A Simple, Physically-Based Soil Moisture Index from SMAP Radiometer Observations

In this paper, a new soil moisture index (SMI), which aims to capture the temporal variability of soil moisture, was developed by using the L-band SMAP radiometer observations. This index is proposed on the basis of two key foundations: 1) vegetation and roughness have similar effects on "depolarization" of microwave emission, while soil moisture enhances polarization differences; 2) vegetation and roughness generally impose positive effects on surface emissivity, while soil moisture and emissivity are negatively correlated. Based on the two physical principles, it is possible to decouple the effects of soil moisture and those of vegetation and surface roughness in a two-dimensional space independent of vegetation type and roughness condition. The proposed SMI was then validated by in-situ measurements from five dense soil moisture networks covering different vegetation and climatic conditions, and also compared with SMAP and ESA CCI official soil moisture products. The results show that the new SMI can well reproduce the temporal dynamic of soil moisture with a very favorable averaged correlation coefficient value of 0.87, higher than that of SMAP (0.80) and ESA CCI (0.69). The unique advantage of the proposed SMI is that it does not need field observations of soil moisture, roughness, or canopy biophysical properties for calibration purposes, and does not involve any empirical coefficients. Therefore, it has great potential to estimate soil moisture variability on a global scale.