The Magic of Water

Soil moisture is one of the components of land-surface evapotranspiration, and is a required parameter for evaporation calculations.

"If there is magic on this planet, it is contained in water," wrote the anthropologist Loren Eisely. He must have been thinking particularly of the alchemy of water on land, through which particular magic, according to some accounts, even the species is supposed to have achieved flesh and blood. It's not a bad idea, considering our rudimentary understanding of the power of soil moisture to influence regional and global climate, and what we know about the circular relationship water in the soil has to that in the atmosphere, given a little vegetation and sunshine.

Soil moisture is one of the components of land-surface evapotranspiration, and is a required parameter for evaporation calculations. Its critical role in the process has been made evident by many studies finding that evaporation and precipitation decrease over time in areas where forest canopy has been reduced. Such overgrowth preserves soil moisture; removing it allows for increased surface albedo as well as a rise in surface temperatures. When the intimacy between soils and vegetation types is disturbed altering soil moisture levels, original plant species cannot always be reestablished under the altered conditions. Winter precipitation is also held in soils, humidifying the atmosphere over summer months.

Scientists (indeed, even ordinary citizens) have known for many years that soil moisture levels and dependent systems have profound effects on climate. The conundrum for climate modelers is that total soil moisture storage cannot be directly measured. While point measurements of soil moisture are taken around the world, it is not possible to measure how much mud there is at any one time on the planet, know its depth or relative dryness. Yet, a realistic initialization of the soil moisture field is required for climate simulations.

This soil moisture data gap prompted members of the Global Energy and Water Cycle Experiment (GEWEX) International Satellite Land-Surface Climatology Project (ISLSCP) and the GEWEX Numerical Experimentation Panel (G-NEP) to establish a pilot program in 1994 for generating global soil wetness fields, as well as snow cover, surface-atmosphere fluxes and runoff, for use in initializing global circulation models.

"There are few adequate data sets for initializing soil wetness in climate models," says director of the Global Soil Wetness Project (GSWP), Paul Dirmeyer, a research scientist at the Center for Ocean-Land-Atmosphere Studies (COLA). "In climate studies, it's very important to have soil wetness, especially in the summertime for the midlatitudes and just about anytime for the tropics and subtropics.

"The idea behind GSWP is to use the highest quality data available to drive the best, state-of-the-art land surface models available in order to get—maybe not perfect data—but the best soil wetness data that has been produced to date."

Land surface modelers involved in GSWP will use the same input data sets, identical soil and vegetation maps, to run their various models. Accordingly, the group selected ISLSCP Initiative I data distributed by NASA's Goddard Earth Sciences Data and Information Services Center (GES DISC) for its global coverage spanning 1987 through 1988, one degree gridded resolution, and for the wealth of geophysical parameters contained in the collection. Output data from all the models will supplement and improve upon some of the Initiative I data, and be at the same global, one-degree resolution.

But besides the advantage of deriving consistent output, "The project is a feasibility study for production of land surface data with models, and to see how well different models compare globally. Most models are rather similar," Dirmeyer says, "but none are perfect." Model intercomparison thus allows for error detection and model refinement, he says.


Dirmeyer, P. A. 1995. Review of meeting on problems initializing soil wetness. Bulletin of the American Meteorologcial Society 76. In press.

International GEWEX Project Office (IGPO). 1995. Global Soil Wetness Project, Ver. 1. WCRP Global Energy and Water Cycle Experiments, Silver Spring, MD.

For more information

NASA Goddard Earth Sciences Data and Information Services Center (GES DISC)

International Satellite Land Surface Climatology Project (ISLSCP)

First ISLSCP Field Experiment (FIFE)

About the remote sensing data used
Data International Satellite Land Surface Climatology Project (ISLSCP)
Parameter soil moisture
DAAC NASA Goddard Earth Sciences Data and Information Services Center (GES DISC)
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