A NASA satellite mission designed to measure near-surface soil moisture content globally using microwave radiometry. SMAP data are vital for drought monitoring, flood forecasting, and agricultural water management.
The Soil Moisture Active Passive (SMAP) mission, launched by NASA in January 2015, was designed to provide global measurements of soil moisture in the top 5 cm of soil at 3-day intervals. SMAP carries an L-band (1.4 GHz) microwave radiometer that measures naturally emitted microwave radiation from the soil, which is strongly influenced by soil moisture content. The original mission design included both a radar (active) and radiometer (passive) instrument; however, the radar failed in July 2015, and the mission has since relied on the radiometer. SMAP provides soil moisture estimates at approximately 36 km resolution from the radiometer alone and 9 km resolution from enhanced products that combine SMAP data with Sentinel-1 SAR observations. The data are critical for improving weather and climate forecasts, as soil moisture influences the partitioning of surface energy into sensible and latent heat fluxes. SMAP soil moisture products are also used in drought monitoring systems, flood prediction models, and crop yield forecasting. Root-zone soil moisture, estimated by assimilating SMAP surface observations into land surface models, provides information about water availability for vegetation that surface measurements alone cannot capture.