Soil moisture is of interest to scientists, weather forecasters, and the Department of Defense, among others. Yet currently it’s not practical to make direct measurements of the variable over all global land regions. Rather, users make “best estimates” based on the history of precipitation and other indirect indicators.
“It’s the best they can do,” says Dara Entekhabi — a professor with joint appointments in civil and environmental engineering and in earth, atmospheric, and planetary sciences — who aims to change that.
The respected hydrologist now leads an international NASA science team set to make the first-ever global observations of soil moisture from space. The data could dramatically impact our understanding of how water weaves through the global ecology, and promise better weather, flood, and drought forecasts as well as predictions of agricultural productivity and climate change.
“Bringing new data sets to work on (a) problem is the way we make quantum leaps in understanding it,” says Entekhabi, who also directs MIT’s Ralph M. Parsons Laboratory for Environmental Science and Engineering, established in the 1950s as a water lab.
The Soil Moisture Active-Passive mission (SMAP), a satellite scheduled to launch in 2014, will provide real data at high resolution, creating a global map of soil moisture every two to three days. It will also tell whether that moisture is frozen or not — a factor key to understanding sources and sinks of carbon in the land biosphere.
Among many applications, the data will inform models key to weather forecasts and climate change. The National Weather Service, for example, currently produces daily estimates of soil moisture across the country. That helps forecasters determine how much rain an area can withstand before the soil is saturated. Coupled with weather radar indicating the amount of expected rain, they then know whether to issue a flash flood warning.
But because the data key to these forecasts are estimates, Entekhabi says, sometimes the predictions are wrong. “SMAP’s direct measurements of soil moisture at high resolution (about 10 kilometers) will therefore lead to more precise forecasts.”
The data will also help constrain current climate-change models, which disagree about which areas across the globe will have more stored soil moisture, and which less. “The SMAP data will narrow the uncertainty of these models,” he says.
The Department of Defense, too, is interested in SMAP. Soil moisture affects everything from low-level fog forecasts to the calculation of the so-called density altitude, or lift capacity of aircraft. Yet it “is among the top (environmental) measurements that they don’t have,” Entekhabi says.
Key to the work are two sensors aboard the satellite. Together they will create soil-moisture maps of higher resolution and accuracy than either can achieve alone. One captures the low-frequency microwaves naturally emitted at the surface of the planet. The other actively beams low-frequency microwaves to the surface and captures what is reflected back.
Soil minerals and water molecules have extremely different microwave emission and reflection properties, Entekhabi explains. That allows detection of the moisture content from space. SMAP captures that data then analyzes it to create the resulting maps.
Entekhabi speaks fondly of MIT Professor Emeritus Peter Eagleson, under whom he received his MIT Ph.D. and whom he calls “the grandfather of modern hydrologic science.” Now Entekhabi and colleagues are continuing Eagleson’s footsteps, poised to make similarly groundbreaking contributions to the field.