tonle Sap, intra-basin uses and inter-basin diversions shall be subject to notification to the Joint Committee.
B. On the mainstream of the Mekong River:
1. During the wet season:
a) Intra-basin use shall be subject to notification to the Joint Committee.
b) Inter-basin diversion shall be subject to prior consultation which aims at arriving at an agreement by the Joint Committee.
2. During the dry season:
a) Intra-basin use shall be subject to prior consultation which aims at arriving at an agreement by the Joint Committee.
b) Any inter-basin diversion project shall be agreed upon by the Joint Committee through a specific agreement for each project prior to any proposed diversion. However, should there be a surplus quantity of water available in excess of the proposed uses of all parties in any dry season, verified and unanimously confirmed as such by the Joint Committee. An inter-basin diversion of the surplus could be made subject to prior consultation.
Article 6. Maintenance of Flows on the Mainstream. To cooperate in the maintenance of the flows on the mainstream from diversions, storage releases, or other actions of a permanent nature; except in the cases of historically severe droughts and/or floods:
A. Of not less than the acceptable minimum monthly natural flow during each month of the dry season;
B. To enable the acceptable natural reverse flow of the Tonle Sap to take place during the wet season; and,
C. To prevent average daily peak flows greater than what naturally occur on the average during the flood season.
The Joint Committee shall adopt guidelines for the locations and levels of the flows, and monitor and take action necessary for their maintenance as provided in Article 26.
Article 26. Rules for Water Utilization and Inter- Basin Diversions. The Joint Committee shall prepare and propose for approval of the Council, inter alia, Rules for Water Utilization and Inter-Basin Diversions pursuant to Articles 5 and 6, including but not limited to: 1) establishing the time frame for the wet and dry seasons; 2) establishing the location of hydrological stations, and determining and maintaining the flow level requirements at each station; 3) setting out criteria for determining surplus quantities of water during the dry season on the mainstream; 4) improving upon the mechanism to monitor intra-basin use; and, 5) setting up a mechanism to monitor inter-basin diversions from the mainstream.
Hydrology of the Tonle Sap system 
The Tonle Sap system has two hydrological features. First, the Tonle Sap lake is the largest freshwater lake in Southeast Asia and acts as a natutal reservoir that may hold up to 20% of the Mekong flow during the wet season. Second, the Tonle Sap, a tributary connecting the Tonle Sap lake with the Mekong at Phnom Penh, reverses its flow seasonally. The flow reversal in the Tonle Sap is unique and caused by the differences between the water levels in the Mekong river and the Tonle Sap lake. On the average, there is about 43 km3 water flowing from the Mekong river annually, accounting for 52% of the system water (35% from tributaries and 13% from rainfall).
Based on the discharges measured at Prek Kdam on the Tonle Sap from 1962 to 1972, the flow reversal in the Tonle Sap has the following features:
▪ On average, the first annual reversal occurs in the first week of June, though it can take place one month earlier or later, reflecting the character and timing of the onset of the Mekong wet season proper. Peak discharges towards the lake usually occur in late August;
▪ This lake inflow phase generally lasts for four months, typically ending at the end of September, though again there is some latitude, of the order of plus/minus two weeks;
▪ Maximum contributions from the lake to the Mekong mainstream take place during November;
▪ In many years the change from the lake inflow to outflow phase is precipitous with a switch of up to 7,500 m3/s from one direction to the other over only a few days; and
▪ March, April and May are characterised by flows towards the Mekong River of only a few hundred m3/s, reflecting dry season conditions and low head differentials.
The effects this natural regulation of the Mekong flood or the temporary storage of flood water in the Tonle Sap lake are positive:
▪ Peak rates of flood discharge in the Mekong are attenuated, thus reducing the severity and extent of potential flood inundation in the Mekong Delta; and
▪ During the first half of the dry season months in particular, the release of water from the Tonle Sap system increases flows downstream of Phnom Penh such that the volumes and reliability of resources available for irrigation are significantly augmented and low flow conditions that are favourable to saline intrusion in the MDV are not as frequent as they would be without this natural flow regulation.
▪ August is typically the month of maximum discharge upstream of the confluence at Phnom Penh Port, with a mean monthly discharge of 32,000 m3/s. Downstream at Tân Châu (in the Mekong River) and Châu Đốc (in the Bassac River) in the delta, mean flows at this time are reduced by 5,000 m3/s and the month of maximum discharge delayed to September due to the attenuation effects of storage in the Tonle Sap system; and
▪ Flows during the dry season between November and February are increased significantly.
In addition, the natural regulation of the Tonle Sap system causes the flood levels in the MDV to rise slowly (less than 10 cm per day) and can be predictable; therefore, residents in the flooded areas have adequate time to prepare.
Maintenance of Flows on the Mekong Mainstream
Regulations for the maintenance of flows on the Mekong mainstream are presented in details in an MRC report entitled Technical Guidelines on Implementation of the Procedures for the Maintenance of Flows on the Mainstream – Working Version . The Technical Guidelines includes the following important parts:
The 1995 Mekong Agreement identifies the acceptable minimum monthly natural flows; the acceptable natural reverse flows; historically severe droughts and/or floods; the dry, wet and flood seasons.
The acceptable minimum monthly natural flows are the acceptable minimum monthly natural flows drung each month of the dry season. The acceptable natural reverse flows are the wet season flow levels in the Mekong river at Kratie that allows the reverse flows of the Tonle Sap river to an agreed upon optimum level of the Tonle Sap lake.
Historically severe droughts occur when the daily updated flows (observed water levels or rated discharge) during the dry season are less than the lower bound of the ARI 1:20 of the historically observed daily flows (usually the period of 1960-2009) at the selected hydrological stations, using smoothed values. Historically severe floods occur when the daily updated flows (observed water levels or rated discharge) during the flood season are higher than the upper bound of the ARI 1:20 of historically observed annual daily peak flows (usually the period of 1960-2009) at the selected hydrological stations.
The dry season lasts 6 months from December 1st through May 31st. The wet season lasts 6 months from June 1st through November 30th. The flood season lasts 4 months from July 1st through October 31st.
Locations of the hydrological stations
In accordance with Article 26.2 of the 1995 Mekong Agreement, nine hydrological stations are selected as the basis for the maintenance of flows on the mainstream. They include Chiang Saen, Vientiane, Khong Chiam, Pakse, Stung Treng, Kratie, Tân Châu and Châu Đốc. Phnom Penh Port, Prek Kdam and Kampong Luong in the Tonle Sap system are selected for the flow reversal in the Tonle Sap.
Determination of flows
Article 6.A – Acceptable minimum monthly flows for each month during the dry season (from December to May): Applied for all hydrological stations, except Kampong Luong and PrekKdam. Agreed to test two options: ARI 1:4 to 1:5 or 80-90% FDC.
Article 6.B – Acceptable reversal flows in the Tonle Sap during the wet season (from June to Novembet): Applied for Kratie on the Mekong and Prek Kdam and Kampong Luong on the Tonle Sap system. The flow volumes during the wet season at Kratie are to be maintained within the upper and lower 90% Confidence Interval (= thresholds band) of probability of exceedance of the Baseline Scenario.
Article 6.C – Prevent average daily peak flows greater than what naturally occur on the average during the flood season (from July to October): Applied for all hydrological stations, except Prek Kdam and Kampong Luong. Mean daily peak flows drung the flood season (from July to October)
Shortcomings in the MRC Technical Guidelines
The first shortcoming is the acceptable minimum monthly natural flows that are defined as the acceptable minimum monthly natural flows during each month of the dry season from December through May. However, the Mekong river in northeast Thailand dropped to the record low levels during the wet season in 2019 and low water levels are observed along the river this year. For that reason, the definition for the acceptable minimum monthly natural flows should be extended to include the wet season months.
The second shortcoming is the complexity and difficulty in using the flow volumes during the wet season at Kratie and accumulated reverse flows at Prek Kdam to determine the flow reversal in the Tonle Sap during the wet season. The flow reversal in the Tonle Sap can be simply and easily determined by using the water levels at Phnom Penh Port, Prek Kdam and Kampong Luong.
The third shortcoming is not taking into consideration flows from the tributaries, especially those with operational dams. The most important is the amount of water released from the most downstream dam on the tributary, such as Nam Ou 1, Nam Ngum 1, Pak Mun and Lower Sesan 2.
The last, but not least, shortcoming is the maintenance of flows of the Mekong river during the flood season (from August to November) at Tân Châu and Châu Đốc to maintain a “moderate flood” in the MDV, if the rainfall in the Mekong basin is favorable. Known as “floating water” by the MDV residents, it can be maintained by allowing the daily water levels at Châu Đốc to fluctuate between the warning level of 3,80 m and the flood level of 4,20 m . The benefits of the floating water have long been recognized by the MDV communities and were verified recently by MRC, with an average annual cost of floods in the Lower Mekong Basin ranging from US $60 to 70 milion and an average annual value of flood benefits ranging from US $8 to 10 billion .
Summary and Conclusion
The Tonle Sap is a unique tributary of the Mekong river that connects the Tonle Sap lake with the Mekong river at Phnom Penh, the capital of Cambodia. The Tonle Sap is the only river in the world with seasonal flow reversal. During the dry season, water flows from the Tonle Sap lake to the Mekong river. During the wet season, when the water levels in the Mekong river are high enough, water from the Mekong river flow into the Tonle Sap lake, expanding its surface area six times and providing ideal spawning and breeding grounds for various fish species. The Tonle Sap lake acts as a natural reservoir regulating the Mekong floods, making these floods moderate and providing supplemental water for the MDV during the dry season.
In 2019, the Tonle Sap delayed its flow reversal and lasted only 6 weeks instead of months. This year, according to the MRC, the Tonle Sap may delay its flow reversal again. Some experts fear that the Tonle Sap may not reverse its flow, threatening the fisheries of the Tonle Sap lake, and in turn, jeopardizing the MDV.
While the 1995 Mekong Agreement contains provisions for the flow reversal in the the Tonle Sap, the MRC technical guidelines for implementation of the provisions are complex and inadequate. To improve the usefulness of the provisions in the 1995 Mekong Agreement, the technical guidelines should be revised to 1) extend the acceptable minimum monthly natural flows to the wet season, 2) use the water levels at Phnom Penh Port, Prek Kdam and Kampong Luong to monitor the flow reversal in the Tonle Sap, 3) take into consideration flows from the main tributaries with operational dams, and 4) maintain moderate floods or floating water in the MDV, a hydrological event as important as the flow reversal in the Tonle Sap.
About the author
Quang M. Nguyen was a professional engineer of the States of Florida and California. He worked for the National Water Resources Commission in Saigon, Vietnam; the Broward County’s Water Resources Management Division in Florida; and the Stetson Engineers Inc. in Los Angeles County, California, specializing in water resources and groundwater contamination. He retired in 2016.
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