Securing water resources amid El Niño: [4] Mekong Delta water reservoir

Long Xuyen: Mekong Delta’s strategic 'water reservoir'

If the prevailing approach in the past was to combat drought and prevent saltwater intrusion at all costs, recent years have seen a gradual shift toward adapting to natural conditions, adopting integrated water resource management, and using water resources more efficiently. This is increasingly becoming the guiding approach across the region.

Against this backdrop, the Long Xuyen Quadrilateral is regarded as one of the areas playing a particularly important role in ensuring water security for the Mekong Delta.

The Long Xuyen Quadrilateral spans An Giang Province and Can Tho City, covering nearly 600,000 hectares of natural area. It is a major rice-producing and freshwater aquaculture region in the Mekong Delta, with annual rice output of nearly 5 million tonnes.

According to Nguyen Huu Thien, an ecological researcher specializing in the Mekong Delta, nature has long endowed the delta with three giant water reservoirs: Tonle Sap Lake in Cambodia, the Long Xuyen Quadrilateral, and the Plain of Reeds. Together, they form a natural hydrological regulation system that helps reduce flooding during the rainy season and replenish river flows during the dry season.

During the annual flood season, when water levels in the Mekong River rise, Tonle Sap Lake can expand from around 2,500 sq km to as much as 12,000 sq km, storing up to 60 billion cubic meters of water. After being absorbed by the lake, the water continues downstream and spreads into the Long Xuyen Quadrilateral.

On the right bank of the Hau River lies the nearly 600,000-hectare low-lying floodplain of the Long Xuyen Quadrilateral. When water from Cambodia and the Hau River flows into the area, water levels can reach 2.5–3 meters, temporarily storing more than 9 billion cubic meters of water, along with large quantities of sediment and natural aquatic resources.

Together with the Plain of Reeds, which covers around 700,000 hectares, these two low-lying areas function as enormous “storage basins” for the delta. Their ability to absorb and regulate floodwaters significantly reduces flood pressure on downstream areas such as Can Tho, Vinh Long and Dong Thap.

During the dry season, water retained in these low-lying areas continues to replenish flows in the Tien and Hau rivers, helping maintain the balance between saline and freshwater zones and limiting saltwater intrusion in coastal provinces.

However, over the years, hundreds of thousands of hectares of enclosed dike systems have been built in the Long Xuyen and the Plain of Reeds to increase rice production, enabling farmers to grow three rice crops a year.

Studies show that the Long Xuyen capacity to absorb floodwaters has declined significantly. In 2000, the area could store around 9.2 billion cubic meters of water, but by 2011, that capacity had fallen to approximately 4.5 billion cubic meters as enclosed dike systems expanded.

When floodwaters no longer have enough space to spread across and be stored in the fields, more water is pushed downstream, increasing the risk of urban flooding. At the same time, reduced freshwater storage means less water is available to replenish dry-season flows, allowing saltwater intrusion to penetrate farther upstream through river mouths.

Proactively storing freshwater to adapt to more extreme climate future

The lessons from historic drought and saltwater intrusion events have shown that protecting natural water-storage areas is not merely a matter of agricultural production, but also a critical factor in ensuring the sustainable development of the entire Mekong Delta.

After years of facing increasingly severe drought and saltwater intrusion, water management practices in the Mekong Delta are undergoing a clear shift. Rather than focusing solely on controlling water through infrastructure, many localities are moving toward a more integrated approach that combines investment in irrigation infrastructure, freshwater storage, production transformation, and enhanced community resilience.

Tran Thanh Hiep, Deputy Director of the An Giang Department of Agriculture and Environment, said that following the merger of An Giang and Kien Giang, the newly expanded An Giang Province has become the country’s largest rice-producing locality, with an annual planted area of more than 1.3 million hectares.

On average, the province cultivates around 450,000-500,000 hectares of rice per crop. It currently has about 550 rice production sub-regions managed under closed irrigation systems, including more than 320 sub-regions protected by fully enclosed dikes, covering approximately 430,000-450,000 hectares.

These systems enable proactive flood control, water regulation and production protection, while providing a foundation for implementing the project to develop 1 million hectares of high-quality, low-emission rice production linked to green growth in the Mekong Delta.

However, amid increasingly erratic climate conditions, An Giang no longer views flood control as its sole objective. The province is gradually shifting toward more flexible water resource management that both ensures agricultural production and preserves the role of natural hydrological regulation.

One of the projects expected to contribute to this effort is the Tra Su-Tri Ton freshwater reservoir, located in the upstream area of the Long Xuyen Quadrilateral.

Under the proposal, the reservoir would cover approximately 3,050 hectares, with a total embankment length of 37.4 km, and would be designed to provide irrigation water for around 30,000 hectares of agricultural land. The project would feature six operating gates, combined with a modern system of pumps and regulating sluices, to make maximum use of floodwaters during the rainy season and supply water during the dry season.

According to An Giang agricultural authorities, the development of large-scale freshwater reservoirs would help regulate water supplies between the flood and dry seasons, reduce pressure on groundwater extraction, ensure domestic water supplies for local communities, and support agricultural production as climate change becomes increasingly extreme.

Beyond investment in large-scale infrastructure, localities across the region have in recent years stepped up the development of centralized domestic water supply systems, extending pipelines to remote and border communities as well as areas that frequently face water shortages during the dry season.

Many households have also taken proactive measures by installing rainwater storage tanks, using large-capacity water containers, adopting water-saving equipment, and changing their domestic water-use habits.

Protecting natural water-storage areas while increasing investment in water regulation infrastructure will help the Mekong Delta strengthen its resilience to El Niño in the near term and to climate change in the longer term.

In agricultural production, An Giang has identified two key groups of solutions. The first is to restructure crops by reducing water-intensive crops, particularly rice grown in areas facing water shortages. These would be replaced with high-value upland crops such as mangoes, bananas, vegetables and sorghum.

The second is to promote the use of water-saving irrigation technologies in agricultural production. Drip irrigation and water-efficient sprinkler irrigation systems are being encouraged among cooperatives, businesses and farming households to reduce costs and conserve water resources while maintaining crop yields.