Soil moisture is the amount of water contained in the soil. From agriculture productivity to flood and drought prediction, soil moisture on Earth plays a key role in understanding our planet. How much do you know about Earth’s soil moisture?
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Although they use different types of technology, NASA's SMAP and Aquarius instruments include radiometers (instruments for measuring the intensity of radiant energy) that operate at the same frequency (i.e., 1.41 GHz). At this frequency, the emission of natural radiation from Earth's surface is affected by both the moisture content of soil and the saltiness of ocean. Continually measuring these properties helps us understand water cycle processes across the globe. For example, SMAP observations help with flood assessment and drought monitoring on land. Over the ocean, Aquarius data are being used to study the outflow of rivers and melting of polar ice, both of which decrease ocean salinity.
Those resources are harder to estimate than rain or bodies of water and snow in plain sight. SMAP’s measurements can help people in these and other parts of the world to understand how much water is at hand and to plan accordingly.
According to the California Department of Water Resources, about 65 percent of California’s water supply comes from the Sierra Nevada Mountains’ snowpack. Winter rain and snow replenish rivers and groundwater for the year. The amount of snow and when it melts in the spring directly affects how much water is available to cities and to the agricultural industries of the Central Valley. The Global Precipitation Measurement Mission Core Satellite has advanced instruments that can see the snow in clouds to add data to hydrologic models necessary for determining freshwater availability for agriculture and other uses.
Soil moisture conditions on the Indian subcontinent vary dramatically as a result of seasonal monsoons. During some months, evaporation of seawater feeds monsoon rains, which saturate Indian soil for weeks. The resulting freshwater discharge from rivers such as the Ganges reduces salinity levels in the Bay of Bengal. Thus monsoons can be seen in sea surface salinity patterns around India: the relatively salty Arabian Sea to the west contrasts sharply with the low-salinity Bay of Bengal to the east.
Soil moisture variations affect the evolution of weather and climate over continental regions. Initialization of numerical weather prediction and seasonal climate models with accurate soil moisture information enhances their prediction skills and extends their skillful lead-times. Improved seasonal climate predictions will benefit climate-sensitive socioeconomic activities, including water management, agriculture, and fire, flood and drought hazards monitoring.
Using precipitation data from satellites such as the Global Precipitation Measurement mission, plus soil moisture data from the upcoming Soil Moisture Active Passive (SMAP) mission, allows scientists to more accurately model potential flood conditions. In addition, the instruments onboard SMAP will have difficulty obtaining a clear observation of the soil surface when it is raining, which is expected to cause errors in the SMAP soil moisture retrievals. The addition of GPM data to the SMAP soil moisture algorithm will provide a more robust approach for assessing the times and locations of rainfall events so that the SMAP mission can correctly interpret the soil moisture data.
In addition to ground-based measurements, NASA uses airborne and spaceborne remote sensing instruments to estimate soil moisture. For example, measurements from NASA’s Airborne Microwave Observatory of Subcanopy and Subsurface (AirMOSS) radar are used to estimate soil moisture profiles, from surface to the root zone, at many representative locations in the North American continent. AirMOSS uses long-wavelength microwave radar to penetrate through the vegetation, and into the soil where plants’ roots are. The radar imagery is converted into estimates of Root Zone Soil Moisture.
In addition to causing deaths, floods are responsible for losses that average billions of dollars per year in the U.S. alone. SMAP will help communities to predict and prepare for flooding to minimize loss of life and property.
The water cycle involves more than evaporation from the oceans and land forming clouds that drop rain or snow on the ground, where it flows across the land before returning to the sea. For example, plants absorb water from the soil to grow but also “transpire” some of it straight back into the air. Animals drink water and eat plants, delivering water back to the ground where it may end up flowing to the ocean or be evaporated as the ground dries. NASA’s Soil Moisture Active Passive (SMAP) spacecraft will measure soil moisture over all of Earth’s land surface that is not frozen every three days. Its data will permit scientists to better understand the details of the water cycle.
By measuring the nature, extent, timing and duration of landscape seasonal freezing and thawing transitions on Earth’s land surface, NASA’s SMAP freeze/thaw measurements will also contribute to understanding how ecosystems respond to and affect global environmental change. SMAP will improve regional mapping and prediction of high latitude ecosystem processes.