Soil and vegetative controls on global soil moisture drydowns using SMAP: Implications for large scale soil hydraulic parameterization

Scaling soil hydraulic properties (SHP) for compatibility with various land-surface and/or earth systems models is a crucial challenge. The direct portability of Representative Elementary Volume (REV) scale-based SHP to coarser scales is restricted by the large differences observed in the soil moisture drydown dynamics at the REV and remote sensing scales. However, recent literature suggests that these differences may be quantifiably attributed to differences in sub-grid scale variability in soil and vegetation. In this study, we present a global parameterization of the seasonal soil moisture drydowns using soil moisture observations from SMAP. Global analysis reveals strong evidence of vegetation dominated control on soil moisture drydowns, especially in Savanna and cropland biomes in response to the seasonal variability in evapotranspiration (ET) and plant photosynthetic activity (FPAR). In areas with sparse vegetation, soil texture (percent clay) constraints the soil moisture drydowns, thus controlling the effective wilting point of the pixel. The effective wilting point is observed to have a strong positive correlation with percent clay of the soil across different landuse classes. We discuss the implications of these observations w.r.t the ongoing efforts of large-scale soil hydraulic parameterization. We propose that in a large-scale inverse-modeling scheme, both static (soil controlled) and the seasonally-varying vegetative control should be considered. For this purpose, remotely sensed soil moisture, ET and FPAR can be used effective indicators of the coupled Soil-Atmosphere-Plant processes at large spatial scales.