Agricultural impact of flooding

I can barely hear Esther Ngumbi over the roar of greenhouse fans as she shows me around her rooftop laboratory in Morrill Hall. The benches are full of tomato plants, and the tomatoes don't look good. Half of the plants are submerged in bins of water. Their leaves are yellow and withering. Some of the dying tomatoes have flowered. I see one or two baby tomatoes on a couple of spindly plants.

This isn't the only torture inflicted on the tomatoes. Someone has tied little baggies to their stems. Inside the bags, fat green caterpillars are chowing down on the tomato leaves.

Entomology professor Ngumbi has questions -- lots of them -- and this is how she's set out to answer some of them. She is purposely flooding the tomatoes to see how they might respond to flooded conditions in farmers' fields -- a scenario that is becoming more common as a result of climate change.

"In nature, there are many stressors on plants during flooding," Ngumbi says. "Once the tomatoes get flooded, they're already weak, so most likely they will be attracting insects, which like to eat weaker plants. We're investigating how the plants deal with the combined stress of flooding and herbivory."

This explains the caterpillars. They are the larval form of Manduca sexta, the tobacco hornworm. They are feasting on one of the two heirloom tomato varieties Ngumbi is using in the experiment: Cherokee purple and striped German.

Half of the tomato plants in the greenhouse are not flooded, allowing the team to compare the stressed plants with those grown in more common conditions. But there are more investigations going on here.

"Also, within this experiment, we're looking at the microbes," Ngumbi says. "We want to understand how the microbial community changes in flooded conditions."

One of Ngumbi's key focuses is how soil microbes influence plant health and productivity. She's fascinated by mycorrhizal fungi, which form intimate associations with plant roots, offering essential elements like nitrogen to the plants in exchange for glucose supplied by the roots.

The tomato plants are all growing in soil from an Illinois farm, but half were also inoculated with mulch from a local farmer who has developed his own recipe for nurturing mycorrhizal fungi in the soil. Ngumbi wants to see if this inoculation makes any difference to the plants' ability to defend themselves from the fat caterpillars.

To measure plant defenses, Ngumbi's team collects samples of gases emitted by the plants and screens them for volatile organic compounds, the chemicals plants use to ward off bugs that would eat them.

Two years later, Ngumbi publishes the results of these and other laboratory experiments. She found that the two tomato varieties differed in gene expression and in the volatile compounds they emitted -- before any intervention. And when flooded, both varieties of tomatoes had very different chemical emission profiles than when grown in normal conditions. Herbivory influenced the production of these volatile compounds, but not as much as flooding did.

Today, the experiments continue, and Ngumbi's interest in the effects of flooding has only intensified. In a new review published in the journal Trends in Plant Research, she spells out the many changes that occur when plants are inundated with water for days or weeks at a time.

"Flooding is different from other climate-related stressors because it deprives plants of oxygen, an essential and indispensable element and substrate for plant growth and development," Ngumbi writes. Flooding disrupts plant metabolism and energy generation. It interferes with photosynthesis. Flooding kills beneficial bacteria and promotes pathogenic microbes in the soil. It also can compromise plants' ability to defend themselves from disease and harmful insects like the tobacco hornworm.

Ngumbi also warns that increased flooding can undermine decades of research aimed at making plants more resilient to climate change. Flooding may thwart efforts to build soil quality and microbial health to make crops more resilient to stressors such as heat and drought. Flooding also may eliminate gains derived from genetic engineering or plant breeding.

With flooding intensity and frequency predicted to increase by roughly 7% for every 1° C increase in global average temperatures, Ngumbi writes, scientists must consider the impacts of floods to "protect the monumental gains made in building climate-resilient crops."

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Sinking land increases risk for thousands of coastal residents by 2050

One in 50 people living in two dozen coastal cities in the United States could experience significant flooding by 2050, according to Virginia Tech-led research.

Published in Nature, the study combines satellite-obtained measurements of sinking land, also known as subsidence, with sea-level rise projections and tide charts to provide a new comprehensive look at the potential for flooding in a combined 32 cities along the Atlantic, Pacific, and Gulf coasts. The study projects that in the next three decades as many as 500,000 people could be affected as well as a potential 1 in 35 privately owned properties damaged by flooding. The study also highlights the racial and socioeconomic demographics of those potentially affected.

"One of the challenges we have with communicating the issue of sea-level rise and land subsidence broadly is it often seems like a long-term problem, like something whose impacts will only manifest at the end of the century, which many people may not care about," said lead author Leonard Ohenhen, a graduate student working with Associate Professor Manoochehr Shirzaei at Virginia Tech's Earth Observation and Innovation Lab. "What we've done here is focused the picture on the short term, just 26 years from now."

Other increases compared to current estimates include:

• Between 500 and 700 more square miles of land flooded
• 176,000 to 518,000 more people affected
• 94,000 to 288,000 more properties exposed with an estimated value of $32 billion to $109 billion

"The whole purpose of this paper is to provide data to support decisions," Shirzaei said. "Every city, every county has a flood resiliency plan in place. They are required by law to create that. But it's likely nobody has received the entire picture until this study, which creates probably the first comprehensive picture of what's happen in the not-too-distant future."

Collaborators on the study include:

• Chandrakanta Ojha of the India Institute of Science Education and Research in Punjab, India
• Sonam Sherpa, a former Ph.D. student at Virginia Tech and a postdoctoral scholar at Brown University
• Robert J. Nicholls of the Tyndall Centre for Climate Change Research at the University of East Anglia, United Kingdom

"This study demonstrates that we can now measure vertical land motion at a sufficient scale to create a useful climate service that supports planning and management decisions on flooding," said Nicholls, a professor of climate adaptation. "This approach has the potential to be applied in any city around the world, really supporting adaptation."

Using highly accurate data points measured by space-based radar satellites, Shirzaei and his research team have built some of the world's first high-resolution depictions of the sinking land along the coast of the entire United States. Their work has previously revealed regions of the Atlantic coast to be sinking by as much as 5 millimeters per year.

This study revealed that 24 of the 32 coastal cities are currently sinking more than 2 millimeters per year and half of those cities have areas sinking more than global seas are rising. These numbers might seem small, but when combined with sea-level rise over time, it adds up to quite a significant shift, according to Ohenhen.

"The analogy I have found that is really helpful in helping people understand this change is to think about a sinking boat," he said. "Imagine you are in that boat with a steady leak, slowly causing the boat to sink. That leak symbolizes sea-level rise or broadly flooding. What would happen if it also starts raining? Even a minor rainfall or drizzle would cause the boat to sink more quickly than you thought it would. That's what land subsidence does -- even imperceptible millimeter land subsidence exacerbates existing coastal hazards."

Along with the new flood projections, the study also revealed the 32 cities have a combined 131 flood control structures, such as levees, berms, or dikes, but that 50 percent of those are located on the California coast. Only three of the 11 Atlantic coast cities studied maintain levees or floodwalls.

"When we looked at it across the board, we found that there is a general unappreciation for flood protection, particularly on the Atlantic coast," Ohenhen said. "And even the levees there often protect less than 10 percent of the city, compared to other cities on the Pacific or Gulf coasts where up to 70 percent is protected."

Another first in the study is the consideration of racial and socioeconomic demographics related to the potentially affected areas.

In some cities examined, particularly those along the Gulf Coast, the potential increased exposure fell disproportionately on racial minorities. In other cities, the properties facing increased exposure were found to generally be of lesser value than the median property value of the area. And in a few cities, New Orleans and Port Arthur, Texas, particularly, these two demographics intersect, showing the areas with greatest potential risk to be disproportionately occupied by people of color who are also at an economic disadvantage when compared to the city as a whole.

"That was the most surprising part of the study," Ohenhen said. "We found that there is racial and economic inequality in those areas in that there was an overrepresentation of historically marginalized groups potentially impacted as well as properties with significantly lower value than the rest of the cities. It really multiplies the potential impact to those areas and their abilities to recover from significant flooding."

Shirzaei said he believes the study as a whole not only provides the clearest picture of potential flooding to date, but also should serve as a call to action for policymakers of those areas.

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Researchers shed light on river resiliency to flooding

Researchers at theUniversity of Nevada, Renohave completed one of the most extensive river resilience studies, examining how river ecosystems recover following floods. They developed a novel modeling approach that used data from oxygen sensors placed in rivers to estimate daily growth in aquatic plants and algae. The researchers then modeled the algal and plant biomass in 143 rivers across the contiguous U.S. to quantify what magnitude of flooding disturbs the biomass and how long the rivers take to recover from floods. Increased understanding of rivers' resiliency is important to maintaining healthy rivers, as human actions can affect flood regimes and change the conditions in rivers for other aquatic life that may rely on algae and plants as a food source.

Assistant Professor Joanna Blaszczak and Postdoctoral Scholar Heili Lowman, both in the University'sCollege of Agriculture, Biotechnology & Natural ResourcesandGlobal Water Centerled the research, which was published in two separate journal articles. The preliminary work, led by Blaszczak andpublished in Ecology Letters last June, first studied six rivers and laid the groundwork and methodology for the second study, which Blaszczak hired Lowman to conduct, examining the 143 rivers. The results of that research were justpublished last month in PNAS (the Proceedings of the National Academy of Sciences).

The research is unique because it estimates changes in biomass in rivers more frequently than ever before without needing to directly sample rivers. This is done by using both data from oxygen sensors placed in the rivers by the U.S. Geological Survey and a population model of algal and plant biomass -- similar to a human population model that models change in the number of people over time, but instead modeling the change in the amount of algae and plants. The oxygen sensors began collecting data in 2007, and the most recent Nevada-led study of 143 rivers includes some data that are for nine years running, among the longest such records on file for rivers across the globe.

"Previously, you would have to go to a river and scrub rocks to measure the algae, and do that several times for an extended period of time in order to estimate changes in biomass growth and loss," Blaszczak said. "This is very time consuming, so the data have been extremely limited relative to how extensive our sensor networks are."

Blaszczak said that with the oxygen sensors that take data as frequently as every five minutes, the team found that they could use statistical models to extract the amount of photosynthesis that occurs daily and estimate daily changes in the amount of biomass in a river over time.

"The dissolved oxygen sensors show the peak during the day, and the low during the night, and from those patterns, you can estimate how much new algae and other biomass grew that day," she said. "With the sensors measuring data continuously in hundreds of rivers for years now, we can get a much bigger, clearer picture. The data is there, and we can use it to model the size of flood needed to disturb the biomass in a river, as well as the rate at which a river recovers from flood disturbances, which can help us manage rivers more effectively."

Getting started

In the first study, Blaszczak used two years of data from oxygen sensors placed in six rivers. She found that she could successfully use this data to model the flood threshold specific to a river that disturbed the underlying biomass, and that generally, the magnitude of flood necessary to disturb biomass and reduce ecosystem productivity was lower than the disturbance flow threshold necessary to mobilize river bed sediment, a metric of disturbance commonly used by those studying rivers. In other words, instead of estimating the disturbance of the river by the movement of the rocks on the river bed, this study used the biology -- the changes in algae and plant growth -- to quantify disturbance to the river and found that the biological disturbance threshold was lower.

"The amount of biomass is important for water quality and a food source for everything that lives in a river," Blaszczak explained, "so it is more important than rock movement, in terms how a river ecosystem is affected by a disturbance."

Blaszczak, a freshwater ecologist, began this work with Robert O. Hall Jr. of the Flathead Lake Biological Station at the University of Montana and enlisted the help of her colleague Assistant Professor Robert Shriver, a plant ecologist, for both research projects to complete the biomass growth modeling. Blaszczak, Shriver, and Lowman all conduct research as part of the College'sDepartment of Natural Resources & Environmental Science, as well as the College'sExperiment Stationresearch unit. The College's faculty often take interdisciplinary approaches to meet research challenges, Blaszczak said.

Expanding out to a continental scale

Blaszczak wanted to delve further by applying this approach to more rivers over a longer period of time to shed light on how various factors may be influencing both a river's thresholds for flood disturbance and its resilience to floods. Thus, she recruited Lowman to embark on the second, more extensive study. Lowman's research examined landscape and river characteristics that affected the rivers' resiliency to floods.

"We've never had such great insight into the resilience of rivers, and because of the amount of data and our modeling, we now understand the natural variation in resilience, and that the widest rivers without dams upstream recover the most quickly," Lowman said.

The fact that wide rivers without dams recover more quickly than wide rivers with dams upstream was not immediately obvious, she said, and is one example of how rivers can be affected and/or managed by our actions. Most of the rivers Lowman researched had three to four years of data, with some having as much as nine years, and a handful having less than a year.

"Having three to four years' worth of data is way more than we've ever been able to use before," Lowman said. "And, we used rivers of various sizes with various climates and land characteristics."

Besides wider rivers without dams being more resilient, Lowman said those rivers that had more frequent floods also tended to recover more quickly.

"It could be that they have had a long history of frequent flooding, so their algae and plant communities have developed the ability to adapt to more frequent disturbance," she said.

Overall, Lowman said the new model results are consistent with other previous approaches. But, she said that some sites took much longer, a month or more, to recover from floods than other sites, regardless of river size.

"It might be the composition of the algal and plant communities, the structure of the river bed, or other factors," she said. "The thresholds and recovery times are very likely partially dependent on the slope, the grain size of the sediment, and possibly other factors that aren't as well documented. Those are some next steps for future research."

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New study examines historical drought and flooding on the Amazon River

Despite the rapid increase in severe flooding, a new paper by researchers from the U of A indicates recent floods and droughts in the Amazon River Basin may have not yet exceeded the range of natural hydroclimatic variability.

Extreme floods and severe droughts on the Amazon River have occurred more frequently in the last 40 years. Eight of the 12 most extreme floods in the 121-year streamflow record at Manaus, located on the Negro River, a tributary of the Amazon River, have occurred in just the last 14 years.

Natural climate variations, deforestation and anthropogenic climate change are all likely contributors to the recent Amazon River level extremes. Despite the rapid increase in severe flooding, a new paper by researchers from the U of A indicates recent floods and droughts in the Amazon River Basin may have not yet exceeded the range of natural hydroclimatic variability.

The American Meteorological Society published these recent findings by Daniela Granato-Souza, a post-doctoral student in geosciences, and David Stahle, a Distinguished Professor of geosciences, in a new paper titled, "Drought and flood extremes on the Amazon River and in northeast Brazil, 1790-1900."

Using tree-ring analysis to reconstruct rainfall totals in the eastern Amazon, along with historical documentary accounts of extreme flooding in Manaus and Santarem, Granato-Souza and Stahle found evidence indicating that Amazon River floods in 1859 and 1892 may have equaled or exceeded recent flooding.

The biggest difference between then and now, though, is the size of the population now living in the flood plain.

"There are millions of people residing in Manaus, the most populous city in northern Brazil, so there is a socioeconomic concern in these regions," noted Granato-Souza. "Usually, poorer people are seen living in risk areas, and with each flood they suffer the consequences of loss and illness. There is a 'normal' for the maximum level that the river reaches, but the most extreme floods exceed this threshold, and studies have shown that this is intensifying."

Granato-Souza added that while they found the recent extreme floods weren't unprecedented, examining the scale of past flooding provides a framework for imagining and anticipating the environmental and socioeconomic consequences of future floods.

And they would likely be devastating for a population living in high-risk areas.

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How quickly does the climate recover?

Climate change is causing temperatures to rise and is also increasing the likelihood of storms, heavy rain, and flooding -- the recent flood disaster in the Ahr valley in Germany is just one such example. What we need to ask ourselves in this connection is how quickly the climate can recover from the warming caused by an increase in carbon dioxide in the atmosphere.

Professor Philip Pogge von Strandmann of Johannes Gutenberg University Mainz (JGU), Germany, set out to investigate this aspect by considering the significant rise in global temperatures of five to eight degrees Celsius that took place 56 million years ago -- the fastest natural period of global warming that has impacted on our climate, known as the Paleocene-Eocene Thermal Maximum (PETM). It was most likely triggered by a volcanic eruption that released huge amounts of carbon dioxide or CO2 into the atmosphere. We know that the higher the temperature, the faster rock will weather, and, in addition, if there is a lot of CO2 in the atmosphere, some of it will react with water, forming carbonic acid -- the very acid that promotes and accelerates the weathering process. Because of the weathering process, this atmospheric carbon will eventually find its way into the seas via rivers, where it binds CO2 as carbonate and form a persistent ocean-based reservoir of carbon dioxide. "Our theory was that if rock weathers faster due to the increased temperatures, it also helps convert a lot of carbon dioxide from the atmosphere into insoluble carbonate in seawater -- meaning that, over the long term, CO2 levels would end up falling again and the climate would ultimately recover," explained Pogge von Strandmann. This effect could have helped to keep the Earth's climate fairly stable over billions of years and it could have even prevented the total extinction of all life on the planet.

Weathering of rocks contributes to climate stabilization

In order to test this theory, Professor Philip Pogge von Strandmann and his team decided to analyze the weathering processes that occurred during the warming event 56 million years ago. Their findings indicate that the theory may well be correct. "Rock weathering during that time increased by 50 percent as a result of global warming; erosion -- the physical part of weathering -- actually tripled. Another consequence of the rise in temperature was that evaporation, rainfall, and storms also increased, which then led to even more erosion. As a result of this increased rock weathering, the climate stabilized, but it took between 20,000 and 50,000 years for this to happen," said Pogge von Strandmann, summarizing the team's findings.

But how did the researchers come to these conclusions? After all, these weathering processes took place 56 million years ago. The answer lies in the rocks themselves. When rocks dissolve, they release lithium -- the isotopes lithium-6 and lithium-7 to be precise -- which escapes into any surrounding water. The proportion of the isotopes lithium-6 and lithium-7 present in water is determined by the type of weathering, in other words, the amount of erosion produced by weathering. Clay, which is found at the bottom of the sea, mainly stores lithium-6, while lithium-7 remains in the water. The research team conducted two types of scientific investigation: They examined marine carbonates that were formed 56 million years ago -- a type of rock that absorbs chemical components from water. They also investigated clay minerals from Denmark and Svalbard, which also formed during this period, looking at the relative proportions of lithium isotopes in these two different kinds of minerals. The researchers were able to use the data obtained to draw conclusions about weathering and climate 56 million years ago. Their results have been published in the journal Science Advances.

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