How much do SMAP brightness temperature observations and gauge-based precipitation data contribute to the skill of the SMAP Level-4 Soil Moisture product?

Authors: Rolf Reichle, Qing Liu, Joe Ardizzone, Wade Crow (USDA), Gabrielle De Lannoy (KULeuven), Jianzhi Dong (USDA), John Kimball (U Montana), and Randal Koster  

Soil moisture provides an important connection between the land surface water, energy, and carbon cycles. By merging Soil Moisture Active Passive (SMAP) satellite brightness temperature (Tb) observations and gauge-based precipitation data into a numerical model of land surface water and energy balance processes, NASA generates the global, 9-km resolution, 3-hourly Level-4 Soil Moisture (L4SM) data product, which is published with ~2.5-day latency to support Earth science research and applications such as drought monitoring.

Using additional model simulations and validation against independent measurements, we find that the SMAP Tb observations contribute more to L4SM surface soil moisture skill than do the gauge-based precipitation data (Figure 1). SMAP’s contribution to L4SM skill is particularly large in poorly instrumented regions, including portions of South America, Africa, and central Australia. Both data sources contribute about equally to L4SM root-zone soil moisture skill and L4SM runoff skill largely stems from gauge-based precipitation data (not shown).

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Figure 1. Surface soil moisture skill difference between the L4SM product and L4SM simulations (top) without SMAP Tb and (bottom) without gauge-based precipitation. Skill is measured as anomaly time series correlation and estimated using independent ASCAT retrievals as Instrumental Variable. Red shading indicates that using SMAP Tb observations (or gauge-based precipitation data) increases the skill of the L4SM product relative to not using these data


Reichle, R. H. et al. (2020), The Contributions of Gauge-Based Precipitation and SMAP Brightness Temperature Observations to the Skill of the SMAP Level-4 Soil Moisture Product, J. Hydromet., in press, doi:10.1175/JHM-D-20-0217.1.

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