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PI-Submitted Research Highlights for
Subsurface Biogeochemical Research Program

Reconciling Observations and Global Models of Terrestrial Water Fluxes

Reed Maxwell
Colorado School of Mines

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

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 (HPC) shows that lateral groundwater flow, not included in previous approaches, may be the missing link to predicting how important plant water use is to the total system.

The Impact
Understanding freshwater flows at continental scales will enable scientists to better predict hydrologic response and manage our water resources. The relative importance of plant transpiration remains one of the largest uncertainties in balancing water at these scales. Improving the large-scale simulation of plant transpiration will allow scientists to better predict and understand how much freshwater is present on our planet.

Summary
Using integrated hydrologic simulations that couple vegetation and land energy processes with surface and subsurface hydrology, the PIs studied the relative importance of transpiration as a fraction of all of 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 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.

BER PM Contact
David Lesmes, SC-23.1, 301-903-2977

Contact
Reed Maxwell
Colorado School of Mines
Rmaxwell@mines.edu

Laura Condon
Syracuse University
lecondon@syr.edu

Funding
This work was supported by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research and Office of Advanced Scientific Computing through the IDEAS project. Simulations made possible through support from Yellowstone at the National Center for Atmospheric Research Computational and Information Systems Laboratory.

Publication
R. M. Maxwell, L. E. Condon, “Connections Between Groundwater Flow and Transpiration Partitioning.” Science (2016). DOI: 10.1126/science.aaf7891

Citation:

R. M. Maxwell, L. E. Condon, "Connections Between Groundwater Flow and Transpiration Partitioning." Science (2016). DOI: 10.1126/science.aaf7891


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.

[Credit Laura Condon, Syracuse, Mary Michael Forrester and Reed Maxwell CSM]

midsized version

A mosaic of plant and water images making up a single leaf overlaid on the continental U.S. illustrates the connection between water, plant function and scale.

[Credit Mary Michael Forrester, CSM]

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