土壤湿度指标（Soil wetness index，soil water index SWI）

ECMWF定义的SWI是前两层相对SM的平均值

The Soil Water Index quantifies the moisture condition at various depths in the soil.

我们这里介绍的不太一样时间上的平均

这里使用的SWI filterSWI exponential smoothing filter¹² (Wagner et al., 1999; Albergel et al., 2008)定义为：

定义式:
$SWI(t)=\frac{{\sum }_{i}S{M}_{sat}(t_i)e^{-\frac{t-t_i}{T}}}{{\sum }_i{e}^{-\frac{t-t_i}{T}}}$

$S{M}_{sat}$是$t_i$时刻土壤retrieval
T代表时间滞后常数
t代表time step

To illustrate the SWI filter, SMAPL3E instantaneous volumetric soil moisture retrievals (from both ascending and descend"ing overpasses) and 3-hourly SMAPL3Eswi time series obtained by application of the SWI filter (with the time lag constant T set to 5 d) for a 2-month period at 34.82° N. 89.44° W.

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1. Hylke E. Beck et al., 2021, Evaluation of 18 satellite- and model-based soil moisture products using in situ measurements from 826 sensors, HESS ↩︎

2. Grillakis, M.; Koutroulis, A.; Alexakis, D.; Polykretis, C.; Daliakopoulos, I. Regionalizing Root‐Zone Soil Moisture Estimates From ESA CCI Soil Water Index Using Machine Learning and Information on Soil, Vegetation, and Climate. Water Resources Research 2021. https://doi.org/10.1029/2020WR029249. ↩︎