Study On The Response Relationship Between Evapotranspiration And Vegetation Change On The Loess Plateau

In order to scientifically evaluate the sustainability of forest and grass measures on the Loess Plateau under the background of climate change and returning farmland to forest and grass,research on the response relationship between evapotranspiration and vegetation change was carried out,and the vegetation restoration threshold and restoration potential that can maintain the sustainable succession of vegetation on the Loess plateau were proposed,which had important practical significance for the vegetation ecological construction of the Loess Plateau in the future.Based on hydrometeorological data from 1960 to 2018,and remote sensing products such as MOD13 A1 EVI,GRACE and PML＿V2 ET from 2000 to 2018,the temporal and spatial variation characteristics of vegetation and evapotranspiration changes were revealed by trend analysis and F-test methods on the Loess Plateau.The influence of meteorological factors on vegetation,and meteorological factors and vegetation on evapotranspiration were proved by correlation analysis and the residual method.Based on the water balance principle and multiple stepwise regression method,the optimal response relationship between evapotranspiration and enhanced vegetation index(EVI)was established to quantitatively estimate the restoration threshold and restoration potential of forest and grass vegetation coverage under different rainfall scenarios on the Loess Plateau.The main results were as follows:(1)The temporal and spatial variation of EVI on the Loess Plateau was significant from 2000 to 2018,and it was sensitive to meteorological factors.The multi-year average EVI of the Loess Plateau was 0.17,showing a spatial distribution pattern of decreasing from southeast to northwest.EVI showed a significant increase trend from2000 to 2018,with an annual growth rate of 0.0024/a.EVI increased significantly in86.96% of the regions(P < 0.05),and the regions with rapid growth were mainly distributed in the southeast of the Loess Plateau.The average annual EVI in different seasons from large to small were summer,autumn,spring and winter,and the growth rates were 0.0046/a,0.0024/a,0.0013/a,and 0.0012/a respectively.The contributions of different land use types to EVI were forest,farmland and shrub and grass from high to low,and there were significant differences in interannual growth rates in different seasons.Rainfall,temperature,sunshine duration and saturated water vapor pressure difference showed an increasing trend,and all showed a positive correlation with vegetation on the Loess Plateau from 2000 to 2018.The correlations between different meteorological elements and vegetation from high to low were rainfall(0.28),temperature(0.25),saturated water vapor pressure difference(0.13)and sunshine duration(0.04).The average effect of human activities on EVI was 0.0011/a.(2)The errors of PML＿V2 ET based on Flux observation and water balance formula verification were 4.5mm /8d and 34.3 mm/a,and the product accuracy was better than MOD16 A2 GF ET.The spatio-temporal variation of ET on the Loess Plateau was significant from 2000 to 2018,which was influenced by vegetation and meteorological factors.The annual average evapotranspiration of the Loess Plateau is445.36 mm,showing a spatial distribution pattern of decreasing from southeast to northwest.ET increased significantly from 2000 to 2018,with an interannual growth rate of 4.47mm/a.ET increased significantly in 62.76% of the regions(P < 0.05).The average annual ET in different seasons from large to small were summer,spring,autumn and winter,and the growth rates were 2.37 mm/a,1.65 mm/a,0.70 mm/a,and-0.35 mm/a respectively.Vegetation,precipitation,temperature and vapor pressure difference were positively correlated with evapotranspiration,while sunshine duration was negatively correlated with evapotranspiration,with correlation coefficients of 0.54,0.30,0.20,0.05 and-0.09,respectively.Non-vegetated underlying surface(soil and water conservation engineering measures)contributed to the increase of evapotranspiration on the Loess Plateau,with a value of 0.43 mm/a.(3)The relationship between evapotranspiration and vegetation response on the Loess Plateau has obvious spatial heterogeneity,which can be divided into: ET was only affected by the linear action of EVI;ET was affected by the linear action of EVI and the interaction between EVI and meteorological elements at the same time;ET was affected by the linear and nonlinear action of EVI;ET was affected by the linear and non-linear action of EVI,and the interaction between EVI and meteorological elements at the same time and accounting for 36.45%,24.33%,24.34% and 14.88% of the loess Plateau,respectively.The R square of the model was 0.66.73.35% of the areas passed the significance test,and the average root mean square error of the model was 44.5 mm.(4)Restoration threshold and restoration potential of vegetation coverage were sensitive to precipitation change on the Loess Plateau.The average restoration thresholds of forest and grass vegetation coverage in wet,normal and dry years were71.5% ± 5.7%,55.6% ± 5.7% and 22.4% ± 5.7% on the Loess Plateau respectively,and the average recovery potential were 9.4% ± 5.7%,-6.4% ± 5.7% and-39.7% ± 5.7%respectively.(5)The restoration potential of forest and grass vegetation coverage on the Loess Plateau was closely related to the change of drought index,and there was obvious spatial heterogeneity.Under the scenarios of wet and dry years,the restoration potential of forest and grass vegetation coverage increased first and then decreased,decreased first and then increased with the increase of drought index.It showed a decreasing trend with the increase of drought index in normal year.Under different precipitation scenarios,the restoration potential of forest and grass vegetation coverage changes most violently in the range of drought index 2.5-4.The restoration potential of shrubby vegetation coverage was greater than that of forest.