Water is everywhere on Earth, and it is a unique molecule that is critical for life. Where, when, and how it moves—the water cycle—is equally critical.
Water falls over Earth’s surface as rain, snow, or ice. From there, it evaporates and returns to the atmosphere; seeps into the ground as soil moisture or groundwater; or runs off into rivers or streams. It continually evaporates from bodies of water, gets transpired from vegetation, sublimes from ice and snow, condenses, and precipitates along these pathways.
In new research, scientists from the U.S. Geological Survey (USGS) showed that there has been an increase in the flow between the various stages of the water cycle over most the U.S. in the past seven decades. The rates of ocean evaporation, terrestrial evapotranspiration, and precipitation have been increasing. In other words, water has been moving more quickly and intensely through the various stages.
“As the planet warms we anticipate that the warmer air, which holds more moisture, will lead to more evaporation and precipitation,” said Tom Huntington, the study’s first author and a research hydrologist at USGS. “If those processes are increasing, it is evidence for an intensifying water cycle. But no one had really shown that trend quantitively.” Until now.
Huntington and colleagues developed a new method for calculating water cycle intensity, specifically looking at stages that occur over and on land. They took the sum of precipitation and evapotranspiration per year to serve as a new indicator of water cycle intensity. Precipitation data came from the PRISM Climate Group, which combined ground observations with sophisticated modeling to create continuous precipitation measurements across the U.S. Evapotranspiration data—the amount of water evaporating from the leaves of plants and from the land surface—was calculated using precipitation and temperature data and a water-balance model.
The map above shows where the water cycle has been intensifying or weakening across the continental U.S. from 1945-1974 to 1985-2014. Areas in blue show where the water cycle has been speeding up—moving through the various stages faster or with more volume. Red areas have seen declines in precipitation and evapotranspiration and experienced less intense or slower cycles. Larger intensity values indicate more water was cycling in that region, primarily due to increased precipitation.
The team also related their measurement of water cycle intensity to how much moisture is stored in the soil. They found the amount of water being stored in the soil during June through August is declining in parts of the southeast, northwest, and upper midwestern U.S.
“We're trying to separate the precipitation totals into what comes out as streamflow, what goes up through evaporation, and what is stored in the soil,” said Huntington. The team found evapotranspiration had decreased in the southeast over the same time period, most likely because of decreased precipitation in those regions.
Huntington said these regions with weakening water cycles and low soil moisture should be carefully tracked over the next few decades because they could become increasingly dry. That would make agriculture more difficult or require more irrigation. On the other hand, too much rain or soil moisture storage, such as in the northeast U.S. or Texas, could lead to increased flooding.
NASA Earth Observatory image by Lauren Dauphin, using data from Huntington, Thomas, et al. (2018). Story by Kasha Patel.