State-of-the-Art and Future Directions in P-Band Radar Remote Sensing of Soil Moisture.

Knowledge of surface-to-depth profiles of soil moisture is key in understanding the water and energy cycle, connections between surface and subsurface process, evapotranspiration and recharge, and interactions between the water and carbon cycles. While near-surface soil moisture has been extensively addressed by several airborne and spaceborne microwave sensors in the past 3 decades, profiles of soil moisture from surface to the root-zone (RZSM) have been among the most poorly observed geophysical quantities on a global scale.

For surface soil moisture, sensors operating at L-band (~ 24 cm, e.g., NASA Soil Moisture Active-Passive (SMAP) satellite) and higher have been successfully used. Observing RZSM and reaching retrieval depths of down to 50 cm requires longer wavelengths, which are capable of deeper penetration into the soil and carrying back information about the soil dielectric constant profile and therefore the profile of water content. Recently, active microwave sensing in the P-band frequency range (430 MHz) has demonstrated substantial success in measuring RZSM. In particular, the NASA Airborne Microwave Observatory of Subcanopy and Subsurface (AirMOSS) has shown retrieval accuracies of better than 0.05 m3/m3 for a wide variety of biomes and landcover types in north America. There are still, however, a few challenges and opportunities for enhancing the RZSM products. These can be divided into the two areas of (1) enhancing the sensors and sensing scenarios, and (2) improving the retrieval algorithms. In the area of sensor enhancement, we are considering systems that provide observations at multiple frequencies, have better calibration characteristics, have a more compact form factor, and/or are of lower cost. In the area of algorithm enhancement, optimal combinations of hydrologic and electromagnetic models are being considered to develop multi-physics retrieval strategies that integrate the physics of soil water movement with those of radar scattering. This talk will provide an overview of current state-of-the-art in both sensor and algorithm technologies of P-band radar for RZSM observations, and will describe the roadmap for advancing both in the next decade.