Analysis of temporal and spatial variations of NDVI in Taihu
Lake Basin
3.1 Temporal and spatial variation characteristics of vegetation NDVI in Taihu
Lake Basin
3.1.1 Temporal variation characteristics of vegetation NDVI in Taihu Lake Basin
In order to study the characteristics of vegetation NDVI change with time in Taihu
Lake Basin, the annual average value of vegetation NDVI from 1998 to 2013 was
selected to represent the annual vegetation NDVI status of the basin (Fig.2). It can be
seen from Figure.2 that the annual average NDVI of vegetation in Taihu Lake Basin
fluctuates between 0.60 and 0.71, showing a significant downward trend, with a
decline rate of 0.51%·a-1. The average NDVI of vegetation decr eased by 11.46% from
0.71 in 1998 to 0.63 in 2013. The minimum and maximum of NDVI appeared in 2009
and 1998, respectively. The interannual variation of NDVI in Taihu Lake basin can be
divided into five rising stages and five declining stages. Among them, the vegetation
coverage in 2000, 2003, 2005 and 2010 was better with peaks, while the vegetation
coverage in 1999, 2002, 2004, 2009 and 2012 was worse with troughs.
3.1.2 Spatial variation characteristics of vegetation NDVI in Taihu Lake Basin
In order to clarify the spatial variation characteristics of vegetation NDVI in Taihu
Lake Basin, the average value of 16 years was calculated according to the annual
average NDVI from 1998 to 2013, and the spatial distribution of vegetation NDVI was
obtained (Fig.3). It can be seen from Figure.3 that the spatial distribution of vegetation
NDVI in Taihu Lake Basin is high in the southwest and low in the East. The average
value of vegetation NDVI in 16 a is 0.65.
The high value areas with NDVI greater than 0.75 in Taihu Lake Basin are mainly
located in the southwest, mainly distributed in Huzhou City and Lin'an City. Huzhou
City is dominated by low mountains and hills, with a forest coverage rate of 48.7%.
The main forest vegetation types are Pinus massoniana, Ph yllostachys pubescens,
Cunninghamia lanceolata, evergreen broad -leaved forest and coniferous broad -leaved
mixed forest, etc(Wu et al. 2002). It plays a positive role in the local vegetation NDVI;
Lin'an City is rich in forest resources, with the forest coverage rate as high as 76.55%.
The vegetation belongs to the subtropical evergreen broad -leaved forest subregion(Qin
and Chen 2010), with high vegetation coverage, so the NDVI value of vegetation is high.
The low value areas with NDVI less than 0.5 are mainly located in the flat delta
plain in the east and the riverside plain in the north, which are mainly distributed in
urban areas. The land use types are mainly urban and rural land, residential land and
other construction land, so the NDVI of vegetation is low.
3.1.3 Analysis on vegetation NDVI change trend in Taihu Lake Basin
Based on the univariate linear regression model, the spati al distribution of
vegetation NDVI interannual variation trend in Taihu Lake Basin was obtained (Fig.4).
The S value ranged from -0.034 to 0.044, which indicated that there were spatial
differences in the change trend of NDVI in Taihu Lake Basin. It can be seen from
Figure.4 that the vegetation NDVI change trend shows a spatial pattern of obvious
increase in the southwest and decrease in the north.
It can be seen from Table 1 and Figure.4 that the vegetation degraded area of Taihu
Lake Basin accounts fo r more than 96% of the basin area, of which the seriously
degraded area accounts for 2.62%, mainly distributed in Huzhou City and other cities
in the southwest; the improved area accounts for about 2%, of which the significantly
improved area is mainly distributed in Shanghai in the east and Suzhou in the middle.
3.2 Influencing factors of vegetation NDVI change in Taihu Lake Basin
3.2.1 Climate factors
In order to clarify the response of vegetation NDVI to climate factors in Taihu Lake
Basin, the correlation coefficient between annual average NDVI of vegetation and
annual precipitation and annual average temperature in Taihu Lake Basin from 1998 to
2013 is shown by histogram (Fig.5). As shown in Figure.6, the annual average NDVI
of Taihu Lake Basin fro m 1998 to 2013 showed no significant positive correlation with
the annual average temperature and annual precipitation, and the correlation
coefficients were 0.45 and 0.50 respectively. Taihu Lake Basin belongs to humid
subtropical monsoon climate zone, wi th abundant precipitation and lakes. Abundant
water resources can promote the growth of vegetation. The temperature in the basin is
within the suitable range for plant growth. In a certain range, the physiological
response and growth of vegetation are acce lerated when the temperature rises.
3.2.2 Human factors
Population growth and economic development are the main human factors affecting
NDVI change in Taihu Lake Basin. Population growth and economic development
promote the expansion of urban scale, th us improving the capacity of cities to
accommodate population (Jiang 2010). The industrial structure and employment structure
of the main cities in the Taihu Lake Basin show the development characteristics of
secondary industry > tertiary industry > primary industry, that is, industry is the pillar
industry of Taihu Lake Basin. However, rapid industrialization and urbanization not
only make great achievements in promoting economic development , but also promote
the transformation of land use types in the basin (Li and Chen 2014). Urban expansion and
the establishment of various development zones occupy woodland and grassland
disorderly, which leads to the destruction and degradation of vegetation around the
city. All kinds of environmental pollution brought by industrial development, such as
air pollution, water pollution and toxic waste pollution, will aggravate global warming,
increase the frequency of extreme bad weather, seriously stress the growth of
vegetation, and reduce the vegetation coverage (Pan et al. 2018). At the same time, Taihu
Lake Basin is one of the busiest areas of trans portation in China. The rapid
development of transportation industry will have a negative impact on the vegetation
around the transportation trunk line (Lv et al. 2017), resulting in serious degradation of
the surrounding vegetation.
4 Discussion
Vegetation coverage can reduce the erosion of soil surface caused by wind, water
and other external forces(Cong et al. 2013), improve soil, and weaken runoff erosion and
sediment transport capacity(Qiu et al. 2019). Therefore, the long -term monitoring of
vegetation coverage is of great significance to the prevention and control of regional
soil erosion, and the normalized difference vegetation index (NDVI) is the most
important index to reflect the vegetation coverage and growth, and the NDVI is mostly
used in the analysis of long-time series of vegetation coverage (Guerric et al. 2011).
SPOT/VEGETATION NDVI data with 1km spatial resolution are selected to study the
vegetation change in Taihu Lake Basin in recent 16 years on a large scale. The results
can better reflect the vegetation NDVI cha nge in the study area as a whole. It is found
that the vegetation in the basin has been significantly degraded in recent 16 years, but
in recent years, with the implementation of ecological restoration project, the
vegetation coverage is recovering . Climate factors such as precipitation and
temperature have no significant effect on vegetation growth , which may be due to the
fact that human activities are the most important factors affecting the vegetation cover
change in Taihu Lake region, among which popul ation growth and economic
development are the main human factors affecting the vegetation cover (Pan et al. 2018).
Population growth stimulates the demand for agricultural products, which may lead to
the reclamation of forest and grassland (Qiu et al. 2019).
At the same time, the Yangtze River Delta has a superior geographical location and
developed economy, and urban expansion is more common and its scale is larger(Zhou
et al. 2014). The expansion of urban construction land constantly encroaches on the
surrounding agricultural and ecological land, resulting in the decrease of vegetation
coverage. At the same time, vegetation restoration measures such as returning
farmland to forest and grassland and constructing ecological forest also have positive
effects on increasing vegetation coverage, improving ecological environment and
reducing soil erosion(Deng et al. 2020). It can be seen that human activities not only
cause vegetation damage, but also actively improve the ecological environment, which
also shows that human activities have an impo rtant dual impact on vegetation cover
change.
Taihu Lake Basin has superior natural conditions and high level of urbanization. It
is of great significance to strengthen the protection of vegetation and the prevention
and control of soil erosion for re gional social and economic development and
ecological security. Therefore, the government should control the population growth,
optimize the employment structure, clarify the boundary of urban growth, curb the
disorderly expansion of cities with compact ur ban form, improve the efficiency of
urban land use, and protect the peripheral grassland and forest (Deng et al. 2020);
continue to implement the policy of " returning farmland to forests ", speed up th e
wetlands project, and continue to strengthen vegetation restoration and ecological
protection. At the same time, for a large number of land disturbance activities or
production and construction projects, supervision should be strengthened to minimize
the damage to the surrounding ecological environment.
5 Conclusion
The results show that the NDVI of Taihu Lake Basin shows a downward trend in the
past 16 years, with a decline rate of 0.51 %·a-1, mainly concentrated in the eastern
region; the spatial distri bution characteristics of NDVI are high in the southwest and
low in the East, with a significant increase in the southwest and a significant decrease
in the north; the interannual variation of NDV I has little correlation with rainfall and
temperature, which may be mainly affected by policy factors and the level of social
economy. It is necessary to strengthen the population control, optimize the industrial
structure, strengthen the protection and restoration of vegetation, protect the
ecological environment while improving people's living standards, and live in harmony
with nature.