Response Of Vegetation Cover Change To Water And Heat Factors And Human Activities In Ningxia

Vegetation is the main body of terrestrial ecosystem.It not only plays a key role in stabilizing ecological environment and regulating global climate,but also provides necessary resources for other organisms through material exchange and energy cycle.It is an irreplaceable part of the ecosystem.Vegetation has obvious soil and water conservation function and growth cycle change,which is affected by many factors.Therefore,to explore the dynamic change law of vegetation cover is an important path for ecological restoration and prevention of soil erosion.At the same time,under the background of global climate change,vegetation cover change information has great theoretical value and practical significance for the assessment of regional environmental conditions,regulation of ecosystem balance and prevention of soil erosion.The Yellow River is the mother river of the Chinese nation.Promoting the ecological protection and high quality development of the the Yellow River river basin is a major national strategy determined by the Party Central Committee with Comrade Xi Jinping as its core,which concerns the great rejuvenation of the Chinese nation.In June 2020,when general secretary Xi Jinping inspected Ningxia,he explicitly instructed to "strive to build an ecological protection zone and a high quality development area in the the Yellow River Valley".Therefore,Ningxia's ecological protection and construction is of great significance.Based on the background of global climate change,MODIS EVI data,meteorological data and land use data were used to study the temporal change,spatial distribution,trend change of vegetation cover in Ningxia from different time scales,such as year,month,half month,season and growing season In order to provide some decision-making reference for Ningxia ecological environment protection,the methods of correlation analysis,partial correlation analysis,time lag analysis,residual analysis and land use type were used to explore the response law of vegetation cover change to water and heat factors and human activities.The main conclusions are as follows:(1)From 2001 to 2018,the vegetation cover in Ningxia improved significantly,and the enhanced vegetation index(EVI)increased from 0.115 to 0.1 7,with an average annual growth rate of 0.0022,among which 2001-2002,2005-2007,2008-2010,2011-2012 and 2015-2018 showed an increasing trend;The vegetation coverage changed from medium low cover and low cover to medium low cover and medium high cover;During the year,the vegetation growth has obvious biological rhythm.The EVI value is relatively gentle from January to March,and increases sharply from April to August until a peak value of 0.23 appears in July.The EVI value drops rapidly from September to December,and the vegetation coverage continues to decline slowly from November to December,and the lowest value of 0.08 appears in January;The variation trend of four seasons and growing season is basically the same as that of the year.The EVI value in summer has the fastest growth rate,with an average annual growth rate of 0.004,and the average annual growth rate in growing season is 0.003.From the regional point of view,EVI value and trend of each period are the highest in the southern mountainous area,followed by the central arid zone,and the lowest in the northern Yellow River irrigation area.(2)The distribution of vegetation coverage in Ningxia presents the characteristics of "high in the north and south,low in the middle".The average value of EVI pixel is 0.14,and the overall average annual growth rate is 0.0021,showing a good improvement trend.During the study period,95.2%of the areas were in the state of increasing vegetation coverage,and the area of vegetation coverage increased by one level was 2.22 × 106 hm2,and the area of two-stage improvement was 2.55 × 106 hm2,and the area increased by three levels or above was 1.2 × 106 hm2,accounting for 38.47%,22.26%and 1.88%respectively.The standard deviation of EVI value in Ningxia is between 16-1632,with an average of 193.The overall volatility and spatial difference are small,and the overall stability of EVI value increases gradually from south to north.The overall Hurst index of Ningxia ranged from 0.11 to 1.02,with an average of 0.52,indicating that the EVI index of the study area has a continuous trend of change,and the vegetation cover change will increase steadily in the future,of which the area of continuous improvement in the future is 1.84 × 106 hn2(27.64%),and the area of continuous improvement was 3.55 × 105 hm2(5.34%),and the largest area of sustained stability was 2.93 × 106 hm2(44.17%),and the area of continued degeneration is 9.75 × 105 hm2(13.69%).From the regional point of view,the vegetation coverage of the North Yellow River irrigation area is the lowest,and the difference between planting area and non planting area is large.The coverage of the former is significantly higher than that of the latter.The average value of the central arid zone is close to that of the north,and the overall distribution is more uniform.The coverage of the southern mountainous area is the highest,and the stability and sustainability are better than the former two areas.(3)The correlation coefficients of monthly EVI with temperature and humidity were 0.76 and 0.54,the partial correlation coefficients were 0.92 and 0.67,and the time lag correlation coefficients were 0.82 and 0.05,respectively,There was no significant correlation between humidity and humidity.The correlation coefficient between EVI and temperature is low in Shapotou District,Helan Mountain and Liupan Mountain,most of which are above 0.8,while it is about 0.7 in northern planting area and southern Jingyuan county.The correlation coefficient between EVI and rainfall is high in Yanchi county and Yongning county,but the correlation coefficient between EVI and rainfall is not high,especially in Shapotou district and Tongxin county.The results showed that 97.57%of the total area had moderate positive correlation and strong positive correlation between temperature and humidity,and the low proportion of other correlation had little effect on the results.Generally speaking,temperature and humidity had a synergistic positive correlation with vegetation growth,but the impact was different,And it has the characteristics of decreasing from south to north.(4)The land use pattern of Ningxia has changed significantly from 2000 to 2018.The area of cultivated land and grassland continued to decrease,while the area of forest land and construction land continued to increase.The area of water area first remained unchanged and then increased,while the area of unused land first increased and then decreased.From the perspective of transfer characteristics,the transfer area of cultivated land was the largest,followed by grassland.The most significant transfer process was from cultivated land to grassland(19.32%),followed by grassland to cultivated land(12.82%).Human activities played a positive role in vegetation restoration.The results show that 93%of the total vegetation cover area is positively affected by human activities,mainly distributed in the southern mountainous area and part of the central arid zone,and 7%of the total vegetation cover area is negatively affected by human activities,mainly located in the northern Yellow River irrigation area.The contribution of temperature and humidity to vegetation coverage was 0.1 in 26.7%,0.15 in 52.6%,0.2 in 26.2%,0.25 in 12.8%and above 0.35 in 3.7%.The contribution of temperature and humidity to vegetation coverage increased gradually from north to South except for the northern planting area.