SMAP will provide a capability for global mapping of soil moisture and freeze/thaw state with unprecedented accuracy, resolution, and coverage. SMAP science objectives are to acquire space-based hydrosphere state measurements over a three-year period to:

• Understand processes that link the terrestrial water, energy and carbon cycles
• Estimate global water and energy fluxes at the land surface
• Quantify net carbon flux in boreal landscapes
• Enhance weather and climate forecast skill
• Develop improved flood prediction and drought monitoring capabilities

Soil moisture is a key control on evaporation and transpiration at the land-atmosphere boundary. Since large amounts of energy are required to vaporize water, soil moisture control also has a significant impact on the surface energy flux. Thus, soil moisture variations affect the evolution of weather and climate particularly over continental regions.

Recent model simulations of the effects of greenhouse gases on climate show that current models agree quite well in predicting temperature change but disagree significantly in predicting surface moisture change and water resource availability. Accurate soil moisture information such as will be available from SMAP will improve the performance of numerical weather prediction models and seasonal climate models and enhance their predictive skill.

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Soil moisture and its freeze/thaw state are also key determinants of the global carbon cycle. Carbon uptake and release in boreal landscapes is one of the major sources of uncertainty in assessing the carbon budget of the Earth system (the so-called missing carbon sink). The SMAP mission will quantify the nature, extent, timing and duration of landscape seasonal freeze/thaw state transitions that are key to the estimation of terrestrial carbon sources and sinks. SMAP freeze/thaw state measurements will also contribute to understanding how ecosystems respond to and affect global environmental change, improving regional mapping and prediction of boreal-arctic ecosystem processes.
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