## 1 Introduction  
Multiple factors, such as water pollution, climate change, and population growth, have led to a growing conflict between water supply and demand, resulting in global drought scenarios gaining more attention. Droughts are one of the most complex natural disasters in the world, with impacts on agriculture, society, the economy, and even ecology [1]. Droughts are characterized by their wide area of influence, long duration, and effect on numerous people. Based on historical records, large-scale droughts can be found on all continents, with drought footprints covering North and South America, Australia, Asia, Africa, and Europe [2]. The spatial and temporal distributions of water resources in China are extremely uneven. North and Southwest China experience droughts with the highest frequency and duration [3]. Severe droughts in Southwest China in the fall of 2009 and the spring of 2010 resulted in a lack of potable water for 21 million people and economic losses of $30 billion [4]. Droughts are one of the main threats to agricultural production in Northeast China, and this risk is growing in intensity [5]. Additionally, there are records of droughts causing the Yellow River to dry up. The difficulty in characterizing droughts stems from the lack of proper understanding of the evolutionary patterns of different droughts [6]. Additionally, information on the precise definition, start, and end of droughts is lacking, providing major obstacles to investigation. Also, the spatial and temporal effects of drought, the instability in their variability, and the anthropogenic factors that trigger droughts are not fully understood [7]. The definitions of a drought are not uniform; a drought can be generally defined as \"a prolonged period of abnormally dry weather with scanty rainfall that causes a serious hydrological imbalance [8]\". Traditional droughts can be divided into four categories: meteorological, hydrological, agricultural, and socio-economic [9; 10]. A meteorological drought is usually defined as inadequate precipitation over an area for a period of time [1], and it is described by the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI) [11; 12]. A hydrological drought is associated with surface water or groundwater deficits; therefore, runoff data are often used to assess hydrological droughts [10; 13]. An agricultural drought is concerned with soil-moisture deficiency with meteorological droughts and climatic factors and their impacts on agricultural production and economic profitability; this drought usually lasts longer than the aforementioned droughts [14]. Droughts have direct and sustained economic and health effects on those engaged in agriculture and indirectly affect non-agricultural populations [15]. Therefore, drought propagation, wherein different types of droughts transform into one another, is possible. In existing studies, drought conditions are generally assessed using drought indicators, which are prime variables for analyzing the effects of droughts and defining different drought parameters, including intensity, duration, severity, and spatial extent [1]. The Palmer Drought Severity Index (PDSI) [16] and SPI [17] are the most commonly known drought indices. The former is widely used, and the latter is simple to calculate and can be used for different time-scales. With the development of drought research, an increasing number of meteorological and hydro-ecological factors have been added to the assessment of drought indicators. On a precipitation basis, the SPEI [18] and Reconnaissance Drought Index (RDI) [19] consider evapotranspiration. Generally, the Standardized Runoff Index (SRI) [20] and the Surface Water Supply Index (SWSI) [21], which consider runoff, are mostly used to assess hydrological droughts. The Crop Moisture Index (CMI) [22] was used to evaluate agricultural droughts. Furthermore, [PERSON] et al.