); [EMAIL_ADDRESS] (S.L)  
Yong Zhou  
College of Grassland Science and Technology, China Agricultural University, Beijing 100193, China; [EMAIL_ADDRESS] (W.Z. ); [EMAIL_ADDRESS] (Y.R. ); [EMAIL_ADDRESS] (Y.Z. ); [EMAIL_ADDRESS] (Y.Z. ); [EMAIL_ADDRESS] (Y.Z. ); [EMAIL_ADDRESS] (S.L)  
Yong Zhou  
College of Grassland Science and Technology, China Agricultural University, Beijing 100193, China; [EMAIL_ADDRESS] (W.Z. ); [EMAIL_ADDRESS] (Y.R. ); [EMAIL_ADDRESS] (Y.Z. ); [EMAIL_ADDRESS] (Y.Z. ); [EMAIL_ADDRESS] (S.L)  
Yong Zhou  
College of Grassland Science and Technology, China Agricultural University, Beijing 100193, China; [EMAIL_ADDRESS] (W.Z. ); [EMAIL_ADDRESS] (Y.R. ); [EMAIL_ADDRESS] (Y.Z. ); [EMAIL_ADDRESS] (Y.Z. ); [EMAIL_ADDRESS] (S.L)  
Yong Zhou  
College of Grassland Science and Technology, China Agricultural University, Beijing 100193, China; [EMAIL_ADDRESS] (W.Z. ); [EMAIL_ADDRESS] (Y.R. ); [EMAIL_ADDRESS] (Y.Z. ); [EMAIL_ADDRESS] (Y.Z. ); [EMAIL_ADDRESS] (S.L)  
Yong Zhou  
College of Grassland Science and Technology, China Agricultural University, Beijing 100193, China; [EMAIL_ADDRESS] (W.Z. ); [EMAIL_ADDRESS] (Y.R. ); [EMAIL_ADDRESS] (Y.Z. ); [EMAIL_ADDRESS] (Y.Z. ); [EMAIL_ADDRESS] (S.L)  
Yong Zhou  
College of Grassland Science and Technology, China Agricultural University, Beijing 100193, China; [EMAIL_ADDRESS] (W.Z. ); [EMAIL_ADDRESS] (Y.R. ); [EMAIL_ADDRESS] (Y.Z. ); [EMAIL_ADDRESS] (Y.Z. ); [EMAIL_ADDRESS] (S.L)  
Yong Zhou  
College of Grassland Science and Technology, China Agricultural University, Beijing 100193, China; [EMAIL_ADDRESS] (W.Z. ); [EMAIL_ADDRESS] (Y.R. ); [EMAIL_ADDRESS] (Y.Z. ); sy20223243 In the last few decades, changes in the water cycle caused by global warming have led to increasing water scarcity pressures worldwide [8]. Water scarcity results in more drought, and more severe droughts may occur in the future. As we all know, drought is a relatively common disaster that causes large economic losses every year. Drought has a significant impact on grassland ecosystems, causing changes in vegetation structure and function [9]. When soil moisture deficit leads to drought, it affects plant growth in the water and vapor transport processes between the soil, plants, and atmosphere [10; 11]. Soil moisture deficit reduces the water potential difference between the soil and plant roots, leading to a decrease in plant water uptake [12]. At the plant level, drought reduces plant water content, inhibits plant translocation from the root zone to the atmosphere, induces stomatal closure, alters assimilation rates, and reduces photosynthesis [10; 13]. Therefore, monitoring meteorological, hydrological, and agricultural drought and their impacts on grassland ecosystems is essential. The relationship between drought and grassland ecosystems has gained strong interest among scientists and managers and has gradually become a research hotspot. Among them, grassland gross primary production (GPP) has received extensive attention in recent decades due to its importance in understanding ecosystem carbon cycling and the simplicity of satellite-based GPP models. GPP of terrestrial ecosystems is the carbon fixed by plants and is important for the global carbon cycle [14; 15]. The traditional method for monitoring ecosystem GPP is the eddy covariance (EC) flux technique [16]. However, the small number of EC flux sites makes it challenging to monitor GPP at the regional scale. Remote sensing (RS) techniques are uniquely positioned to monitor GPP at a wide spatial scale [17]. Many studies have shown that vegetation indices (VIs) such as Normalized Difference Vegetation Index (NDVI) can accurately estimate terrestrial ecosystem GPP [18; 19; 20]. However, during drought events, vegetation photosynthesis or productivity typically declines [21]. VIs often represent the greenness of vegetation, which cannot promptly capture the changes in photosynthesis caused by drought, resulting in a lag effect and affecting the accuracy of GPP estimation using VIs [22].