The memory of soil moisture measured by the SMAP satellite

Global soil moisture map (Image MIT/NASA/JPL-Caltech)
Global soil moisture map (Image MIT/NASA/JPL-Caltech)

An article published in the journal “Nature Geoscience” describes an analysis of data collected during the first year of NASA’s SMAP satellite’s mission. The results were surprising, especially because the data about the soil’s upper layer has a kind of memory of weather, more than you might think from theoretical models or from incomplete surveys carried out prior to this mission.

The SMAP (Soil Moisture Active Passive) satellite was launched on January 31, 2015 with the goal of monitoring the moisture in the surface layer of the soil, to be precise 5 centimeters (2 inches). It’s a thin layer but is critical because it’s the one where vegetation grows, including our food. The moisture in that layer is a very small part of the water existing on Earth but it’s essential for plants.

Completeness and continuity of moisture monitoring by the SMAP satellite are by far the best ever obtained. Now this allowed to make initial assessments through the analysis of the data collected during the first year of the mission and they will be useful in refining climate models, in weather forecasting and to monitor agricultural activities.

The MIT and NASA’s JPL team that conducted the analysis had a few surprises. The greatest was given by the memory effect for weather anomalies. For the first time thanks to these global measurements it was possible to quantify this type of effect.

In essence, when rains are more or or less strong than normal their effects have a greater persistence than expected, lasting for a few days instead of only for a few hours. On average, about one-seventh of the amount of rainfall is still present in those 5 centimeters of soil three days after the fall and persistence is greater in drier regions.

The analysis also showed a feedback effect that can amplify the effects of drought and floods. When moisture evaporates from wet soil it cools it during this process but when the soil becomes too dry this effect diminishes and this can lead to a hotter weather and heat waves that make the drought worse. It’s a type of effects hypothesized but never observed directly before.

These are important findings that will allow to significantly improve various climate models. Two years after its launch, the SMAP satellite confirms that its mission is crucial to better understand a variety of phenomena taking place on the planet that have direct effects on its inhabitants’ lives. Surely the next studies will provide other results at least as useful.

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