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Climate Change

Salty Rice: Tasty Treat, or Damaging Effect of Sea Level Rise?

Journal Article: Smajgl, A., et al., 2015. Responding to rising sea levels in the Mekong Delta. Nature Climate Change, 2469. DOI: 10.1038. 1-8.

Vietnamese farmers work in rice fields during the wet season. (Photo: Insiders Asia)

Vietnamese farmers work in rice fields during the wet season. (Photo: Insiders Asia)

Sea level rise is an undeniable fact. It is one of the major challenges of climate change that developing countries must cope with. Most people associate sea level rise with flooding and property loss, and tend to think about the damage that the water causes to roads, buildings, and beaches. However, it is not just the presence of water itself that causes damage. Along with the seawater comes an increased abundance of salt in the groundwater – called “salinity intrusion” – and it can be devastating to agriculture in coastal communities.

In the Mekong Delta of Vietnam, salinity intrusion has been a problem for decades. Farming villages and the national government have a history of searching for the best ways to fight this phenomenon. Agriculture, specifically rice production, is a staple of these villages. Rice is the backbone of Vietnamese national food stability and makes up a large portion of overseas export. If agricultural production is compromised, farmers, villages, and the nation as a whole could be in trouble.

Rising sea levels cause the salty ocean water to infiltrate groundwater aquifers near the coast, which allow the contaminated water to travel far inland. This increases the salinity of groundwater in farming villages used for irrigation of agricultural fields. Excess concentrations of salt in the groundwater will damage crops and hinder their growth. Saline intrusion is a particular problem in Vietnam’s dry season, which lasts from October to May. With lowered precipitation rates, there is less fresh water available to flush the salty water out of the groundwater aquifers. During the dry season, about 1.3 million hectares of the Mekong Delta land is affected by this salty groundwater.

Salt intrusion due to sea level rise has already caused agricultural productivity to decrease and rice production to decline, especially in strains of rice that have low salinity tolerance (Figure 1). For the coast of Southern Vietnam, sea level is expected to rise 30 cm by 2050. This is only going to make the salt intrusion problem worse.

Figure 1: It is clear that saltier soil means fewer rice crops, especially the rice most sensitive to salinity. This shows the relationship between soil salinity and yields of rice varieties with variable salinity tolerances from 2005-2010. (Smajgl 2015)

Figure 1: It is clear that saltier soil means fewer rice crops, especially the rice most sensitive to salinity. This shows the relationship between soil salinity and yields of rice varieties with variable salinity tolerances from 2005-2010. (Smajgl 2015)

In the past, the Vietnamese government has constructed embankments, sea-dykes, and sluice gates to fight the effects of salinity intrusion on agricultural production. These are referred to as “hard policy” options. One such strategy being heavily promoted is the addition of tributary dams that would feed freshwater into the Mekong Delta during the dry seasons. Releases of freshwater from these upstream hydropower dams would aid in flushing the saline water back towards the coasts. So far, tributary dams are operating in Laos, Cambodia, and Thailand. China is operating and planning tributary dams in the upper Mekong Basin as well. The drawback to these dams is that the flow of the rivers will be diminished during the wet season, when they would usually have a very strong flow. This would limit salt flushing and also delay the flood pulse in the delta that is critical for rice production. The dams are also costly to build, coming in between 5-8 billion US dollars.

Though many are in favor of these hard-policy options, “soft-policy” options are also gaining attention. Communities on the delta could adapt by – for one example – introducing crop varieties that are more resistant to saline conditions. Supporters of this adaptive strategy argue that it would come at much lower costs than the dams and embankments designed to actively fight off the salinity intrusion, which will only get worse as the sea level continues to rise. In addition, dams cause a reduction in the nutrient-rich sediment that flows from upstream; this sediment helps fertilize the fields and increases crop yield. Farmers may suffer economic losses with more dams being built, and would require the fertilizer to be brought in from elsewhere.

Figure 2: Salinity intrusion (indicated by an increasing red coloration) modeled for 30 cm of sea-level rise, development of all planned upstream reservoirs and irrigation schemes, and an increase in dry years. (Smajgl 2015)

Figure 2: Salinity intrusion (indicated by an increasing red coloration) modeled for 30 cm of sea-level rise, development of all planned upstream reservoirs and irrigation schemes, and an increase in dry years. (Smajgl 2015)

While it might make sense to implement a balance of hard- and soft-policy strategies, most people have supported going one way or the other. This has led to many uncoordinated development plans vying for attention as the most cost-effective and long-lasting solution. Studies such as this one led by Smajgl are attempting to model the different solutions and their outcomes (Figure 2), in order to shed some light on the debate between the hard- and soft- policies. Costs, groundwater hydrology, sea level rise, and even household behavior are factors in their models.

So far, Smajgl and his team argue that an ensemble of hard and soft policies would provide effective results, depending on the location. The results of the models show that what may work best in one region of the delta may not work so well in another. There appears to be no be-all-end-all solution. The main reason for these differences is the human component in agriculture. Being able to model this component is key to understanding what tactics to use and where.

The Mekong Delta is one of the most vulnerable deltas on the planet to climate change and its effects. However, Vietnam is just one example of a nation being put at risk by sea level rise. The models being used here to find solutions to the salinity intrusion problem can be used in other places, and likely will come up with different results. It is a race against the climate clock to find how to best adapt to these climate changes, and to then use that knowledge to face future, global challenges.

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