# Response of Grassland Vegetation Growth to Drought in Inner Mongolia of China from 2002 to 2020
## 1 Introduction  
Drought is a common hydrometeorological phenomenon and a significant environmental risk that increases vegetation mortality and reduces vegetation productivity and sequestration of carbon [1; 2; 3; 4]. For the past several decades, the frequency and severity of drought events have shown a rapidly increasing trend with rising temperature [5; 6; 7]. Earth system models also project a vast expansion of regions often affected by extremeand severe droughts in the coming decades [8; 9]. Grassland areas cover approximately 25% of the Earth's land surface, yet they store approximately 34% of global terrestrial carbon [10; 11; 12]. Grasslands are among the most fragile ecosystems and are highly susceptible to droughts [10; 11]. Recently, some studies have indicated that drought is the key factor influencing grassland ecosystem dynamics [11; 13; 14]. Therefore, it is important to study the response of grassland vegetation growth to drought under the background of global warming. Remote sensing observations have proven to be invaluable in monitoring drought characteristics and quantifying its impact on grasslands over varying time intervals and spatial scales [15; 16]. Recently, remote sensing approaches have been developed, and vegetation indices (e.g., the Normalized Difference Vegetation Index (NDVI), Enhanced Vegetation Index (EVI), and Near-Infrared Reflectance of Vegetation (NIRv)) and drought indices (e.g., the Temperature-Vegetation Dryness Index (TVDI), Soil Water Deficit Index (SWDI), Standardized Precipitation Evapotranspiration Index (SPEI), etc.) have been widely applied to assess the effects of drought events on grassland ecosystems [3; 17]. Traditionally used vegetation indices (such as NDVI, EVI, and NIRv) do not directly reflect changes in actual photosynthesis in grassland ecosystems when water stress occurs [11; 18; 19]. Some studies have also used Solar-induced Chlorophyll Fluorescence (SIF) to monitor vegetation response to drought events at different spatial and temporal scales [11; 17; 20; 21]. Unlike traditional vegetation indices, SIF is directly related to the actual photosynthetic function of plants and can reveal rapid changes in water stress within the vegetation canopy [22; 23]. However, the degree of change in photosynthesis or productivity is highly sensitive to different drought levels [10; 24]. Generally, extreme and severe drought events can result in additional reductions in grassland photosynthesis or productivity due to increased mortality and physiological function recession [24]. Previous studies have primarily focused on the effect of specific drought levels on grassland, leaving the response of grassland ecosystems to different intensities of drought largely unexplored. Moreover, different types of grassland ecosystems exhibit varying responses to droughts. Therefore, a more precise investigation into the spatiotemporal evolution patterns of grassland ecosystems and their response to different drought levels is warranted. Inner Mongolia is highly responsive to global climate change, with more than 74% of its area covered by various types of grasslands [25; 26]. This region has experienced increased temperatures, reduced precipitation, and more frequent droughts over the past few decades [26]. Understanding the spatial and temporal patterns of grassland NDVI, EVI, GOSIF, and GPP and their responses to different drought levels in Inner Mongolia is of great significance to enhancing ecological security in Northern China [26]. The primary objective of this study was to quantify the spatiotemporal variation in the characteristics of grassland ecosystems and their response to different intensities of drought in Inner Mongolia over the past two decades (2002-2020) using NDVI, EVI, GOSIF, GPP, SPEI, and soil moisture (SM) data. Additionally, we assessed variations in NDVI, EVI, GOSIF, and GPP losses in different grassland types under the same drought severity levels.