| Climate change and human activities have a profound impact on land cover,vegetation structure and hydrological processes,resulting in dramatic changes in the global land surface over the past four decades,posing serious challenges to the sustainable development of human economic,social and ecological systems.With the launch of the "future earth science program",the coupling between human activities and the natural environment on different spatial scales is becoming the forefront of international earth system science.The Loess Plateau is one of the regions with the highest concentration of population,resources and environmental contradictions in China.It used to be the area with the most severe soil and water loss in the world,but recently the vegetation cover has been significantly improved and the total amount of sediment imported into the Yellow River has decreased sharply.There is no scientific consensus on how land use change drives vegetation cover change,and the impact of vegetation cover change on regional hydrological processes has not been clear.We used meteorological data of 65 meteorological stations,ecological construction data of 11 sample basins,information on ecological construction of provinces and regions and land use data of the fourth period were selected.We used remote sensing evaporation model to estimate the regional actual evapotranspiration(ET),combined with the long time series of remote sensing vegetation Leaf area index(LAI)products.In order to understand the environmental change and realize the sustainable development of the region,we analyzed the change of land use pattern,explored the temporal variation characteristics of vegetation and the evaporation and made the attribution analysis with the theories and methods of mathematical statistics,remote sensing and geographic information technology and geography.The main conclusions are as following:(1)On the basis of understanding the situation of regional ecological control,the data of soil and water conservation measures,vegetation restoration,silting dam construction and sediment transport are analyzed comprehensively,and the ecological construction effect of this area is evaluated.The results showed that the area of water conservation measures and vegetation cover have changed significantly since 2000,and the number of small and medium-sized dams and backbone dams in the region have increased before 2000 and then decreased after 2000,so that the ecological construction has achieved remarkable results.The annual sediment transport volume decreased by 62.5%from 2000 to 2012 compared with it in 1980-1999.The average annual sediment transport volume dropped from 636.1 × 106 t before 2000 to 238.7× 106 t after 2000.This is mainly due to the control of soil and water loss by vegetation restoration in the region,as well as the comprehensive effect of silting dam to block sand and silt to improve the ecological environment.(2)Based on the comprehensive analysis of the temporal and spatial change characteristics of land use and the dynamic attitude of land use in the fourth period of the loess plateau,it is found that the land use types in this area are mainly grassland,arable land and forest land.From 1990 to 2015,the land use was dominated by unchanged forest land,grassland,arable land and other land use types,accounting for 88.29%of the total area.There were only 11.71%of the land use type have changed,among which the conversion from arable land to grassland was the most obvious,accounting for 2.94%.During the whole study period,the order of the rate of land use change were other land>cultivated land>grassland and>woodland,and the land use change was the most active from 2000 to 2010.(3)Combining LAI,meteorological factors and land use data of the loess plateau from 1982 to 2015,the spatial and temporal change pattern and driving factors of LAI were explored.The results showed that the vegetation pattern in this area has changed significantly and was characterized by stages.During the whole time period,LAI increased significantly with 0.0065 yr-1(p<0.001),and LAI increased faster after vegetation restoration(2000-2015)than before vegetation restoration(1982-1999).The spatial change trend of LAI was gradually decreasing from southeast to northwest,and the areas with significant increase are concentrated in the Yulin and Yan 'an in northern Shaanxi and the southern plain area.On the whole,LAI is mainly green,in which the proportion of green,yellow and stable is 73.64%,1.75%and 24.61%,respectively.The net growth of LAI in Shaanxi,Shanxi,Inner Mongolia and Gansu was significant,and the cumulative growth area accounted for 86.23%of the total growth area.The influence of climate factors on LAI is spatially heterogeneous,LAI changes are mainly affected by rainfall in the desert areas in northwest China,central Shaanxi,most parts of Gansu and some parts of Shanxi.At the junction between Shanxi and Shaanxi and most parts of Shanxi,LAI is mainly affected by temperature,and the overall LAI change is more significantly affected by rainfall.In the land use change,the types of unchanged forest land,grassland and arable land dominate the regional LAI change.Recently,the large-scale vegetation construction projects have effectively improved the vegetation cover,which is the main driving force of regional LAI change.(4)Based on the remote sensing vegetation index model,ET was estimated from 1982 to 2015 on the loess plateau.The simulation results were verified by means of water balance and remote sensing evapotranspiration,and the spatial and temporal variation characteristics of ET in the region were quantitatively analyzed.The effects of climate change and human activities on actual evapotranspiration were separated by means of first-order differential detrend and multiple linear regression analysis.The results showed that the actual evapotranspiration in the region shows significant changes and stage differences,ET in the study area increased significantly with 2.23mm yr-1(p<0.001)from 1982 to 2015,especially after vegetation restoration(2000-2015).The space variation characteristics of actual evaporation from the southeast to the northwest had diminishing and strong spatial heterogeneity,it showed a trend of a significant increase in the actual evaporation northern Shaanxi,south of Shaanxi and Shanxi,eastern part of the farming area in Gansu and other regions,the actual evaporation presented a wide range and drastically change trend after the vegetation restoration(2000-2015)especially.Recently,the large-scale vegetation construction has improved regional vegetation conditions and led to the growth of actual evapotranspiration.The influence of meteorological factors on actual evapotranspiration showed obvious spatial difference,which was manifested as rainfall>sunshine hours>temperature>wind speed.In the change of land use,the invariable types of forest land,grassland and arable land dominated the change of regional ET.The relative contribution of human activity and climate change to the actual evapotranspiration trend was 73.5%and 26.1%respectively,indicating that human activity was the leading factor of the actual evapotranspiration change in the region. |
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