Soil Moisture: A Tiny Share of Earth's Water That Shapes Farms, Weather and Climate

About 97 percent of Earth's water is in the oceans. More sits in lakes, rivers, ice and clouds. Almost invisible from space is the water held between grains of soil. It is a tiny amount by comparison, but it decides when, where and what farmers plant, it affects the weather, and in the far north it matters for the global climate.

Measuring it from space

Satellites measure soil moisture in two ways.

• Active radar sends microwaves at the ground and reads the echo, which changes with wetness. It gives detail over small areas (high resolution) but is less accurate. One such sensor flies on the European Metop satellites.
• Passive radiometers pick up the microwaves the soil gives off naturally, which also vary with wetness, to estimate the water in the top few inches. They are more accurate but see a broader area at once. The European Space Agency's SMOS mission has carried one since 2009, and NASA flew them on Aqua (2002) and Aquarius (2011). NASA's SMAP, launched in January 2015, carried both, but its radar stopped sending data a few months after launch.

Active and passive ways of sensing soil moisture from space. NASA Earth Observatory illustration by Joshua Stevens.

Together with ground sensors, these build a growing global record, with the aim of a standard set of measurements for the whole planet.

Farms and drought

Farmers and ranchers need to know more than whether plants have water: whether fields are too muddy for a tractor, how much water crops will have from sprouting to harvest, what harvests will look like worldwide, and how all that may shape trade and food aid. Soils differ too: fine clay and silt hold more water than sand and gravel.

Soils of the United States. Fine-grained soils hold more water than coarse ones. NASA Earth Observatory maps by Joshua Stevens, using CONUS-SOIL data.

A ground sensor covers less than a square meter, so some states have built networks. Oklahoma's Mesonet, conceived after a disastrous flood struck Tulsa in 1984, added soil moisture sensors in 1996. Its more than 100 stations measure at depths down to 60 centimeters by giving the soil a pulse of heat and watching how its temperature changes, and report every 30 minutes. It is one of 31 networks with 1,479 stations in North America, still a sparse web for a continent.

Soil moisture stations across the United States, with readings from the Coastal Sage UCI station in California. NASA Earth Observatory map and chart by Joshua Stevens, using the TAMU North American Soil Moisture Database.

Computer models can fill the gaps, but gathering their inputs can take two or three months. "We have to get soil moisture information to the agriculture community, and the only way to do that is from satellites," said Susan Moran, a hydrologist with USDA's Agricultural Research Service. She noted that the drought then gripping the western United States was not the longest or driest on record, but none had been so hot; heat and lack of water together were pushing soil moisture to unprecedented lows.

Weather

Soil moisture is a thin reservoir in the root zone that plants and evaporation return to the air. Globally, this evapotranspiration supplies more than 60 percent of the rain and snow that fall on land. Forecasters learned its importance in July 1993, said Patricia de Rosnay of the European Centre for Medium-Range Weather Forecasts. After six months of extreme rain and snow on the central United States, the Upper Mississippi had its worst flooding on record. Existing models ignored the water stored in the soil and evaporating back, but a new model tested that very month did not, and came closer to reality.

The link is strongest in hot spots, about 10 percent of Earth's surface according to NASA's Randy Koster: transition zones neither too dry for evaporation to matter nor so humid that the air limits it. They include the U.S. Midwest, northern India and the African Sahel.

N’Djamena, Chad, on the Logone River in the Sahel, October 20, 2015. NASA Earth Observatory image by Joshua Stevens, using Landsat data from the USGS.

The northern thaw

In the boreal lands of Alaska, Canada, Siberia and Scandinavia, frozen soil water is out of plants' reach until spring, when within a few weeks the land thaws, greens and fills with life. To a microwave radiometer, frozen ground looks like a desert and thawed ground looks wet, a contrast satellites have tracked for more than 30 years. John Kimball of the University of Montana and colleagues found that Northern Hemisphere soils thawed for as many as 7.5 more days in 2008 than in 1979, mainly because spring thaw came earlier.

Ice and frozen land advancing and retreating over a year. NASA Earth Observatory image by Joshua Stevens, using NASA Blue Marble data.

That matters for carbon. Natural processes remove half of each year's carbon emissions, and the boreal region, about 15 percent of the land, holds more than 30 percent of the carbon on land. Longer growing seasons could make it a bigger carbon sink; burning, decay and thawing permafrost could make it a source. "There is debate as to how stable that soil will be with continued global warming," Kimball said.

Coastal erosion exposes ice-rich permafrost on Alaska’s Arctic Coastal Plain. Photo by Brandt Meixell/USGS.

Sources

• NASA Earth Observatory, "A Little Bit of Water, A Lot of Impact"; rewritten in hubnx's own words. Images from NASA and the USGS.