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.

中文翻译：

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土壤盐度动态影响基于辐射计的全球农田土壤水分反演

土壤盐分影响微波辐射计观测的方式与土壤湿度 (SM) 类似。如果不考虑土壤盐分，基于辐射计的 SM 产品的质量可能会受到损害。本研究使用基于 X、C 和 L 波段辐射计的 SM 产品对全球农田进行基于三重配置 (TC) 分析的土壤盐分效应的幅度、频率依赖性、季节模式和影响因素。多种产品证实土壤盐度动态造成 0.005–0.01 cm 的不确定性 $^{3}~\cdot $厘米 $^{-3}$到总 SM 反演误差预算，与土壤盐浓度成正比，在 L 波段较高，在 X-/C 波段较低。当降雨驱动显着的土壤盐分动态时，春季和夏季的土壤盐分效应更强。当盐被抬升到表土上并被微波辐射计检测到时，SM 反演的不确定性随着非降水天气的持续时间而单调增加。尽管盐分效应总体较弱，但随着土壤盐化，X-/C 波段产品似乎比 L 波段 SM 产品反应更早，这可能是由于复杂的土壤-盐水-水介电特性和较浅的微波穿透深度。在土壤水分主动被动 (SMAP) SM 反演算法中忽略损失因子也可能会增强土壤盐分效应。这些不确定性本质上是随时间变化的湿偏差，