Adapting to rising salinity in the Mekong Delta
Dernière mise à jour : 6 févr. 2020
Real rice granary of Asia, the Mekong Delta is the heart of Vietnamese agriculture, mainly oriented towards rice growing, aquaculture and arboriculture. However, this area is threatened by the slow but progressive salinisation of its soils and waters, leading to yield losses and production difficulties. ACIAR (Australian Center for International Agricultural Research) in partnership with local farmers and researchers has therefore addressed this problem to identify a sustainable rice and shrimp farming system that is under threat.
Rice and shrimp rotation threatened
The association between rice and shrimp in the Mekong Delta is not new, it is at least 40 years old. This virtuous combination - the rice is cultivated during the rainy season when fresh water is abundant, and shrimp is bred during the dry season (December to April) when the salt concentration in the water increases - seemed to last.
However, the effects of climate change, with a reduced rainy period from May to November, but also poor water management choices in the delta threaten this synergistic system. Freshwater availability is low even in the rainy season for rice and water temperatures become too high for shrimp in the dry season. In times of drought, water scarcity is increased, leading to saltwater upwelling and thus salinization of the waters and soils of the Mekong Delta. This was particularly visible during the 2015/2016 crop season, which was subject to severe drought.
The shrimp breeding has also been affected by a reduction in soil and water quality leading to the emergence of certain diseases.
However, climate change is not the only cause of this decrease in available freshwater. Man is also directly responsible because he has built dams to recover energy upstream.
It is therefore necessary to be able to adapt to the decreasing availability of fresh water and the increasing salinization during the dry period.
ACIAR, promoter of a research project
In partnership with local authorities in the Mekong Delta and universities such as Can Tho University (CTU), ACIAR has therefore chosen to help farmers adapt to the decreasing availability of freshwater. For this purpose, a selection of test plots and test farmers were selected by the researchers in collaboration with the local authorities. These parcels are located in the South of the Delta at Ca Mau.
Province of Ca Mau in Vietnam
ACIAR then met with the farmers before choosing them definitively. Begun in 2014, the first phase ended in 2017 for a total investment of €1.3 million (US$1.46 million). A renewal took place until 2019 to track changes in practices and performance through the project and increased resilience to high salinization events.
To be able to compare the actual contribution of the ACIAR techniques, geographically close negative controls are also monitored. As a result, they have not changed their farming practices. A total of 5 hectares of plots are cultivated by 6 farmers benefiting from ACIAR techniques.
Partnerships with extension agencies and farmers have facilitated the transfer of information, created a climate of trust between farmers and researchers, and enabled the project to test management practices on farms rather than under controlled laboratory conditions.
Alternate Wet and Dry irrigation is a water management system in which rice fields are not continuously submerged but can dry intermittently during rice cultivation.  AWD is suitable for lowland rice growing areas where soils can be drained every 5 days. In addition, this technique is used in the dry season to drain fields, and not in the wet season when rainfall is too high.
The AWD system is simple, a pipe (sometimes made of bamboo) is placed so that the bottom 20 cm of the perforated part remains below the ground surface and the unperforated 10 cm above the surface. The perforations allow water to flow from the ground into the tube, where a scale is used to measure the depth of water below the ground surface. So we have the depth of the water table.
After irrigation in the field, water depth gradually decreases due to evapotranspiration, infiltration and percolation. When the water level drops 15 cm below the soil surface, the field should be irrigated to a depth of 5 cm. During the flowering phase of the rice, the field must be kept flooded. After flowering, in mid-season and late season (grain filling and maturation stages), the water level is allowed to drop below the soil surface to 15 cm before re-watering.  Rice fields therefore dry sparingly.
This technique is not feasible on all topographies on all soils and climatic conditions. In the end, it saves freshwater resources, without losing yield on rice while reducing the amount of fertilizer added.
At the same time, farmers have turned to shrimp, which is much more profitable than rice. However, the ecological, structural and social benefits of rice are being advanced by researchers to encourage farmers to keep their rice production. Indeed, rice mulching blocks some pests, for example. In the end, it seems that farmers have perceived the mutual benefit of rice and shrimp farming in the Mekong Delta.
The combination of rice and shrimp seems to be a permanent rotation in these regions of the Mekong. New techniques, such as AWD, can make it possible to keep these crops even in the context of salinization thanks to better water management. Other adaptation projects have been developed through ACIAR, such as a question on the different possible rotations and a possible change of a rice rotation to another crop if the climatic conditions become more advantageous for the latter.
Since we were unable to visit the project site, we lack a lot of data to conclude on the well founded of this adaptation technique.
 Journal of Agricultural Science - Vol. 3, No. 3; September 2011 “Achieving More with Less Water: Alternate Wet and Dry Irrigation (AWDI) as an Alternative to the Conventional Water Management Practices in Rice Farming” by Tejendra Chapagain (Corresponding author) & Andrew Riseman