Obtaining daily root zone soil moisture using a calibration-free approach

Root Zone Soil Moisture (RZSM) plays a critical role in the regional and global water cycle. The distribution of RZSM influences the incidence and intensity of floods and droughts, and has a range of ecohydrological implications including on crop productivity and the growth and sustainability of trees. Despite its importance, obtaining gap-free daily moisture data remains challenging. For example, remote sensing-based soil moisture products often have gaps arising from limits posed by the presence of clouds and satellite revisit period. Here, we retrieve a proxy of daily RZSM using the Surface Flux Equilibrium theory. Our method is calibration-less, parsimonious, and only needs widely available meteorological data and standard land-surface parameters. Evaluation of the retrievals at Oklahoma Mesonet sites shows that our method, overall, matches or outperforms widely available RZSM estimates from three markedly different approaches, viz. remote sensing data based Atmosphere-Land EXchange Inversion (ALEXI) model, the Variable Infiltration Capacity (VIC) model, and the Soil Moisture Active Passive (SMAP) mission root zone moisture data product. RZSM from our method could serve as a more accurate and temporally-complete alternative for a variety of applications including mapping of agricultural droughts, assimilation of RZSM for hydrometeorological forecasting, and design of optimal irrigation schedules.