Research: Plant processes could be important in predicting drought development
Stanford, US: Plants play an important role in shaping Earth's weather and climate because they act as physical links between the ground and the sky. Stanford University researchers have revealed how a closer look at the inner workings of plants may be able to improve model predictions of some devastating global disasters.
Flash droughts, which develop quickly and deplete water availability in a matter of weeks, are linked to changes in evapotranspiration, which is the process by which plants move moisture from their roots to the air. Water lost to the atmosphere through evapotranspiration is sometimes considered "lost," so accurate calculations of this loss can be critical to understanding the impacts on water resources and ecosystems.
Researchers calculated changes in evapotranspiration during droughts that occurred globally from 2003 to 2020 by analyzing satellite data of both precipitation and belowground moisture. The study, published in Nature Climate Change reveals more information about evapotranspiration's role in these catastrophic events.
"When water is already limited, the evapotranspiration will continue to make the water loss happen even faster -- and that will make the drought become more severe in a much shorter time period," said lead study author Meng Zhao, a postdoctoral researcher in Earth system science in the Stanford Doerr School of Sustainability. "We have a very big challenge in predicting flash droughts and the underestimation of water loss could be a major obstacle in that prediction."
Droughts with a rapid onset and intensification can have a devastating impact on vulnerable communities and food production, as seen in the 2012 Central Great Plains flash drought, which cost more than USD 30 billion in damages. To improve models, the researchers say they need to include a hidden element in the evapotranspiration process: how plants change the structure and pathways in the soil surrounding their roots.
"We found that the model error seems to be explained by the way plants change how particles are arranged in the soil," said senior study author Alexandra Konings, an assistant professor of Earth system science. "As a result of these changes to the soil, water flows through the soil differently, changing where and how much water is available for plants to take up and transpire."
Plants respond to droughts in the same way that people do, with varying diets, exercise habits, and sleep hours based on available resources. Stomata, or tiny pores in leaves that release water, can close, but not all plants do so equally or at the same rate. Drier atmospheres have a greater ability to pull water out of the land through evapotranspiration during drought, causing it to increase; however, if the stomata close sufficiently, evapotranspiration will be reduced relative to non-drought times.
"There's such a diversity of ways that plants operate that it can be really hard to fully understand, predict, and quantify in the models," Konings said. "And unfortunately, if this increase in evapotranspiration is happening more often than we realize, it's intensifying the effect of the drought; there's even less water in the soil than we realize because more is being lost to the atmosphere."
Current Earth system models show increases in evapotranspiration, in which stomata are more open, occurring about 25% of the time during droughts. Yet according to the researchers' new estimate, it occurs about 45% of the time. "This underestimation is particularly large in relatively drier climate and lower biomass regions," the study authors write.
Researchers calculated global evapotranspiration by combining observations of water storage from the Gravity Recovery and Climate Experiment (GRACE) satellites with precipitation data from the Global Precipitation Climatology Project.
A variety of factors influence whether a given drought in a specific location leads to high evapotranspiration and has the potential to develop into a flash drought. The authors discovered that dry soils are an important control. They also discovered that current models do not account for the effect of roots on how water moves through soils. This resulted in errors in the simulations of soil dryness and, as a result, evapotranspiration.
"We knew that there were problems with the models, but I was really surprised at how off they were," Konings said. "My personal hope is that other folks in the community who are building different models use the lessons from our paper."
A transferrable approach
To understand current and future water resources, improved model representations of soil moisture impacts on evapotranspiration, soil structure effects on water transfer, and plant traits are required. While the researchers did not calculate how these new evapotranspiration measurements might affect future climate scenarios, which are expected to bring more frequent and severe droughts, they do believe the findings should be easily transferable to other models. Furthermore, no on-the-ground resources are required because it is based on satellite data.
"You can clearly see that the models underestimate the evapotranspiration increase during droughts for arid and semi-arid regions," Zhao said. "That means our understanding of this phenomenon is especially poor in regions that are already suffering from environmental injustice issues -- I think our work can help improve the knowledge of these regions that are already water-stressed."
