Tracking the Flow of Surface Water on a Warming Planet
Keith Cherkauer can tell an astonishing amount about global warming by studying mud. Trudging through rural Indiana farmland early this spring, for instance, Cherkauer, an agricultural and biological engineer at Purdue University, paid close attention to how his boots sank a bit more than an inch into the thawing mud before hitting solidly frozen soil below. The way surface water travels under precisely such conditions is a source of both fascination and concern for Cherkauer. For the past decade, he has carefully monitored soil temperatures and patterns of freezing and thawing in the upper Midwest of the United States to better understand how the region is being affected by climate change.
Cherkauer's research—some of the most detailed of its kind yet undertaken—has pinpointed exactly what global warming looks like on the ground in Indiana, Illinois, Minnesota and Wisconsin. In the process, he has also demonstrated a surprising finding: many areas in the region are experiencing more frozen soil as temperatures rise—a potentially worrisome trend for farming in the upper Midwest.
"I want to know everything I can about the pathways water takes through the environment," Cherkauer says. For him the quest has been nothing short of a lifelong pursuit that originated, perhaps, in a kind of osmosis. As the son of a geologist who studied groundwater flows into and out of the Great Lakes, Cherkauer says he spent virtually every summer of his youth from the age of seven traveling with his family to sites in the U.S. to measure stream depths and rate of flow. Although as an undergraduate he considered a career in aerospace engineering, Cherkauer says the burgeoning potential of remote sensing drew him back to the study of water, which played such a large role in his childhood.
Cherkauer's research in the upper Midwest focuses primarily on winter and spring water flows. For much of the year, when the soil is warm and dry, he explains, precipitation simply percolates down into the ground. But the picture gets substantially more complex as the ground freezes and thaws in the winter and spring. Soil ice reduces the infiltration of melting snow into the ground and, especially in flat areas, he says, when the soil finally thaws, "water sinks into the ground as though you had pulled the plug on a drain."
To analyze the situation, Cherkauer draws upon weekly measurements of soil temperature and snow depths at an array of spots around the region. Combining these field observations with remote sensing data from satellites, he develops a detailed grid to track the movement of surface water over the course of the winter and spring.
Drawing upon his own measurements as well as detailed soil temperature records from the area since the early 1960s, Cherkauer's evidence shows clearly that annual average temperatures in the upper Midwest have risen and winters are getting shorter. Potentially most significant, though, is a steep decline in snow cover in some areas. Snow cover strongly affects the way water moves in the spring; a blanket of snow roughly 8 inches or more in depth insulates the soil and keeps it from freezing solid. Less snow, Cherkauer explains, can often lead to more frozen soil because "a thinner snow pack means more exposure to cold winter air temperatures."
The ironic result demonstrated by his research is that because of decreasing snow cover, some areas of the upper Midwest will see more frozen soil in the winter as overall temperatures warm. This can be problematic because, if the ground is frozen when surface water flows, the conditions can lead to flooding and erosion. Because the region is relatively flat, increases in soil frost can lead to larger stream flows of spring runoff across frozen flood plains. Not only can this mean localized flooding in the spring but, even more worrisome to Cherkauer, "an increase in freezing and thawing in the soil breaks the bonds that hold soil aggregates together," making it more likely that topsoil can be carried away in surface water.
Cherkauer's research is notable for its combination of sophisticated hydrological models with a solid, worm's eye view of ground-based observational data. Not only has his research deepened our understanding of how climate change is affecting the upper Midwest of the United States, but it is also helping Cherkauer and others refine our hydrological modeling capability and build confidence in its projections. This will be especially important in future research in areas such as Africa and China, where explains, "we won't have the luxury of long-term observational data to build upon."