Observing and understanding the climate change and human impacts on rice agriculture in the Mekong delta to support mitigation and adaptation measures
Thuy Le Toan1, Alexandre Bouvet 1, Stephane Mermoz 2, Hoa Phan 1, Thu Trang Le 1, Hironori Arai 1, Thierry Koleck 3, Lam Dao Nguyen 4, Nguyen Quoc Khanh 5
1. Centre d’Etudes Spatiales de la Biosphère, Toulouse, France
2. GlobEO, Toulouse, France
3. CNES, Toulouse, France
4. Vietnam Space Center, Ho Chi Minh city, Vietnam
5. Vietnam Remote Sensing Department, Hanoi, Vietnam
The countries in the Mekong region and in particular the deltas in the region are among the world most exposed to the threats of climate change, which are being worsened by stressors from fast growing population. In the last decades, the frequency and intensity of inundations, droughts, salinity intrusion have increased, causing negative impacts on different sectors, the most important being rice agriculture, which plays a central role in the social and economic development of the countries in the region. This is the case of the Vietnam Mekong Delta.
In order to support stakeholders to define adequate adaptation and mitigation strategies, it is essential to develop methods and to conduct studies, to monitor the impacts of climate change and human pressure on rice agriculture, in order to advance our understanding on the causes of the changes observed on rice land.
In most research work, the focus has been mainly on the impacts of climate change on crop productivity. However, for crop production, the impact could be more important on the crop harvest area which undergoes significant changes in the last few years, resulting from reduction of the number of crops per year, conversion of rice crop into other land use (aquaculture) or other perennial or annual crops. The drivers of these changes could be either the impacts of climate change on the habitat suitability for rice crop, or/in conjonction with the socio economic conditions in the region, both could be at the origin of farmers’s adaptation measures.
With the unprecedented observations collected by Sentinel-1 satellites since 2015, the role of EO data is reinforced in the monitoring of the full agricultural dynamics at national scale down to the single fields. In this study, the dynamic information on rice area estimates, rice growth stage and agricultural practices, as well as on the extent of seasonal flood on the Mekong Delta, are produced and the changes observed on rice land in the last 6 years are derived.
Inter-annual changes in land use, changes in rice harvest area, change in cropping density, and changes in crop calendar have been quantified and mapped. The most significant changes are the losses of dry season rice harvest area observed in 2016 and 2020. Other changes included rice fields converted into aquaculture, turning from triple rice crop to double crop, or having shifted calendar -up to 2 months from the traditional calendar. These changes appeared in scattered fields or small parts of administrative units, and are apparently decided by farmers as autonomous adaptation to climate change effects.
The changes have been analysed, in combination with climatic and environmental data, to improve our understanding on the drivers of these autonomous adaptation.
To understand the drivers of these changes, linked either to climate or socio-economic factors, the changes are analysed against a range of data including climatic data, sea level rise, flood extent, drought index , salinity intrusion…. The analysis has pointed out that most observed changes can be explained primarily by climatic factors. For example the loss of rice harvest area in coastal region in spring 2016 and spring 2020, following El Niño events, has been found in regions having high drought index and high saline water concentration. In regions impacted by upstream flooding, the flood extent and duration are larger and longer than before during the flood season, changes are observed in the calendar of the crops before and after the flood season. In the regions impacted by drought, salinity intrusion or flood, a number of rice fields were converted into shrimp farms, or let fallow during the critical season.
In order to support the stakeholders for longer term mitigation and adaptation measures, we estimate the suitability for rice cultivation in the future, based on the projections of climatic factors under different climatic scenarios for 2030 and 2050 developed in the Gemmes Vietnam project (Espagne et al., 2021).
In particular, the projected flood maps have been established, based on the projected elevation map of the Mekong Delta (Minderhoud et al., 2019), and taking into account the cumulative effect of Sea Level change and land subsidence, the latter being caused mainly by groundwater extraction. The results show that without adaptation, a large part of the rice land will fall below sea level. For example 34% of rice area in the Mekong Delta will be lost by submersion in 2050, for a Sea level rise of 25 cm. The first mitigation measure would be to halt groundwater extraction rate at the impacted provinces. The adaptation measure would be building hard infrastructure to protect the land from inundation, but in this case, there is a need to consider socio-economic and ecological factors.
The projection of saline water intrusion (Eslami et al., 2021) has been used to map the projected rice land which will become less suitable for rice cultivation in 2030 and 2050, The impacted area, with the water salinity exceeding 2‰, amounts 10.5% of the dry season rice area. The mitigation measure would be to reduce sediment starvation from upstream hydropower dams and excessive sand mining, which increase saline water intrusion. The adaptation measure would be the use of salt resistant species, or conversion of rice cultivation into other land use, e.g. aquaculture.
Finally, the ambition of our studies is to provide tools to support stakeholders in Vietnam for devising adaptation and mitigation measures, to simulate on the effects of each of the relevant driving parameters. Further works need to be done integrating socio-economic factors.
The study benefits from the results of the works conducted in the ESA GEORICE project, the AFD GEMMES project, and the CNES VIETSCO project.
ESA-GEORICE : https://www.globeo.net/georice
Eslami, S., Hoekstra, P., Minderhoud, P. S. J. ., Trung, N. N., Hoch, J. M., H.Sutanudjaja, E., Dung, D. D.,
TranQuang, T., Voepel, H. E. , Woillez, M.-N. and van der Vegt, M. (2021). Projections of salt intrusion in a mega-delta under climatic and anthropogenic stressors. Nature Communications Earth & Environment, 2(1), 1-11. doi:10.1038/s43247-021-00208-5
Espagne E. (ed.), T. Ngo-Duc, M-H. Nguyen, E. Pannier, M-N. Woillez, A. Drogoul,T. P. L. Huynh, T. Le Toan , T. H. Nguyen, T. T. Nguyen, T. A. Nguyen, F. Thomas,C. Q. Truong, Q. T. Vo, C. T. Vu. 2021. Climate change in Viet Nam- Impacts and adaptation. A COP26 assessment report of the GEMMES Viet Nam project. Paris. Agence Française de Développement, 1 November 2021.
Minderhoud, P. S. J., Coumou, L., Erkens, G., Middelkoop, H., & Stouthamer, E. (2019). Mekong delta much lower than previously assumed in sea level rise impact assessments. Nature communications, 10(1), 1-13.