Satellite-based Surface Energy Flux/Soil Moisture Estimation
Soil Moisture Retrievals
Research is also being done on a retrieval of soil moisture (SM) using surface flux estimates from satellite-based thermal infrared (TIR) imagery and the Atmosphere-Land-Exchange-Inversion (ALEXI) model (Hain et al. 2008, 2010). The ability of ALEXI to provide valuable information about the partitioning of the surface energy budget, which can be largely dictated by soil moisture conditions, accommodates the retrieval of an average soil moisture value from the surface to the rooting depth of the active vegetation.
For this method, the fraction of actual to potential evapotranspiration (fPET) is computed from an ALEXI estimate of latent heat flux (LE) and potential evapotranspiration (PET). The ALEXI-estimated fPET can be related to a fraction of available water (fAW) in the soil profile. Four unique fPET to fAW relationships are proposed and validated against Oklahoma Mesonet soil moisture observations within a series of composite periods during the warm seasons of 2002-2004. Based on the validation results, the most representative of the four relationships is chosen and is shown to produce reasonable (mean absolute errors values less than 20% with respect to fAW) SM estimates when compared to Oklahoma Mesonet observations. Quantitative comparisons between ALEXI and modeled SM estimates from the Eta Data Assimilation System (EDAS) are also performed to assess the possible advantages of using ALEXI SM estimates within numerical weather predication (NWP) simulations. This TIR retrieval technique is advantageous over microwave techniques because of the ability to sense SM that extends into the root-zone layer. SM conditions can also be retrieved over dense vegetation cover and is available on spatial resolutions on the order of the native TIR imagery. A notable disadvantage is the inability to retrieve SM conditions through cloud cover. See “Soil Moisture Assimilation Research.”