For more than a decade, U.S. Forest Service and Chinese scientists have collaborated to understand how human activities affect carbon and water cycles in managed ecosystems. Working through the U.S.-China Carbon Consortium, scientists share data from a network of eddy covariance flux towers across the two countries. The towers measure the flow of water vapor, carbon dioxide, and other gases above the forest canopy.
A recent study, led by Chunwei Liu, an assistant professor at Nanjing University of Information Science and Technology, and coauthored by Ge Sun and Steve McNulty of the Southern Research Station (SRS) Eastern Forest Environmental Assessment Center, explores seasonal water demand in forests, grasslands, and other ecosystems.
In order to measure water stress over large areas and different types of forests, the scientists used the crop coefficient method. The crop coefficient method has been widely used in agriculture to estimate how much water crops will need. Sun and his colleagues are among the first to apply it to forests.
The scientists used eddy flux tower data from 81 sites in the FLUXNET La Thuile dataset to develop monthly, seasonal, and annual models for seven cover types—including broadleaf, evergreen, and mixed forests. The models were based on location, amount of rainfall and other forms of precipitation, and a common measure of forest growth or health known as leaf area index.
The crop coefficient describes the difference between the actual amount of water lost to the atmosphere and the amount of water that could be lost if there was unlimited water available. The ratio of the actual and potential amounts of water loss shows when ecosystems are experiencing water stress.
“Our synthesis studies extend the applications of the traditional crop coefficient methods to natural ecosystems,” says coauthor and research hydrologist Ge Sun. “The study offers practical ways for estimating the seasonal dynamics of ecosystem water use and stress.”
These seasonal changes in forest water use and stress could help land managers deal with prolonged or severe drought conditions or evaluate trade-offs between managing for water supply or carbon storage.
The research builds on a previous study that combined eddy flux data from more than 200 sites with remote sensing products and statistical models to create simple, powerful evapotranspiration models for seven different land cover types—a significant refinement of previous models that lumped land cover types together.
The studies refine our understanding of the connections between forest management practices, carbon and water cycles, and changing climate conditions, and land managers may also be able to use the crop coefficient method to measure water use at watershed or regional scales.