NM WRRI Scientist Invited to SMAP Science Team Meeting
by Catherine Ortega Klett, NM WRRI Program Manager
Dr. Tom Schmugge was invited to the 11th SMAP (Soil Moisture Active Passive) Science Team meeting at the Jet Propulsion Laboratory (JPL) in Pasadena CA on April 17–19, 2018. He was invited because of his early research on the use of microwave sensors for soil moisture remote sensing in the 1970s. This was the third anniversary of SMAP’s satellite launch in 2015 and marked the end of the prime operation phase of the satellite. It was important for scientists to show significant results from the satellite during this three-year period to insure its continued support. Based on the presentations at the meeting, the scientists succeeded.
SMAP has two instruments, a microwave radiometer operating at 1.4 GHz and a radar operating at 1.2 GHz. The radiometer is the primary instrument for soil moisture sensing and the radar is there to obtain higher spatial resolution images of the surface. Unfortunately, the radar failed after about three months. The remote sensing of soil moisture relies on the unique dielectric properties of water at microwave frequencies ‒ it is this property that makes microwave ovens so effective.
In addition to SMAP, the European Space Agency launched the SMOS (Soil Moisture & Ocean Salinity) satellite mission in 2009. Thus, there are two space missions for the purpose of observing global soil moisture. The basic characteristics of the two systems are given in the figure below, which is a snapshot of a slide presented at the meeting by Dr. Jeff Walker of Monash University in Australia:
The shortcoming of the microwave radiometer approach is the rather poor spatial resolution of the soil moisture observations (~40 km). The higher resolution radar was to be used to improve this spatial resolution as indicated for SMAP. After the radar failed, the team has been attempting to use radar from other satellites, but the lack of co-incident observations has made this difficult. The figure below from the talk of Dr. Thomas Jackson on the USDA/ARS Hydrology and Remote Sensing Laboratory presents global results from the two satellites for a week in early June 2015.
These figures show reasonable agreement, but there are a few apparent differences. While these global results are interesting, it was the application of the SMAP data to regional scale watersheds that Dr. Schmugge found most impressive. Working with the time series of soil moisture data from SMAP and a water balance analysis for the San Francisco watershed in eastern Arizona, Dr. Randy Koster and co-workers at NASA's Goddard Space Flight Center came up with very reasonable estimates of the monsoon season precipitation for 2015 through 2017. Their estimates of stream flow for the basin were not as good, but still impressive considering that only the SMAP soil moisture data were used. They also made the point that this watershed yielded better agreement than some of those made in more humid conditions.
Two other applications of SMAP data were discussed at the meeting, namely the inclusion of SMAP data in the Drought Monitoring Index and also its use by USDA's Foreign Agriculture Service's Global Crop Assessment. The concern with these applications is the possible lack of continuity because neither SMAP nor SMOS are research satellites, and so may not be available in the future.