Temporal gap-filling of 12-hourly SMAP soil moisture over the CONUS using water balance budgeting

Temporal gaps in satellite-based soil moisture (SM) products are a persistent issue. This study presents an entirely observation-based method to derive volumetric SM content for filling gaps in Soil Moisture Active Passive (SMAP) retrievals. Using a water balance equation, 12-hr topsoil water amount variations are determined based on observed precipitation from the Global Precipitation Measurement Mission (inflow) and a hydrologic loss function (outflow) built on SMAP dry-downs. A temporally seamless SM product, composed of SMAP dry-downs and precipitation-driven moisture approximations, was generated as a secondary outcome in determining optimal water balance parameters. This data set maintains the original SMAP SM dynamics with a median Pearson correlation (R) of 0.69 and an unbiased root-mean-square error (ubRMSE) of 0.05 m3/m3. Using these parameters and available SMAP observations, a 12-hourly SM product was produced over the conterminous United States. Validated against in situ measurements, this 12-hourly SM product exhibits good performance with a median R of 0.63 and captures most SM peaks induced by heavy rainfall. A time series examination revealed the produced 12-hourly SM product closely corresponds to in situ SM variations and outperforms two other SMAP-based 12-hourly SM products gap-filled using temporal linear interpolation and a three-dimensional smoothing approach, especially during sparse SMAP data periods. The proposed scheme s validity is further verified by the comparable performance of the exclusive filled-on SM estimates. Utilizing the 12-hourly SM data set and its paired hydrologic losses could enhance the quantification connections among the hydrologic components and benefit the understanding of land-surface hydrology.