09/01/2016

Reconciling Observations and Global Models of Terrestrial Water Fluxes

Water table depth and groundwater flow are key to understanding the amount of water that plants transmit to the atmosphere.

Image depicting traditional land surface models vs integrated hydrologic models

Modeling Water Movement. This conceptual diagram compares two approaches for modeling water movement above and below the land surface. Traditional land surface models simplify the system by solving it as a set of discrete columns without lateral groundwater flow, while integrated hydrologic models connect three-dimensional flow in the subsurface with processes at the land surface.

Image courtesy Laura Condon, Syracuse University; Mary Michael Forrester and Reed Maxwell, Colorado School of Mines

The Science

Plants are one of the largest water users on land, and, through transpiration, they move more water into the atmosphere than streams or rivers move across the landscape. Unlike stream flow, which can be easily observed, measuring and simulating the amount of water plants transmit to the atmosphere is a significant challenge. A new modeling study using high-performance computers (HPCs) shows that lateral groundwater flow, not included in previous modeling approaches, may be the missing link to predicting how important plant water use is to the total hydrologic system.

The Impact

The relative importance of plant transpiration remains one of the largest uncertainties in balancing water at continental scales. Improving the large-scale simulation of plant transpiration will enable scientists to better predict hydrologic response and manage water resources, as well as predict and understand how much freshwater is available globally.

Summary

Using integrated hydrologic simulations that couple vegetation and land-energy processes with surface and subsurface hydrology, the researchers studied the relative importance of transpiration as a fraction of all the water moving from the land surface to the atmosphere (commonly referred to as transpiration partitioning) at the continental scale. They found that both the total flux of water and transpiration partitioning are connected to water table depth. Because of this connection, including groundwater flow in the model increases transpiration partitioning from 47% (±13%) to 62% (±12%). This finding suggests that groundwater flow, which is generally simplified or excluded from other continental-scale simulations, may provide a missing link to reconciling observations and global models of terrestrial water fluxes.

Principal Investigator(s)

Reed Maxwell
Colorado School of Mines
Rmaxwell@mines.edu

Laura Condon
Syracuse University
lecondon@syr.edu

Related Links

Funding

This work was supported by the Office of Biological and Environmental Research (BER), within the U.S. Department of Energy (DOE) Office of Science, and the Interoperable Design of Extreme-scale Application Software (IDEAS) project of the Office of Advanced Scientific Computing Research (ASCR), within the DOE Office of Science. Simulations were made possible through support from Yellowstone at the National Center for Atmospheric Research Computational and Information Systems Laboratory.

References

Maxwell, R. M., and L. E. Condon. “Connections between groundwater flow and transpiration partitioning.” Science 353(6297), 377–80 (2016). [DOI: 10.1126/science.aaf7891]