The world’s freshwater lakes are a crucial natural resource. Although they hold just over a quarter of a percent of Earth’s freshwater in total (most freshwater resources are associated with ice sheets and groundwater), that amount makes up about 87% of available freshwater on the surface. These lakes support ecosystems and human societies around the world.
Rising temperatures are causing massive evaporation challenges, and a new global-scale study from the International Atomic Energy Agency (IAEA) published in Nature Communications offers a new way to standardize estimates of lake evaporation using a proven method that measures the concentration of water isotopes in the body of water.
“Before this study, lake evaporation estimates were difficult to reliably establish because of a wide range of different physical measurements and temporal data collection methods,” said Yuliya Vystavna, first author of the study and an isotope hydrologist at IAEA. “Each country also applies different monitoring techniques and hydrologic models, which result in the well-known and long-standing problem of incomparable lake evaporation estimates around the world.”
The study’s isotope input prediction model, based on 60 years of IAEA Global Network of Isotopes in Precipitation (GNIP) data, quantifies watershed inputs, Vystavna explained. The model “can be applied to lakes to calculate how much each lake had evaporated, simply by measuring each lake’s water isotope composition. Isotope analysis of lake water is low cost and easy to sample, and the isotope data produced [are] globally comparative.”
“The main point is that water in lakes starts with a given ratio of heavy and light isotopes,” explained Peter R. Leavitt, Canada Research Chair in Environmental Change and Society and professor of biology at the University of Regina who was not involved in the study. “This initial ratio depends on many features of the climate, landscape, and lake itself.
“As water evaporates,” Leavitt continued, “the light isotopes are lost preferentially from the lake to the atmosphere, and the lake becomes ‘enriched’ in heavy isotopes. Therefore, if you know what the initial regional isotope ratio is, you can also estimate how much water must have been lost to the atmosphere via evaporation.… The more evaporation that occurs, the more the water in the lake becomes enriched with heavy 2H and 18O isotopes.”
The IAEA analysis revealed that overall, about 20% of water inflow in lakes is lost to evaporation, and about 10% of lakes in arid regions are experiencing extreme evaporative losses—defined as more than 40% of their total inflow.
The researchers had a robust collection of lakes to analyze, as well as a robust collection of online databases. They used artificial intelligence to evaluate the stable isotope composition of 1,257 lakes from 91 countries and integrated data sets, including IAEA’s Global Network of Isotopes in Rivers.
“By incorporating large online climatic data sets,” Vystavna said, “we gained a better understanding of which climatic variables are presently the most important drivers of lake evaporation and how these factors differ spatially around the world.”
Leavitt was impressed by the scope of the study. “It compiles water data and evaporation data from nearly the entire planet, with notable gaps only at very high latitudes or in regions of particularly low human population density. These data will be incredibly valuable as a reference data set for future water and climate studies, including those which are based on ground-level studies, models, and satellite observations. The beauty of the data is [they’re] all collected, analyzed, and processed the same way, meaning [they don’t] suffer from methods-related noise that we see in smaller-scale studies where each local study is done a bit differently. Really, the project is the result of decades of enlightened thinking about global processes by the authors and their collaborators.”
Researchers found that precipitation, salt content, wind speed, relative humidity, and solar radiation are predominant controls on lake isotope composition and evaporation. They also accounted for a lake’s surface area, landscape, and bathymetry when predicting evaporation processes.
“The paper underlined the fact that lake ecosystems are always vulnerable to water loss by evaporation,” Leavitt said. “Without a constant supply of inflow, they would simply disappear, as we have seen with the Aral Sea, Great Salt Lake, and other iconic sites of lake dewatering. What the authors show, however, is that the controls of evaporation and thus the water balance of lakes are highly dependent on multiple factors, and not just air temperature.”
Traditionally, comparing evaporation losses for lakes has been difficult because lake characteristics (size, depth, surrounding land use, whether the catchment was created naturally or as a reservoir by people) differ so radically. However, Vystavna said, “isotopes integrate all those parameters. Isotopes reveal the history of lakes and provide a record of change, serving as a proxy of hydrological history.”
—Issa Sikiti da Silva (@sikitimedia), Science Writer