## 1 Introduction  
The terrestrial water balance is vulnerable to hydrological disequilibrium, and this increases the likelihood of droughs when there is greater warming over land areas than the oceans [1]. Drought is a natural disaster phenomenon of water deficits due to prolonged atmospheric dryness, and drought events can adversely affect ecosystems and limit socio-economic sustainable development [2]. In general, droughs are characterized by a slow onset, often taking several months or longer to attain notable extents and intensities. However, the atmospheric circulation features associated with extreme dryness can persist for several weeks, accompanied by high temperatures and evapotranspirationcapacity, resulting in a swift depletion of soil moisture (SM), and this can subsequently trigger a rapid onset of drought [3; 4; 5; 6]. The rapid onset of drought, known as a flash drought, can cause crops to deteriorate rapidly, with potential impacts on agriculture, fire potential, and livestock health [7; 8; 9; 10; 11]. The National Integrated Drought Information System (NIDIS) is part of the National Oceanic and Atmospheric Administration (NOAA) and defines flash drought and describes its impacts and forecasting methods ([[https://www.drought.gov/what-is-drought/flash-drought](https://www.drought.gov/what-is-drought/flash-drought)]([https://www.drought.gov/what-is-drought/flash-drought](https://www.drought.gov/what-is-drought/flash-drought)), accessed on 5 July 2024). The potentially destructive impacts of flash droughts on agricultural production and socio-economics may exceed those of slowly evolving droughts [12; 13]. Well-studied cases of flash droughts include the central U.S. drought that began in May 2012 [14], the South African drought event that began in November 2015 [15], and the southern China drought that began in the late summer of 2019 [16]. Flash droughts exhibit a strong correlation with high evapotranspiration capacity and low SM triggered by heatwaves, causing severe damage to ecosystems [4; 17; 18]. Overall, there is an urgent need to precisely detect flash droughts and evaluate their progression and ecological impacts, especially in agricultural regions. SM dominates the drying stress on ecosystem production [19], so SM can be used as a key variable to monitor flash droughts [20]. Some scholars analyzed the spatio-temporal characteristics of flash droughts using surface or root zone SM [21; 22; 23; 24]. The currently available SM data were obtained from remote sensing products, reanalysis products, and model simulation datasets. Most available SM datasets lack sufficient soil stratification information or spatial resolution. For example, the European Space Agency Climate Change Initiative for Soil Moisture (ESA CCI SM, satellite-based SM) mainly provided SM information for the surface layer [21; 25]; the fifth-generation reanalysis dataset from the European Centre for Medium Range Weather Forecasts (ERA5) and the data from the Global Land Data Assimilation System (GLDAS) mainly provide SM information (about four soil layers) with a spatial resolution of about 0.25\({}^{\circ}\)[18; 26; 27; 28]. For analysis, the SM characteristics at regional scales with high spatial resolution and finer soil stratification are required. Drought is more destructive to vegetation with shallow root depths, especially crops. Scholars applied remotely sensed vegetation indicators, including the normalized difference vegetation index (NDVI) and the leaf area index (LAI), to assess the response patterns of vegetation ecosystems to drought conditions [29; 30; 31; 32]. However, traditional vegetation indices have a delayed response to droughts, making it difficult to monitor the effects of flash droughts on vegetation in a timely manner [3; 13]. Solar-induced chlorophyll fluorescence (SIF) is fluorescence emitted at 650-800 nm wavelength after the absorption of photosynthetically active radiation by vegetative chloroplasts under sunlight irradiation [33].