| Land surface vegetation change is an important representation of land use/cover change,which affects land surface energy balance and water cycle mainly by changing geophysical properties such as land surface albedo,vegetation coverage and soil moisture.Since the 1970 s,the central Ningxia desert savanes is given priority to with bean Caragana is a perennial deciduous shrub Caragana Korshinskii large-scale plantation cultivation,in the area,degradation of grassland restoration of soil and water conservation and desertification control plays an important role in such aspects,but at the same time it appeared in the local different levels of water-born dysfunctional relationship,destroy the secondary problems,such as hydrological cycle.The improvement of ecosystem quality and function is the core goal of current and future ecological construction in desert steppe region,especially the reconstruction of regional ecological construction route around artificial vegetation construction and regional hydrological cycle rebalancing.Therefore,it is necessary to accurately evaluate the effects of Caragana Korshinskii introduction on regional vegetation and hydrological processes,and provide scientific basis for regional ecological construction planning and artificial vegetation management.NDVI and ET are important indicators reflecting vegetation status and water consumption at regional scale,and are often used for overall identification and analysis of regional carbon-water balance changes.In view of this,the temporal and spatial distribution characteristics of NDVI and ET in desert steppe area of central Ningxia were studied by using unary linear regression analysis,pixels-by-pixel correlation analysis and multiple regression residual analysis,and the effects of climate change and human activities on regional vegetation change were quantified.At the same time,based on medium and high resolution multi-source satellite remote sensing image data,combined with field survey data,the temporal and spatial distribution and coverage characteristics of artificial Caragana Korshinskii forest in the region were explored,and the relative contribution of Caragana Korshinskii plantation to regional NDVI-ET was identified by analyzing the contribution of different vegetation types to NDVI-ET in the region.The main results and conclusions are as follows:(1)The overall vegetation coverage of desert steppe in the middle of Ningxia is relatively low.The annual NDVI average is 0.06-0.82,and the regional average is only 0.30.The average annual evapotranspiration ranges from 75-383mm,with an average of 106mm.The high ET value region is consistent with the high NDVI value region.Regional NDVI and ET showed an overall trend of continuous growth,with an average increase of 0.073/10 years in NDVI.The increasing trend of regional evapotranspiration was mainly distributed in the southeast region,while the decreasing area was mainly distributed in the northwest region.There was a strong correlation between REGIONAL NDVI,ET and precipitation from 2001 to 2020,and there was an obvious spatial difference.(2)The multiple regression residual analysis shows that from 2001 to 2020,the proportion of regions with significant vegetation increase caused by climate change and human activities is close to 95%,and the contribution rate of human activities is between 80%and 100%in most regions,while the contribution rate of climate change is less than 20%.There were relatively few regions where vegetation increased significantly due to human activities or climate change,indicating that human activities were the dominant factor of regional vegetation change.(3)The distribution extraction results of Caragana korshinskii based on high-resolution images showed that Caragana korshinskii was widely distributed in the region,with patches of different sizes,showing contiguous or patch distribution and strong spatial heterogeneity.The differential index constructed by spectral differential method was effective in the extraction of regional sparse vegetation.The study showed that the area of regional artificial Caragana korshinskii forest increased continuously from 1986 to 2020,but the change of Caragana korshinskii forest coverage showed strong spatial and temporal heterogeneity in different periods due to the influence of the growth stage and environmental factors.(4)The increase of vegetation coverage led to the increase of evapotranspiration,and Caragana korshinskii had different effects in different periods.In 2005,Caragana korshinskii had low coverage and low evapotranspiration.With the time going by,NDVI and evapotranspiration consumption in Caragana korshinskii region increased gradually,and the correlation was gradually strengthened.The analysis results of NDVI and ET change rate show that the increase of regional NDVI has a significant impact on regional evapotranspiration.The effect of Caragana korshinskii on NDVI and ET changed from low AND low NDVI to low and medium high NDVI,and ET was always in the low value area with the extension of planting years.(5)The contribution analysis of different vegetation types to NDVI and ET in typical regions showed that the regional NDVI and ET had a significant trend of change from 2001 to 2020,and the center of GRAVITY of NDVI and ET scatter plot moved to the direction of high NDVI and HIGH ET,and the change was significant after 2010.The contribution of regional farmland to NDVI and ET was low before 2010,but after 2010,it mainly contributed to the high value of regional NDVI and ET.The distribution area of forest is small in the region,but NDVI is always in the high area of 0.6,and evapotranspiration is 200-300,and the change is not obvious in the past 20 years.The main contribution of grassland in the past 20 years was the increase of regional NDVI,and the NDVI center shifted from 0.2 to 0.35 in the scatter plot.The change of Caragana korshinskii in the last 20 years was mainly reflected in the increase of NDVI,but there was no obvious promoting trend for the evapotranspiration,and the evapotranspiration of Caragana korshinskii in the distribution area was around 100mm.In conclusion,vegetation cover and evapotranspiration in the desert steppe area of central Ningxia Showed a trend of continuous growth in the past two decades.Changes in vegetation cover and evapotranspiration were mainly driven by human activities such as farmland development and artificial vegetation introduction,and were less affected by climate change.Compared with natural grassland,the continuous introduction of artificial Caragana korshinskii did not change the overall pattern of REGIONAL NDVI-ET,nor was it the key factor driving the change of regional ET. |
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