| After the implementation of the policy of“Grain for Green”in the Loess Plateau,the change of evapotranspiration caused by large-scale vegetation restoration has a significant impact on the regional hydrological cycle process.To provide scientific basis for comprehensive and reasonable assessment of the ecological effect,water resources carrying capacity and the impact of regional hydrological cycle,In this study,different typical vegetation types(grassland in small watershed of suideqiaogou and shrub in yaogushan of Ansai)were selected as research objects on the point scale,Dynamic monitoring was carried out on a continuous long-term scale to study the coupling relationship between evapotranspiration resolution and root water absorption characteristics,and the actual evapotranspiration of remote sensing inversion products was compared with the actual evapotranspiration observed on the point scale,and the quantitative relationship between the point scale and the real evapotranspiration on the regional scale was discussed.Based on MODIS evapotranspiration products,the variation of evapotranspiration on the regional scale in the Loess Plateau was quantitatively analyzed.The main results are as follows:(1)In the Loess Hilly and Gully region,the water absorption of grass and shrub roots of two typical vegetation types showed a step-by-step pattern of soil water decrease during the day,and the soil water remained basically unchanged at night;the evapotranspiration of sunny and cloudy days in different seasons had significant differences,showing a"single peak"curve characteristic of low in the morning,low in the evening and high in the noon.(2)The actual evapotranspiration of grassland and shrub in Loess Hilly and Gully area is significantly related to soil moisture content and temperature,and the daily ETa change rule of grassland and shrub in growing season is basically the same as that of daily average temperature;under the influence of sunshine intensity,the daily evapotranspiration of sunny day is significantly higher than that of cloudy day in four seasons.(3)On the point scale,the ETa value observed is generally slightly higher than that retrieved by MODIS,but there is a significant strong correlation between MODIS-ET products and ETa value of soil moisture meter.The correlation coefficient R~2 is 0.75-0.82,and RMSE value is less than 9.3 Mm,MRE value is less than 0.2mm,RPD is close to 2,P value is less than 0.01,which shows that the regression relationship can be used as a conversion function of point scale and regional scale Evapotranspiration to some extent.(4)The spatial distribution of annual average actual evapotranspiration in the Loess Plateau from 2001 to 2017 shows an increasing trend from northwest to Southeast,with the annual actual evapotranspiration increasing significantly at the rate of 8.23 mm/a,However,the spatial distribution characteristics of annual average actual evapotranspiration in 2001-2017 are basically consistent with that of annual average evapotranspiration in the Loess Plateau,and the order of seasonal evapotranspiration from large to small is summer>autumn>spring>winter;The annual actual evapotranspiration of different land use types was as follows:forest land(418.30 mm)>farmland(320.26 mm)>grassland(279.15 mm)>green land construction(221.30 mm);The actual evapotranspiration of different basins is as follows:Weihe River(388.26 mm)>Beiluo River(367.99 mm)>Jinghe River(326.16 mm)>Yanhe River(275.51 mm)>Wuding River(197.45 mm)>Kuye River(164.73 mm)>Huangfuchuan River(153.71 mm).(5)The actual evapotranspiration on the Loess Plateau has a significant linear relationship with NDVI,rainfall,temperature and sunshine hours.Since 2000,the increase of NDVI and rainfall on the Loess Plateau has led to a significant increase in evapotranspiration,while the decrease of sunshine hours has led to a decrease in evapotranspiration on the Loess Plateau. |
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