“Connections between groundwater flow and transpiration partitioning” by Reed M. Maxwell, Laura E. Condon; Science 22 Jul 2016: Vol. 353, Issue 6297, pp. 377-380 DOI: 10.1126/science.aaf7891. Soil evaporation and plant transpiration often produce more water than surface flow from streams and rivers over the continental United States. This makes these processes very important for overall water flow. Understanding these fundamental processes and their drivers is essential to understanding the hydraulic cycle, fresh water availability and ultimately impact on our planet’s climate.
Though when we talk about greenhouse gases we talk in terms of carbon dioxide equivalents, water vapor is the most important and abundant greenhouse gas in the atmosphere. Human activities produce only a small increase in water vapor through combustion processes and irrigation. Atmospheric warming leads to an increase in water vapor since a warmer atmosphere holds more moisture, and warming increases evaporation which increase water vapor in the atmosphere leading to more warming. This is part of the “feedback loop” cited by climate scientists.
Now in a peer reviewed paper published last month, Reed Maxwell, Professor of Hydrology and the Director of the Integrated GroundWater Modeling Center at the Colorado School of Mines, and Laura Condon now of Syracuse University have found that groundwater not only moderates evaporation and plant transpiration, but may also increase the partitioning of the processes. In their latest paper the scientists found that not only is latent heat flux and transpiration partitioning connected by groundwater depth, but that lateral groundwater flow has an impact on evaporation and plant transpiration. Lateral flow of groundwater has not been thought significant by climate scientists .
Using an integrated hydrology model run on a supercomputer that Dr. Maxwell and other scientists developed a groundwater model called ParFlow, they were able to fully integrate shallow groundwater equations and the land surface processes of evaporation and plant transpiration. Usually climate models ignore lateral groundwater movement in their predictions of increased water vapor in the atmosphere. Drs Maxwell and Condon found a significant increase in water vapor supplied from transpiration when they included lateral groundwater flow in their model which was confirmed against real observation data. The increase was over 30%. What this means is that groundwater flow and management practices will impact the modeling of climate change.
Changes in water table depth due to the widespread worldwide overdrawing of groundwater documented by the GRACE project may have a profound influence on climate. GRACE. the Gravity Recovery and Climate Experiment, satellite mission from the National Aeronautics Space Administration (NASA) has been collecting data for more than a decade. Two papers from a group of researchers assembled from the University of California- Irvine, National Taiwan University, and National Center for Atmospheric Research, Boulder Colorado and the Hydrological Sciences Branch at NASA Goddard Space Flight Center have found that more than one third of Earth's 37 largest groundwater basins are using up their groundwater faster than it is being replaced.