Tracing the movement of water

I study the drinking habits of plants,” says Stephen Good, a hydrologist in the Biological and Ecological Engineering department at OSU. Some plants pace themselves and sip steadily, while others are downright bingers, guzzling water all at once. Good is interested in how those natural tendencies respond to changes in the environment. His research involves measuring the soil moisture content in croplands over time, which provides insight into how and when specific plants consume water.

Understanding the conditions in which crops take up water can help growers create more efficient irrigation systems. It can help predict how plants will respond to droughts and floods. It can also help model future changes in ecosystems based on changing weather patterns and plant growth. In fact, Good is finding that vegetation affects the weather more than you might think.

Let’s begin with the water cycle that we all learned about in grade school. It lumped together several different processes under the single heading “evapotranspiration.” These include evaporation from lakes, rivers, and soil, as well as a somewhat different process—transpiration— by which plants emit water vapor through their stomata, much like we perspire through our pores. Scientists like Good are developing new methods that can separate the component of water delivered by transpiration and measure its contribution to the water cycle. In a recent study using data from NASA’s Aura satellite, Good estimated that transpiration surprisingly accounts for nearly two-thirds of all evapotranspiration globally. That’s a lot of exhaling plants.

But you might wonder: How exactly can you trace the origin of a drop of water?

Water consists of hydrogen and oxygen atoms; and within those atoms, there can be various numbers of neutrons. Atoms of the same element with different neutron counts are called isotopes. For example, three isotopes of hydrogen are abundant in nature: protium (with zero neutrons), deuterium (with one neutron), and tritium (with two neutrons). Heavier isotopes of hydrogen and oxygen (those with higher neutron counts) are the first to fall from clouds, which means they accumulate in higher percentages along coastlines and are more available for uptake by plants and animals. By measuring the ratio of each isotope in a water sample, Good can determine the origin of any given droplet.

Interestingly, water retains its isotope ratio even after a water molecule has made its way through the life cycle of a plant or animal. In crime forensics, water molecules extracted from a murder victim’s hair can be used to trace the victim’s movements before death. Isotope ratios in animal fur, scales, or feathers can be used to trace migration patterns, to determine whether a goose flew down from Canada, or a shrimp was caught in the Gulf of Mexico.

The most far-reaching use of isotope ratios, however, comes back to Good and his transpiration modeling. Good employs various methods to collect data, such as remote sensing with radar and satellite, to take measurements of soil moisture content on a global scale. With this, he develops a worldwide picture of plants consuming water in specific regions at specific times in relation to specific weather. In addition to plant intake, Good’s isotope research reveals how much atmospheric moisture is drawn from transpiration.

Combining these data sets, Good creates maps he calls isoscapes, which look something like shaded relief topo maps, with the location of a specific isotope ratio assigned to a specific shade of grey on the map. Such maps provide a sharper, more detailed image of the water cycle by tracing the isotopic journey of each molecule through the guzzling and sipping of plants. As plants take up water through roots and release water vapor through transpiration, they are both responding to and affecting atmospheric changes. This is important information as Good and colleagues develop models of water availability under various climate scenarios to help us weather the future.