Applicability research of optical remote sensing derived indices in soil moisture downscaling based on SiB2 simulation

There are many indices, e.g. SEE (Soil Evaporation Efficiency), TVDI (Temperature Vegetation Dryness Index) and ATI (Apparent Thermal Inertia), usually used in downscaling of coarse-resolution soil moisture derived from microwave remote sensing platforms including AMSR-E, FY-3B, SMOS and SMAP so on. However, which index is optimal and what is its applicative conditions still keep unknown. This paper attempts to use SiB2 (Simple Biosphere Model 2) to simulate the soil surface temperature, canopy temperature, evaporation, evapotranspiration, and soil moisture, and explore the relationships between various indices and soil moisture to determine their relevance and the applicability. Simulation shows that apparent thermal inertia is not applicable under the condition of higher vegetation coverage (e.g. FVC > 0.65), while the soil evaporation efficiency has a higher correlation with soil moisture under dense vegetation coverage and is recommend to be used for downscaling of soil moisture.