Sensors in space to map Earth's water cycle

May 2007


Soil moisture and freeze/thaw measurements are essential to the accuracy of weather forecasts and predictions of global carbon cycle and climate. Yet scientists have no operational network for gathering this data as they do for rainfall, atmospheric winds, humidity and temperature. As a result, weather forecasts are often inaccurate and climate projections uncertain, particularly in the prediction of precipitation and evaporation. In fact, the degree of uncertainty is so great that global warming projections don’t even agree on the sign of regional water cycle change.


MIT scientists are developing the observing systems and algorithms required for global monitoring of soil moisture that would go beyond the point-by-point, field gathering methods now in scattered use. In 2000, Professor Dara Entekhabi, recently appointed director of the Parsons Laboratory for Environmental Science and Engineering at MIT, led a group of about 30 scientists and engineers from universities, NASA and industry to work on a satellite project as a solution.

The project, called the Hydrosphere State Mission (Hydros), was selected by NASA from among 20 competing satellite proposals and scheduled for a 2009 launch. But NASA cancelled the project abruptly in 2005 when funding for its earth sciences missions was diverted to the Moon-Mars Initiative. However, in a report released last month, the National Academy of Sciences recommended the soil moisture measurement project become a top priority for NASA. Recently NASA began engineering studies and other activities related to the definition of system performance requirements.

The Hydros team’s design gathers both passive and active low-frequency microwave measurements on a continuous basis, essentially creating a map of global surface soil moisture. A 6-meter deployable mesh antenna on a satellite will gather data across a swath of 1,000 kilometers, creating ribbons of measurements around the globe and completing the cycle every few days.

In addition to measuring soil moisture, Hydros will detect if the surface moisture is frozen. In forests, the freeze/thaw state determines the length of the growing season and the balance between carbon assimilation into biomass and landscape loss of carbon due to respiration. The result of this balance can tell scientists if a forest is a net source or net sink of carbon.


One mission obstacle that remained was integrating the two types of measurements the satellite would gather: passive measurements collected by radiometer, and active collected by radar. The radiometer measurements provide highly accurate data at a course resolution of 40 kilometers. The radar measurements provide much higher resolution (3 kilometers), but lower accuracy. In April 2007, Susan Dunne, Entekhabi and Eni Njoku published a paper in IEEE Transactions on Geoscience and Remote Sensing that describes the mathematical solution to this problem. The authors present an algorithm that results in a combined resolution of 6 kilometers. The system incorporates soil physics models to infer root zone soil  moisture to a depth of 1 meter, as well.


Entekhabi and Mahta Moghaddam have begun studies on the generation of sensors beyond those proposed for Hydros that would be capable of mapping recharge into groundwater aquifers from space. In “Mapping Recharge From Space: Roadmap to Meeting the Grand Challenge” (Hydrogeology Journal, April 2007), they propose futuristic systems for mapping recharge, a key to the sustainable use of aquifers.


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