ECMWF定義的SWI是前兩層相對SM的平均值

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

我們這里介紹的不太一樣時間上的平均

這里使用的SWI filterSWI exponential smoothing filter¹ (Wagner et al., 1999; Albergel et al., 2008)定義為：

定義式:
$SWI(t)=\frac{{\sum }_{i}S{M}_{sat}(t_i)e^{?\frac{t?t_i}{T}}}{{\sum }_i{e}^{?\frac{t?t_i}{T}}}$

$S{M}_{sat}$是$t_i$時刻土壤retrieval
T代表時間滯后常數
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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Evaluation of 18 satellite- and model-based soil moisture products using in situ measurements from 826 sensors ??

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