## 1 Introduction  
Evapotranspiration is the evaporation of water from water bodies, soils, and vegetation surfaces and the transpiration of water from plants. As a key element and an important link in the hydrological cycle, it affects the water cycle, plant growth and development, energy cycle, and terrestrial water distribution in surface ecosystems and has a significant impact on the availability of runoff water resources. The study of the temporal and spatial variations in evapotranspiration is useful for understanding the mechanisms of water resource evolution and the rational utilization of water resources [1, 2]. The evapotranspiration process can be divided into canopy interception evaporation, soil evaporation, and vegetation transpiration [3]. Canopy interception evaporation is the first link in the redistribution of precipitation by vegetation and is mainly influenced by the wetting condition of the leaf surface; soil evaporation is the process by which water in the soil enters the atmosphere from the soil surface through ascent and vaporization, and is influenced by both the surface and internal soil water content; vegetation transpiration refers to the process of water loss from plants in the form of water vapor through leaves, which is mainly influenced by vegetation morphology, vegetation growth, water supply, air temperature and air humidity [4; 5]. Vegetation, as an important component of the subsurface, is a major factor influencing actual evapotranspiration. The vegetation index allows us to monitor seasonal, interannual, and long-term changes in vegetation structure, phenology, and biophysical parameters. NDVI is one of the most widely used vegetation indices and is related to biological characteristics such as leaf area index of vegetation canopy, vegetation cover, vegetation condition, and biomass. The study of [PERSON], [PERSON] [6] showed a good correlation between NDVI and vegetation primary productivity. [PERSON], [PERSON] [7] found that NDVI is sensitive to multi-temporal (seasonal) vegetation changes, land cover changes, and changes in biophysical parameters, and can be used to monitor and assess spatial and temporal changes in vegetation quantity and condition. NDVI is a good indicator of vegetation growth and vegetation cover changes and can be effectively used for monitoring surface evapotranspiration, which is widely used in studies related to evapotranspiration. [PERSON], [PERSON] [8] constructed the relationship between cumulative NDVI and transpiration in six arid zones of India, starting from the analysis of the relationship between vegetation index and plant dry matter production; [PERSON], [PERSON] [9] used the Shuttleworth-Wallace model and NOAA-AVHRR NDVI data to estimate the potential evapotranspiration in the Mekong River basin; [PERSON], [PERSON] [10] proposed a method combining meteorological and NDVI data to simulate actual evapotranspiration of various terrestrial ecosystems; and [PERSON], [PERSON] [11] and [PERSON], [PERSON] [12] found that there was a high correlation between NDVI and evapotranspiration. Global climate change, urbanization, afforestation, and the construction of water conservancy projects have changed the hydrological balance of the basin, and the trend of aridification in the Yellow River Basin is obvious [13]. The Yiluo River is one of the ten tributaries of the Yellow River and the largest tributary below the Sanmencia Reservoir of the Yellow River. The Yiluo River Basin belongs to the area with abundant precipitation in the Yellow River Basin, it is located in the center of the Yellow River Basin and is an important water source in the middle reaches of the Yellow River. The topography of the Yiluo River Basin is higher in the upper reaches and lower in the lower reaches, the terrain is steep, the surface types are complex, and the water resources are unevenly distributed in time and space, which have a profound impact on the production and living conditions in the basin and even in the lower reaches of the Yellow River [14].