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News: NASA Soil Moisture Radar Ends Operations; Mission Science Continues
News: NASA Focused on Sentinel as Replacement for SMAP Radar 

Science Overview

SMAP's spaceborne Earth-observation mission will enable 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-forecast and climate-forecast skills
  • Develop improved flood-prediction and drought-monitoring capabilities

Water and Energy Cycles, Weather, and Climate

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 data from SMAP, will improve the performance and enhance the predictive ability of numerical weather-prediction models and seasonal climate models. Soil moisture is a key control on evaporation and transpiration at the land-atmosphere boundary. Because vaporizing water requires large amounts of energy, soil-moisture control also has a significant impact on the surface energy flux. Soil-moisture variations affect the evolution of weather and climate, particularly over continental regions.

Carbon Cycle and Ecosystems

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.

SMAP Goals

The SMAP Project is designed to collect measurements of surface soil moisture and freeze-thaw state, together termed the hydrosphere state. Soil moisture is defined in terms of volume of water per unit volume of soil. Freeze-thaw state is defined as the phase of the water contained within the landscape including soil and vegetation. To meet the goals of science and applications users, SMAP must:
    • Resolve hydrometeorological water and energy flux processes and extend weather and flood forecast skill, spatial resolution of 10 km and temporal resolution of 3 days are required.
    • Resolve hydroclimatological water and energy flux processes and extend climate and drought forecast capability, spatial resolution of 40 km and temporal resolution of 3 days are required.
    • Quantify net carbon flux in boreal landscapes, spatial resolution of 3 km and temporal resolution of 2 days are required. In addition, the SMAP mission will validate a space-based measurement approach that could be used for future systematic hydrosphere state monitoring missions.

Baseline Science Requirements

The SMAP science objectives lead to the following requirements for the baseline science mission:

  • The baseline science mission shall provide estimates of soil moisture in the top 5 cm of soil with an error of no greater than 0.04 cm3/cm3 (one sigma) at 10 km spatial resolution and 3-day average intervals over the global land area, excluding regions of snow and ice, frozen ground, mountainous topography, open water, urban areas, and vegetation with water content greater than 5 kg/m2 (averaged over the spatial resolution scale).
  • The baseline science mission shall provide estimates of surface binary freeze-thaw state in the region north of 45N latitude, which includes the boreal forest zone, with a classification accuracy of 80% at 3 km spatial resolution and 2-day average intervals.
  • The baseline science mission shall collect space-based measurements of soil moisture and freeze-thaw state for at least three years to allow seasonal and interannual variations of soil moisture and freeze-thaw to be resolved.
  • The SMAP project shall conduct a calibration and validation program to verify data delivered meets the above requirements.
Synthetic aperture radar SMAP Earth's water cycle infographic
Earth's water cycle involves the transfer and storage of water in the atmosphere, on the planet's surface, underground, and by life in its many forms.

Synthetic aperture radar SMAP water supply
Water availability is changing as a result of global climate change. SMAP data will help researchers understand how these changes affect water supply and food production.

Synthetic aperture radar SMAP global carbon cycle infographic
The global carbon cycle is the complex interaction of different carbon-based gases taking place among Earth’s atmosphere, land, and oceans.
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