| Impacted by China’s returning farmland to forests and grasslands,the phenomenon of greening of the Loess Plateau has become prominent,attracting continuous attention from the international academic community.It is of great significance to investigate the co-evolution mechanism of vegetation coverage index and precipitation change for clarifying the attribution of the greening of the Loess Plateau,promoting the ecological governance effect of the Loess Plateau,and even ensuring the water and eco-environment safety in arid areas.The Loess Plateau is one of our nation’s most ecologically vulnerable regions.The region’s surface vegetation is responding quickly and closely to climate change as a result of the substantial soil erosion.Since the founding of the People’s Republic of China,the state has implemented a series of soil and water conservation and ecological comprehensive management measures,such as converting farmland back to forest(grass),outlawing grazing,enhancing sloping farmland,and constructing siltation DAMS,in an effort to change the Loess Plateau’s traditional appearance of "yellow sand all over the sky and gully everywhere." Particularly,the strategy of converting farmland back to grass(forest)has produced impressive outcomes.The Loess Plateau’s vegetation cover has altered substantially over the past 20 years,and the plateau is still getting greener,drawing interest from all around the world.The Loess Plateau’s policy of converting cropland to forest or grassland,as well as the long-term thorough control of soil and water loss,are all believed to be intimately related to this occurrence.This conclusion has been largely accepted by the scholarly community.However there are also modest differences in how much of a contribution it makes.Although some academics have highlighted the link between changes in precipitation and vegetation on the Loess Plateau,the proof for this claim or the outcomes of further research are still somewhat lacking.Thus,in order to improve and restore the ecological environment in the Loess Plateau,it is necessary to do additional analysis and demonstration on the influence of hydrometeorological change on vegetation cover change.The scientific justification and physical attribution of the greening of the Loess Plateau are also supported by the scholarly references and data it offers.In light of this,this study systematically investigated the spatio-temporal co-evolution law of vegetation coverage and hydrometeorological factors in the Loess Plateau between 1982 and 2015.The data used in this study included hydrometeorological data such as precipitation,evaporation,and air temperature,land use/cover type data,and raster data of NDVI and DEM.The significant factors that significantly influenced the change in vegetation coverage on the Loess Plateau were identified based on the analysis of the response degree of vegetation coverage to the influencing factors,and the impact of various precipitation levels on the change in NDVI in the Loess Plateau was investigated.The following research findings are pertinent:(1)The spatiotemporal evolution characteristics of vegetation coverage show a significant increasing trend from 1982 to 2015 in the Loess Plateau,mainly due to the decrease in cultivated land and the increase in grassland area.After 1999,the vegetation coverage increased at a rate of 0.0055 per year,with similar trends observed in non-reforestation(grass)areas,where the NDVI index increased at an average rate of 0.005/a,and more significantly in reforestation(grass)areas with an increase rate of up to 0.0074/a.Therefore,it is objectively evident that there has been a significant improvement in vegetation coverage in the Loess Plateau during the statistical period,with a stronger phenomenon of "greening" in reforestation(grass)areas.(2)The spatiotemporal evolution characteristics of hydro-meteorological elements are as follows: From 1982 to 2015,precipitation on the Loess Plateau showed an overall upward trend.Results of the Mann-Kendall test showed a decreasing trend in precipitation before 2001,and its spatial distribution maintained significant spatial consistency with inter-annual changes of NDVI.The temperature on the Loess Plateau showed an overall upward trend during the statistical period,but from 1999 to 2015,the temperature in most areas has shown a decreasing trend.The regional evaporation rate showed an overall upward trend,but in areas with significantly increased vegetation cover,there was a significant decreasing trend in evaporation rate after 1999.The sunshine duration in the region showed an overall downward trend,and before 1998,there was a pattern of increasing sunshine duration in the south and decreasing in the north.However,after 1999,the sunshine duration in the entire study area almost showed a decreasing trend;the average wind speed in the Loess Plateau shows significant spatial heterogeneity,with a decreasing trend in the mean sequence.The area of increase shifted from the central region to the east and west around 1998.The relative humidity in the Loess Plateau shows an overall fluctuating downward trend,but since 1999,the significant increase zone is mainly located in the western hilly-gully region,plateau gully region,northern sandy-windy region,and agricultural irrigation area.The average air pressure in the Loess Plateau shows a trend of first increasing and then decreasing.The annual variation of air pressure in various geomorphic zones is relatively stable.The M-K test results show that the annual average air pressure has significantly increased in the east and west,while there is a significant downward trend in the central part.(3)Identification of hydrometeorological elements that contribute to changes in vegetation cover: Using a geographic detector,potential influencing factors of NDVI(Normalized Difference Vegetation Index)in non-reforestation(grass)areas of the Loess Plateau were systematically identified for two periods: 1982-1998 and 1999-2015.The results show that NDVI is positively correlated with precipitation and relative humidity,particularly in the 30-50 mm precipitation range,where the vegetation cover index increases rapidly;while negatively correlated with sunshine hours and evaporation.Compared with other factors,each hydrometeorological factor has a markedly different impact on NDVI;however,natural geographical factors have a relatively stable influence on vegetation in the Loess Plateau,without significant temporal variation.According to previous studies,climatic factors have an increasingly strong impact on NDVI.However,precipitation elements consistently play a significantly bigger role in NDVI changes in the research area than other influencing factors,making them the main factor determining NDVI changes.Instead of being independent or just additive,any two influencing elements have an interaction that significantly amplifies the interaction.In the Loess Plateau region,evaporation is another important hydrological and climatic factor that affects NDVI changes,and it is one that needs careful consideration.(4)Analysis of the correlation between vegetation coverage and precipitation characteristics: Statistical methods such as detrended cross-correlation analysis and multiple linear regression analysis were used to study the spatiotemporal synergistic evolution characteristics and correlation effects between NDVI indicators and precipitation in nonreforested(grass)areas of the Loess Plateau.The results show that changes in precipitation are the most important contributing factor to the greening of the Loess Plateau.This process is driven by comprehensive soil erosion control,returning farmland to forests and grasslands projects.Attention should be paid to this aspect in future research.At the same time,there is spatial heterogeneity in the impact of different levels of precipitation changes on NDVI in nonreforested(grass)areas of the Loess Plateau.Precipitation changes in the range of 30-50 mm are the key factors for significant improvement in NDVI. |
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