Spatial associations between NDVI and environmental factors in the Heihe River Basin

doi:10.1007/s11442-019-1676-0

Abstract

Abstract:

The Heihe River Basin is located in the arid and semi-arid regions of Northwest China. Here, the terrestrial ecosystem is vulnerable, making it necessary to identify the factors that could affect the ecosystem. In this study, MODIS-NDVI data with a 250-m resolution were used as a proxy for the terrestrial ecosystem. By combining these with environmental factors, we were able to explore the spatial features of NDVI and identify the factors influencing the NDVI distribution in the Heihe River Basin during the period of 2000-2016. A geographical detector (Geodetector) was employed to examine the spatial heterogeneity of the NDVI and to explore the factors that could potentially influence the NDVI distribution. The results indicate that: (1) the NDVI in the Heihe River Basin appeared high in the southeast while being low in the north, showing spatial heterogeneity with a q-statistic of 0.38. The spatial trend of the vegetation in the three sub-basins generally increased in the growing seasons from 2000 to 2016; (2) The results obtained by the Geodetector (as denoted by the q-statistic as well as the degree of spatial association between the NDVI and environmental factors) showed spatial heterogeneity in the associations between the NDVI and the environmental factors for the overall basin as well as the sub-basins. Precipitation was the dominant factor for the overall basin. In the upper basin, elevation was found to be the dominant factor. The dominant factor in the middle basin was precipitation, closely followed by the soil type. In the lower basin, the dominant factor was soil type with a lower q-statistic of 0.13, and the dominant interaction between the elevation and soil type was nonlinearly enhanced (q-statistic = 0.22).

Key words: NDVI, environmental factors, vegetation, Geodetector, q-statistic, spatial heterogeneity, Heihe River Basin

YUAN Lihua, CHEN Xiaoqiang, WANG Xiangyu, XIONG Zhe, SONG Changqing. Spatial associations between NDVI and environmental factors in the Heihe River Basin[J].Journal of Geographical Sciences, 2019, 29(9): 1548-1564.

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Category	Factor	Factor unit	Source
Climate
	Precipitation	mm	Heihe River Data Organization (http://www.heihedata.org/)
	Temperature	℃	Heihe River Data Organization (http://www.heihedata.org/)
	Specific humidity	g/kg	Heihe River Data Organization (http://www.heihedata.org/)
Topography
	Elevation	m	Derived from DEM data (http://www. heihedata.org/ data/ea5a9bba- 20c3-40c8-8013-584e0e55a952)
	Aspect	-	Derived from DEM data
	Slope	°	Derived from DEM data
Soil property
	Soil type	-	Heihe River Data Organization (http://www.heihedata.org/)

	Value	Sub-types	Percentage of sub-types (%)	Types	Percentage of types (%)
NDVI slope	(0.001,0.033]	Moderately increased	3.5	Increased	10.6
	(0,0.001]	Slightly increased	7.1	Increased	10.6
	[-0.001,-0]	Slightly degraded	20.8	Degraded	89.4
	(-0.001,-0.020]	Moderately degraded	68.6	Degraded	89.4
Significance level	<5%	-	-	Significance	54.4
Significance level	>5%	-	-	Non-significance	45.6
The overlay result of the slope and M-K test	(0.001,0.033]∩<5%	Moderately increased with significance	52.2	Increased with significance	53.0
	(0,0.001]∩<5%	Slightly increased with significance	0.8	Increased with significance	53.0
	(0.001,0.033]∩>5%	Moderately increased with non-significance	19.3	Increased with non-significance	37.0
	(0,0.001]∩>5%	Slightly increased with non-significance	17.7	Increased with non-significance	37.0
	(-0.001,-0.020]∩<5%	Moderately degraded with significance	1.4	Degraded with significance	1.4
	[-0.001,-0]∩<5%	Slightly degraded with significance	0	Degraded with significance	1.4
	(-0.001,-0.020]∩>5%	Moderately degraded with non-significance	2.1	Degraded with non-significance	8.6
	[-0.001,-0]∩>5%	Slightly degraded with non-significance	6.5	Degraded with non-significance	8.6

	Factor /Interaction	q-statistic
First ranking factor	Precipitation	0.53
Second ranking factor	Elevation	0.52
Third ranking factor	Soil type	0.50
First interaction	Soil type∩Humidity^a	0.68
Second interaction	Soil type∩Elevation^a	0.634
Third interaction	Soil type∩Precipitation^a	0.629

Region	Individual factor				Interaction
Region	First ranking factor	q-statistic	Second ranking factor	q-statistic	First ranking interaction	q-statistic	Second ranking interaction	q-statistic
Upper basin	Elevation	0.30	Humidity	0.27	Elevation∩Humidity^a	0.52	Elevation∩Precipitation^a	0.46
Middle basin	Precipitation	0.36	Soil type	0.33	Soil type∩Humidity^a	0.55	Precipitation∩Soil type^a	0.50
Lower basin	Soil type	0.13	Precipitation	0.05	Precipitation∩Soil type^b	0.22	Soil type∩Humidity^b	0.18