A novel remote sensing monitoring index of salinization based on three-dimensional feature space model and its application in the Yellow River Delta of China

Figures & data

Figure 1. Overview of the study area.

Table 1. List of Landsat images and its detail information.

Id	Time	Sensor	Resolution/m	Id	Acquisition date	Data type	Resolution/m
1	1984.05.30	TM	30	21	2004.04.19	TM	30
2	1985.03.14	TM	30	22	2005.04.22	TM	30
3	1986.04.18	TM	30	23	2006.04.09	TM	30
4	1987.04.21	TM	30	24	2007.03.27	TM	30
5	1988.04.07	TM	30	25	2008.04.14	TM	30
6	1989.04.10	TM	30	26	2009.04.01	TM	30
7	1990.03.12	TM	30	27	2010.05.06	TM	30
8	1991.04.16	TM	30	28	2011.03.22	TM	30
9	1992.04.02	TM	30	29	2012.04.01	ETM+	30
10	1993.05.23	TM	30	30	2013.05.30	OLI	30
11	1994.04.24	TM	30	31	2014.05.01	OLI	30
12	1995.03.26	TM	30	32	2015.05.04	OLI	30
13	1996.05.31	TM	30	33	2016.03.03	OLI	30
14	1997.02.11	TM	30	34	2017.04.23	OLI	30
15	1998.04.19	TM	30	35	2018.03.25	OLI	30
16	1999.04.06	TM	30	36	2019.04.13	OLI	30
17	2000.04.08	TM	30	37	2020.04.15	OLI	30
18	2001.03.10	TM	30	38	2021.04.18	OLI	30
19	2002.04.14	TM	30	39	2022.04.05	OLI	30
20	2003.04.01	TM	30

Figure 2. Principle of Feature space.

Table 2. Formula of surface parameters.

Surface parameters	Formula
NDVI (Sun et al. 2018)	$\text{NDVI}=(\text{Nir}-\text{Red})/(\text{Nir}+\text{Red})$
MSAVI (Ma et al. 2018)	$\text{MSAVI}=(2\text{Nir}+1-\sqrt{{(2\text{Nir}+1)}^2-8(\text{Nir}-\text{Red})})/2$
SI (Lu et al. 2020)	$\text{SI}=\sqrt{\text{Blue}\times \text{Red}}$
Albedo (Guo et al. 2022c)	$\text{Albedo}=(0.356\times \text{Blue}+0.130\times \text{Red}+0.373\times \text{Nir}+0.085\times {\text{Swir}}_1+0.072\times {\text{Swir}}_2-0.0018)/1.016$
WI (Hu et al. 2019)	$\text{WI}=0.1446\times \text{Blue}+0.1761\times \text{Green}+0.3322\times \text{Red}+0.3396\times \text{Nir}\mathrm{ }-0.6210\times {\text{Swir}}_1-0.4186\times {\text{Swir}}_2$
IFe2O3 (Peng et al. 2013)	${\mathrm{I}}_{{\text{Fe}}_{{}^2}{\mathrm{O}}_{{}^3}}=\frac{\text{Red}}{\text{Nir}}$

Note: Blue, Green, Red, Nir, Swir₁ and Swir₂ refer to the reflectance of blue band, green band, red band, near infrared band, and short infrared bands, respectively.

Figure 3. Three-dimensional feature spaces that composed of different surface parameters (a) MSAVI-Albedo-IFe2O3; (b) NDVI-WI- IFe2O3; (c) NDVI-Albedo- IFe2O3; (d) MSAVI-Albedo-SI; (e) NDVI-Albedo-SI; (f) NDVI-WI-SI; (g) MSAVI-WI-IFe2O3; (h) MSAVI-WI-SI.

Figure 4. Spatial distribution of levels of soil salinization in three-dimensional feature space. (a) Non salinization; (b) Slight salinization; (c) Mild salinization; (d) Severe salinization.

Figure 5. Establishment of 3-D feature space monitoring index model of salinization.

Figure 6. Spatial distribution of different salinization models (a) MSAVI-Albedo-IFe2O3; (b) NDVI-WI- IFe2O3; (c) NDVI-Albedo- IFe2O3; (d) MSAVI-Albedo-SI; (e) NDVI-Albedo-SI; (f) NDVI-WI-SI; (g) MSAVI-WI- IFe2O3; (h) MSAVI-WI-SI.

Table 3. Accuracy comparison of different salinization monitoring index models.

Feature space	Formula	R^2	RMSE (g/kg)
MSAVI-Albedo-IFe2O3	$\text{MAI}=\sqrt{{\mathrm{(}\text{MSAVI}-1\mathrm{)}}^2{+\mathrm{(}\text{Albedo}-1\mathrm{)}}^2{+\mathrm{(}\mathrm{I}}_{{\text{Fe}}_{{}^2}{\mathrm{O}}_{{}^3}}{)}^2}$	0.93	0.678
NDVI-WI- IFe2O3	$\text{NWI}=\sqrt{{\mathrm{(}\text{NDVI}-1\mathrm{)}}^2{+\text{WI}}^2{+\mathrm{(}\mathrm{I}}_{{\text{Fe}}_{{}^2}{\mathrm{O}}_{{}^3}}{)}^2}$	0.83	0.799
NDVI-Albedo- IFe2O3	$\text{NAI}=\sqrt{{\mathrm{(}\text{NDVI}-1\mathrm{)}}^2{+\mathrm{(}\text{Albedo}-1\mathrm{)}}^2{+\mathrm{(}\mathrm{I}}_{{\text{Fe}}_{{}^2}{\mathrm{O}}_{{}^3}}{)}^2}$	0.81	0.987
MSAVI-Albedo-SI	$\text{MAS}=\sqrt{{\mathrm{(}\text{MSAVI}-1\mathrm{)}}^2{+\mathrm{(}\text{Albedo}-1\mathrm{)}}^2{+\text{SI}}^2}$	0.79	1.124
NDVI-Albedo-SI	$\text{NAS}=\sqrt{{\mathrm{(}\text{NDVI}-1\mathrm{)}}^2{+\mathrm{(}\text{Albedo}-1\mathrm{)}}^2{+\text{SI}}^2}$	0.76	1.356
NDVI-WI-SI	$\text{NWS}=\sqrt{{\mathrm{(}\text{NDVI}-1\mathrm{)}}^2{+\text{WI}}^2{+\text{SI}}^2}$	0.74	1.647
MSAVI-WI- IFe2O3	$\text{MWI}=\sqrt{{\mathrm{(}\text{MSAVI}-1\mathrm{)}}^2{+\text{WI}}^2{+\mathrm{(}\mathrm{I}}_{{\text{Fe}}_{{}^2}{\mathrm{O}}_{{}^3}}{)}^2}$	0.67	1.966
MSAVI-WI-SI	$\text{MWS}=\sqrt{{\mathrm{(}\text{MSAVI}-1\mathrm{)}}^2{+\text{WI}}^2{+\text{SI}}^2}$	0.61	2.354

Figure 7. Accuracy verification of the feature space model.

Figure 8. Interannual variation of average salinization monitoring index.

Figure 9. Spatial distribution of salinization in Yellow River Delta.

Figure 10. Change trend and intensity of salinization during 1984-2022.

Figure 11. Migration trajectory of salinization gravity center during 1984-2022.

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Data availability statement

The data that support the findings of this study are available from the corresponding author, Guo B, upon reasonable request.