The West Coast of the United States is no stranger to massive floods caused by atmospheric rivers. These powerful storms bring much-needed moisture to the region, but also pose a significant threat to communities and ecosystems. A new study has shed light on the hidden trigger behind these devastating events: wet soils that cannot absorb more water when a storm hits.

The research, published in the Journal of Hydrometeorology, analyzed over 43,000 atmospheric river storms across 122 watersheds on the West Coast between 1980 and 2023. The findings reveal that flood peaks were 2-4.5 times higher on average when soils were already wet. This means that even weaker storms can generate major floods if their precipitation meets a saturated Earth.

Lead author Mariana Webb, completing her Ph.D. at DRI and the University of Nevada, Reno, explained that flooding from any event is not just a function of storm size and magnitude but also depends on what’s happening on the land surface. The study demonstrates the key role that pre-event soil moisture can have in moderating flood events.

Interestingly, flood magnitudes do not increase linearly as soil moisture increases. There’s a critical threshold of soil moisture wetness above which you start to see much larger flows. This research also untangled the environmental conditions of regions where soil moisture has the largest influence on flooding.

In arid places like California and southwestern Oregon, storms that hit when soils are already saturated are more likely to cause floods. In contrast, in lush Washington and the interior Cascades and Sierra Nevada regions, watersheds tend to have deeper soils and snowpack, leading to a higher water storage capacity. Although soil saturation can still play a role in driving flooding in these areas, accounting for soil moisture is less valuable for flood management because soils are consistently wet or insulated by snow.

The study highlights the importance of integrating land surface conditions into impact assessments for atmospheric rivers. Webb worked with DRI ecohydrologist Christine Albano to produce the research, building on Albano’s extensive expertise studying atmospheric rivers, their risks, and their impacts on the landscape.

While soil saturation is widely recognized as a key factor in determining flood risk, Mari’s work helps to quantify the point at which this level of saturation leads to large increases in flood risk across different areas along the West Coast. Advances in weather forecasting allow us to see atmospheric rivers coming toward the coast several days before they arrive. By combining atmospheric river forecast information with knowledge of how close the soil moisture is to critical saturation levels for a given watershed, we can further improve flood early warning systems.

Increased monitoring in watersheds identified as high-risk, including real-time soil moisture observations, could significantly enhance early warning systems and flood management as atmospheric rivers become more frequent and intense. By tailoring flood risk evaluations to a specific watershed’s physical characteristics and climate, the study could improve flood-risk predictions.

The accumulation of microplastics in our environment is a growing concern. These tiny particles have been found to harm marine life, contaminate food chains, and even enter our own bodies through various pathways. However, predicting where these particles will accumulate and therefore where remediation efforts should focus has been difficult due to the many factors contributing to their dispersal and deposition.

New research from MIT shows that one key factor in determining where microparticles are likely to build up is related to the presence of biofilms. These thin, sticky biopolymer layers are shed by microorganisms and can accumulate on surfaces, including riverbeds or seashores. The study found that when these particles land on sediment infused with biofilms, they are more likely to be resuspended by flowing water and carried away.

The research involved a flow tank with a bottom lined with fine sand, sometimes mixed with biological material simulating natural biofilms. Water mixed with tiny plastic particles was pumped through the tank for three hours, and then the bed surface was photographed under ultraviolet light that caused the plastic particles to fluoresce, allowing a quantitative measurement of their concentration.

The results revealed two different phenomena affecting how much plastic accumulated on the different surfaces. Immediately around the rods simulating above-ground roots, turbulence prevented particle deposition. Additionally, as the amount of simulated biofilms in the sediment bed increased, the accumulation of particles also decreased.

The researchers concluded that the biofilms filled up the spaces between the sand grains, leaving less room for the microparticles to fit in. The particles were more exposed because they penetrated less deeply into the sand grains, making them easier to resuspend and carry away by the flowing water.

This research provides a “nice lens” to offer guidance on where to find microplastic hotspots versus not-so-hot areas. For example, in mangrove ecosystems, microplastics may accumulate preferentially in the outer edges, which tend to be sandy, while the interior zones have sediment with more biofilm. This suggests that the sandy outer regions may be potential hotspots for microplastic accumulation.

The work was supported by Shell International Exploration and Production through the MIT Energy Initiative. While other factors like turbulence or roughness of the bottom surface complicate this, it provides a framework to categorize habitats and prioritize monitoring and protection efforts.

The mysterious zone deep inside the Earth has long been shrouded in uncertainty. For over 50 years, researchers have been puzzled by the strange behavior of earthquake waves in the so-called D” layer, around 2700 kilometers beneath our feet. However, a new study by geoscientists led by Motohiko Murakami, Professor of Experimental Mineral Physics at ETH Zurich, has finally solved this mystery.

Murakami’s team discovered that perovskite, the main mineral of the Earth’s lower mantle, transforms into a new mineral near the D” layer under extreme pressure and high temperatures – so-called “post-perovskite.” This change was initially thought to explain the strange acceleration of seismic waves. However, further research revealed that the phase change alone is not enough to accelerate earthquake waves.

Using sophisticated computer models, Murakami’s team found that depending on the direction in which the post-perovskite crystals point, the hardness of the mineral changes. Only when all the crystals of the mineral point in the same direction can the seismic waves be accelerated – as observed in the D” layer at a depth of 2700 kilometers.

In an unusual laboratory experiment at ETH Zurich, Murakami has now proven that post-perovskite crystals align themselves in the identical direction under enormous pressure and extreme temperatures. This has led to the conclusion that solid mantle rock flows horizontally along the lower edge of the Earth’s mantle, a phenomenon known as mantle convection.

This groundbreaking discovery not only solves the mystery of the D” layer but also opens a window into the dynamics in the depths of the Earth. It is now possible to begin mapping the currents in the Earth’s deepest interior and visualize the invisible motor that drives volcanoes, tectonic plates, and perhaps even the Earth’s magnetic field.

Murakami emphasizes that this discovery shows that the Earth is not only active on the surface but also in motion deep inside. This newfound understanding has significant implications for our comprehension of the Earth’s internal dynamics and its role in shaping the planet’s surface features.