Evolutionary Characteristics Of Hydrological Fluxes On The Loess Plateau Under Anthropogenic And Climate Change Environments

Evapotranspiration and terrestrial water storage are important components of the regional water cycle process,and the changes of both can reflect the dynamic balance between water balance elements(precipitation,runoff,evapotranspiration)in different time periods in the region,and are also effective indicators to assess the regional water resources status.With the implementation of the project of returning farmland to forest and grass,the vegetation cover of the Loess Plateau has increased significantly,while the global climate continues to warm,which has an important impact on the regional ecological environment,water cycle processes,total hydrological and water resources and spatial distribution patterns.Based on MODIS 16A2 ET and CSR Mascon data,this paper analyzes the temporal distribution characteristics and spatial distribution patterns of actual evapotranspiration and terrestrial water storage from 2001 to2020 on the Loess Plateau during different growth and development stages of vegetation from2003 to 2020.The direct and indirect effects of meteorological factors and NDVI on ET and TWSC were explored by using through-path analysis,and the influencing factors of water depletion on the Loess Plateau were clarified by combining with ERA5-LAND runoff data.The main research findings are as follows:(1)Using Landsat 8 image data to verify the applicability of MODIS16A2 ET data in the Loess Plateau,the ET values in the study area increased in the annual,growing and non-growing seasons and the growth rate was annual > growing season > non-growing season;the spatial distribution pattern of ET in the annual and growing seasons was decreasing from southeast to northwest,and the overall trend of the region was increasing.The annual ET values in the nongrowing season are the lowest in the year,and the higher values are in the west and south;the annual average actual evapotranspiration in the year and growing season are forest land >cropland > grassland > construction land > water area > unused land,and the annual average actual evapotranspiration in the non-growing season are forest land > grassland > cropland >unused land > construction land > water area;the Hurst index indicates that the future actual evapotranspiration has a continuous decreasing trend in general.(2)The NDVI in the study area increased during the 20-year period and the rate of increase was growing season > annual > non-growing season,and the average NDVI decreased from southeast to northwest in each time period;precipitation,temperature and wind speed showed an increasing trend in the three time periods,relative humidity showed a negative increasing trend in the three time periods,sunshine hours showed a decreasing trend in the annual and growing season,and an increasing trend in the non-growing season.The correlation analysis was used to analyze the correlation between individual factors and ET.NDVI,precipitation,temperature and wind speed,which were significantly correlated with each other,were selected and the direct and indirect effects of each factor on ET changes in the Loess Plateau were calculated based on the path analysis.In the direct effect,all four factors are positively driven and NDVI > wind speed > precipitation > temperature during the whole year,and all four factors are positively driven and NDVI > precipitation > wind speed> temperature during the growing season,while precipitation is negatively driven during the non-growing season,and the rest are positively driven and temperature > wind speed > NDVI> precipitation;in the indirect effect,wind speed has the most prominent effect on ET than other factors during the whole year and the growing season.Among the indirect effects,the effect of wind speed on ET is most prominent in the whole year and the growing season,and the effect of NDVI on ET is most prominent in the non-growing season.(3)TWSC in the Loess Plateau showed a significant decreasing trend from 2003 to 2020,and TWSC was in deficit since 2006;the multi-year average TWSC showed deficit in all three time periods,and the deficit was most serious in the growing season,and the rate of deficit was the largest in the non-growing season.The spatial distribution of TWSC in the Loess Plateau region decreases from west to southeast in all three periods,and the rate of decline in the eastern region is higher than that in the western region.Based on the correlation analysis,we determined that NDVI,wind speed and sunshine hours had significant effects on the TWSC,and calculated the direct and indirect effects of each factor on the ET change in each time period.Among the direct effects,sunshine hours are the positive drivers,NDVI and wind speed are the negative drivers,and NDVI > sunshine hours> wind speed;among the indirect effects,wind speed has the most prominent effect on TWSC than other factors in the three periods.The proportion of water consumption caused by actual evapotranspiration ranged from 70 to 90%,indicating that vegetation transpiration increased significantly due to vegetation restoration,and a large number of shallow-rooted crops were transformed into deep-rooted crops,resulting in increased water consumption in the area and large depletion of groundwater bodies,which eventually caused the terrestrial water reserves in the study area to The deficit of terrestrial water reserves in the study area is serious year by year.