Monitoring crops on La Palma

The crops at the foot of the Cumbre Vieja volcano, 90% of which are banana plantations, have been exposed to volcanic ash fallout for over two months. What economic losses will this cause farmers on the island of La Palma and how will they recover and adapt to this long eruption? Pierre Delmelle, a specialist in these matters, went to the island to assess the situation.

As the Cumbre Vieja volcano has continued to erupt on the island of La Palma in the Canary Islands, Prof. Delmelle, a member of the Faculty of Bioengineering and the Earth and Life Institute, has visited the site. His research focuses on the vulnerability of agriculture to volcanic hazards. “The agricultural sector is generally the sector most affected by a volcanic eruption,” he explains, “sometimes even jeopardising an entire region’s food security.” How do rural communities whose income derives mainly from agriculture at the foot of volcanoes manage to recover from the often considerable economic losses caused by an eruption? How can we predict these economic losses and the resilience of the societies concerned? These are the questions at the heart of Prof. Delmelle’s research. 

100 million cubic metres of ash!

To answer these questions and develop models capable of predicting impacts, it’s necessary to characterise and compare agrarian systems in the vicinity of volcanoes in different regions of the world. After working in the Philippines, Ecuador and Iceland, Prof. Delmelle recently returned from a week of monitoring crops ravaged by the Cumbre Vieja eruption. Eruptions are rare in Europe, and there are few opportunities to observe their consequences in real time. Hence the interest in observing what’s happening on the spot and collecting information on the ground. “More than 1,000 hectares have been buried under lava flows, mainly crops grown in a fertile valley to the west of the volcano,” he says. “Agriculture will probably not be possible in this valley for decades or even centuries while the slow processes of soil formation do their work.” The almost daily fallout of volcanic ash is another scourge for La Palma’s agriculture. “The ash is dispersed by the wind and reaches areas further away from the volcano, which are not affected by the lava flows. It’s estimated that approximately 100 million cubic metres of ash have fallen on the island since the eruption began on 19 September 2021. This is equivalent to the volume of 40,000 Olympic swimming pools.”

Vulnerability: many factors to consider

Traditionally, the vulnerability of crops affected by volcanic ash fallout during an eruption is assessed on the basis of the thickness of the deposit. “But there are other characteristics of the ash hazard that need to be taken into account when determining the vulnerability of crops,” Prof. Delmelle explains, “such as the granulometry of the ash, the type of crop that is exposed, and the local climate.” A priori, the fine ash particles that are typically emitted by highly explosive volcanic eruptions tend to be more retained on the surface of leaves compared to coarser grained ash. However, the crop’s morphology and growth stage, and the shape, surface and inclination of the leaves, are all factors that modulate how long volcanic ash remains on the foliage. “The Cumbre Vieja eruption is only slightly explosive and the ash that is emitted is comparable in size to sea sand. On La Palma, the vast majority of crops are banana trees with large, relatively smooth leaves. Contrary to what was expected, we observed that banana foliage retains the ash, and even an intense rain doesn’t remove it from the surface of leaves. This simple field observation reinforces the idea that in order to be able to reliably estimate the vulnerability of crops exposed to volcanic ash fallout, it’s imperative to combine measurements and knowledge from different disciplines: volcanology, agronomy and botany.”

Is there fluorine in the ash?

In parallel with his research into the Cumbre Vieja eruption’s impact on La Palma agriculture, Prof. Delmelle and INVOLCAN (Instituto Volcanologicó de Canarias) study the surface properties of the ash emitted by the volcano. “I’ve received a series of samples collected since the eruption’s first day. We need to measure the fluorine content in these materials and the fraction of this fluorine that could be rapidly mobilised in the environment.” Indeed, the fluorine that’s sometimes found on the surface of volcanic ash is potentially toxic to humans, livestock and the soil-plant system. “If soluble fluorine is present on the surface of the ash particles, it’s released in the presence of water: for example, when rainfall percolates through the deposit. On La Palma drinking water is pumped from underground galleries, but it’s important to know if the surface water used for irrigation and livestock is contaminated with volcanic fluorine. This information is needed to implement adequate preventive measures.” Indeed, the ingestion of fluoride-contaminated plants or water by animals could be problematic. “My role is to determine whether fluorine is present in volcanic ash and, if so, assess its chemical form(s) in order to estimate the potential risk to livestock and crops exposed to ash fallout.”

More about Pierre Delmelle’s research

Published on December 02, 2021