Soil Salinity Dynamics Impairs Radiometer-Based Soil Moisture Retrieval Over Global Cropland

Soil salt affects microwave radiometer observations in a similar way to soil moisture (SM). Without consideration of soil salt, the quality of radiometer-based SM products might be impaired. This study explores the magnitude, frequency dependence, seasonal patterns, and influence factors of soil salinity effects based on triple collocation (TC) analyses using X-, C-, and L-band radiometer-based SM products over global cropland. Multiple products corroborate that soil salinity dynamics contributes an uncertainty of 0.005–0.01 cm $^{3}~\cdot $ cm $^{-3}$ to the total SM retrieval error budget, proportional to soil salt concentration, higher at L-band and lower at X-/C-band. Soil salinity effects are stronger in spring and summer when rainfall drives significant soil salinity dynamics. SM retrieval uncertainty increases monotonously with the duration time of nonprecipitating weathers when salt is uplifted onto topsoil and sensed by microwave radiometers. Despite of overall weaker saline effects, X-/C-band products seem to respond earlier than the L-band SM products as soils become salinized, probably due to complicated soil-salt-water dielectric properties and shallower microwave penetration depths. The neglect of loss factor in soil moisture active passive (SMAP) SM retrieval algorithm might also enhance soil salinity effects. These uncertainties are essentially time-varying wet biases, and dynamic soil salinity maps are needed for bias-correcting current L-band and future P-band SM products.