1. Introduction
Drought, a globally common natural hazard and one of the most damaging environmental disasters resulting from climate variations, has threatened human life, property safety, and ecosystem at large [
1,
2,
3]. For example, drought impedes the growth and development of crops and reduces crop yields. Similarly, severe drought may cause short or long-term water crisis and affect human society safety and security. Against the background of global warming, severe drought events have occurred throughout China, the damages through crop yield reduction, economic loss, safety of our society and others caused by drought cannot be neglected [
4]. Therefore, it is important to quantify the problematic influence of drought hazard in China.
There are several types of droughts depending on the water deficit patterns including meteorological, agricultural, hydrological, and socio-economic [
5]. Meteorological drought refers to a lack of precipitation over a region for a period of time [
6]. The most obvious manifestation is the continuous below normal rainfall over a certain period. Agricultural drought refers to soil moisture that cannot meet the water demand of plants at different growth stages [
7]. Hydrological drought refers to a period with inadequate surface and subsurface water resources for established water uses of a given water resources management system [
6]. Socio-economic drought is associated with failure of water resources systems to meet water demands and thus associating droughts with supply of and demand for an economic good (water) [
6]. Continuation of meteorological to agricultural or hydrological drought involves a suite of processes including occurrence, development, and propagation.
In view of the definition of water cycle and drought, when precipitation is lower than normal value, it will cause meteorological drought, while the occurrence of agricultural or hydrological drought is not only related to precipitation reduction but also affected by land surface evaporation and other factors. Thus, the occurrence of agricultural and hydrological drought can be regarded as the inheritance and development of meteorological drought, and there is time lag between meteorological, agricultural, and hydrological droughts [
8,
9,
10]. Considering multi-timescales, it is important to understand the relationship between the three drought types for investigating the process of drought transmission.
Although there were some previous studies which compared different types of drought [
11,
12], information on the propagation characteristics among different types of droughts are scarce [
10,
13]. Drought indicators are commonly used to identify the occurrence of a current drought event and its severity. These can also be used to assess past events and forecast future events. To date, more than 150 drought indicators are proposed or developed to characterize a specific degree of wetness and dryness [
14]. Recently, the drought characteristics were investigated by using some remote sensing (RS) based indicators. For example, Gidey et al. [
15] compared RS-based agricultural drought indicators of Vegetation Health Index (VHI), Normalized Difference Water Index (NDWI), and Normalized Difference Vegetation Index (NDVI) with Standardized Precipitation Index (SPI). They reported a significant and positive correlation between VHI and SPI at the three-month timescale. Ezzine et al. [
16] proposed the Standardized Water Index (SWI) based on Normalized Difference Water Index (NDWI) and compared with the SPI based on meteorological drought and the Standardized Vegetation Index (SVI) based on Normalized NDVI. They also tested the consistency of the three drought indexes under different land use types during a period of 15 years (1998–2012) and reported a stronger agreement between SVI and SPI in autumn and winter than that between SPI and SWI. A satisfactory correlation between SWI and SPI was also reported [
16]. Li et al. [
11] analyzed and compared the meteorological and agricultural drought characteristics in Qinghai-Tibet Plateau by calculating Standardized Precipitation Evapotranspiration Index (SPEI) and Temperature Vegetation Dryness Index (TVDI) with the dry area data from 2001 to 2014 [
17]. They reported a deterioration of drought situation in Qinghai-Tibet Plateau from 1971 to 2014, and an improved performance of SPEI over TVDI.
To study the propagation processes, Huang et al. [
10] studied the transmission time from meteorological to hydrological drought and the influencing factors in Weihe River Basin using SPI and Soil Runoff Index (SRI) as the meteorological and hydrological drought indicators, respectively. They observed that the propagation time was affected by seasons and had a strong relationship with atmospheric circulation factors (such as El Niño Southern Oscillation and Arctic Oscillation) and underlying surface conditions. Wu et al. [
18] used SPI and SRI and reported that the average duration and severity of hydrological drought in the Loess Plateau was greater than that of meteorological drought. They found that the maximum duration of hydrological drought was five days in the Loess Plateau. Hisdal et al. [
19,
20] used empirical orthogonal function, Monte Carlo simulation, and probability and reported that the frequency and duration of meteorological drought showed opposite relationship with hydrological drought. Similarly, Wu et al. [
13] established a non-linear relationship between meteorological and hydrological drought for intensity and duration using a binary relationship model. They observed that the operation of the reservoir shortened the transmission process.
Several studies investigated the variation features of meteorological, agricultural, and hydrological droughts. For example, Wang et al. [
21] explored the effects of climate change on the duration, intensity, and frequency of meteorological, agricultural, and hydrological droughts over 1991–2000 and 2091–2100 using Global Climate Models’ emission scenarios. Duan et al. [
22] analyzed the impacts of climate change on meteorological, agricultural, and hydrological droughts in the future. Both studies pointed out that even if the characteristics of meteorological drought were stable, the future climate change would have greater impacts on agricultural and hydrological droughts. Wu et al. [
13] developed a relational model between meteorological and hydrological drought duration and intensity. Drought characteristics at different timescales also differed. For example, Vicente-Serrano et al. [
23] used different scales of SPI and analyzed the differences of the patterns of drought in the Iberian Peninsula. Pasho et al. [
24] and Xu et al. [
25] reported variable influences of different timescales of droughts on plant growth. The possible connections between the timescales of meteorological and agricultural (or hydrologic) drought are crucial. We can also further improve our understanding of the propagation characteristics of drought using lag time by analyzing and comparing the drought characteristics of different drought types. However, there is no information on the spatial distribution of meteorological, agricultural, or hydrological droughts and their interrelations at different timescales across China. Additionally, most previous research has focused on describing the characteristics of single drought type. The comparative studies of different drought types and their connections are limited since it required much more data, more indices, and more complicated methods.
The long-term goal of this work was to simultaneously investigate three types of droughts and reveal the drought propagation features across China. The specific objectives were to: (i) characterize spatiotemporal variations of precipitation (PPT), soil water storage (SWS), and baseflow-groundwater runoff (BGR) and their dominant scale of occurrence; (ii) compute multi-timescale SPI, standardized soil moisture index (SSI), and SRI based on PPT, SWS, and BGR data and analyze characteristics of three types of droughts; and (iii) identify mutual relationships and propagation features among different types of droughts at different timescales. The study was carried out at seven climatic regions covering mainland China and aimed at generating information to support policy development in managing and coping with drought hazards.
4. Discussion
Information on the spatial and temporal distribution of drought is critical to understand the hazards associated and develop strategies against it. This requires detailed information on the temporal and spatial changes of determining factors including PPT, SWS, and BGR [
2,
67,
68] and the probability density functions can provide information on their variability. This study quantified the variations and probability density functions of PPT, SWS, and BGR for further computing drought indices of SPI, SSI, and SRI. This provided rational references for obtaining close-to-real drought indices which decreased to the least. However, many studies computed the standardized drought indices using the original density functions. For example, Yao et al. [
42], Sun et al. [
69], and Lai et al. [
70] used the acquiescent distributions and calculated the SPEI, SPI, SSI, and SRI and evaluated the drought occurrence features without selecting the most suitable probability density function. Unlike the current study, inclusion of a fixed function without considering data specificity may have introduced some errors in drought indicators estimated in the previous studies.
Similarly, previous studies mostly focused on the comparison between meteorological and agricultural (or hydrological) droughts [
13,
71,
72]. The studies of drought propagation were limited to some extent due to data limitation and technology complexity. Li et al. [
73] studied meteorological and hydrological droughts in the Red River Basin of Yunnan Province in China using SPEI and streamflow drought index. They reported that the hydrological drought lagged meteorological drought by 1–8 months. Ma’rufah et al. [
71] analyzed the relationship between SPI and VHI. They reported that meteorological and agricultural droughts were more intensive during the El Niño years. In addition, the meteorological drought mainly occurred during June and November whereas the agricultural drought mostly occurred from August to November. Huang et al. [
10] explored the transmission time from meteorological to hydrological droughts and the influencing factors in Weihe River Basin of China and underlying surface condition based on Fu [
74]. The results showed that the propagation time in spring and summer was shorter than that in autumn and winter. In addition, El Nino and Southern Oscillation had great influences on the propagation time. Wu et al. [
13] proposed a theoretical model of the propagation from meteorological to hydrological drought. However, due to the limitation in number of stations, the theoretical model cannot be widely used in other regions. In this research, the most suitable propagation time from meteorological to agricultural or to hydrological droughts was investigated, which improved our understanding to respond to timely propagation of meteorological drought to agricultural or hydrological droughts. This also provided critical information in drought forecasting or assessment of the impacts of drought on crop and livestock productions.
Since the propagation of meteorological to agricultural and hydrological droughts is non-linear and complex [
37], the response of meteorological drought to hydrological cycle differed for different sub-regions. Not only is the propagation time of drought useful, the factors that affect the propagation time of drought is also important. Although we investigated the characteristics and propagation of different types of drought, the frequency, duration, and magnitude, which depicted more respects of drought occurrence based on the run theory have not been systematically revealed considering the limitation of the article length. In future studies, the analysis of the driving factors of drought formation and propagation or run theory-based drought variable analysis are recommended. In addition, the quantitative description of drought propagation time and the factors influencing drought transmission will be indispensable to develop reasonable measures in tackling and adapting to drought.
Drought is caused by several complicated processes changing generally from meteorological to agricultural to hydrological drought with some intrinsically related process. By comparing the correlation coefficients in different areas of different drought indices, we monitored different types of drought. It implied that we must take measures to cope with local drought. For example, precipitation varies in different locations of China [
75]. When the precipitation is low or the continuous evapotranspiration is large, southern China with a large correlation between different drought types should adapt farmland irrigation and river basin water storage at an early stage to avoid the impact of agricultural hydrological droughts.
5. Conclusions
Drought is very complex and one of the most damaging natural disasters. The interconnection between different types of droughts, their spatiotemporal distribution and propagation characteristics at multiple scales, and the dominant time lag are critical for developing policies and strategies for responding to drought hazards. This study provided a comprehensive analysis and assessment of meteorological, agricultural, and hydrological droughts, their spatiotemporal distribution, and propagation characteristics at multiple time scales. Three drought indices, SPI, SSI, and SRI representing meteorological, agricultural, and hydrological droughts were calculated based on PPT, SWS, and BGR characteristics. Strong spatial variability in the PPT, SWS, and BGR and the drought indices were observed across mainland China with distinct time lags.
The temporal variations of 12-month SPI, SSI, and SRI over 1948–2010 indicated generally increasing risks of drought in six out of seven sub-regions but were not always consistent in denoting the dry/wet spells for different sub-regions. For all of SPI, SSI, and SRI, more decreasing (significant or not) trends were observed in the eastern half of China, indicating the aggregation of metrological, agricultural, and hydrological droughts, especially SSI and SRI. The meteorological drought was worsened mainly in southwestern China, while the agricultural and hydrological droughts were worsened in northwestern China. The main periods of agricultural and hydrological droughts were larger than for meteorological droughts, and the spatial distribution characteristics of the main periods of agricultural and hydrological drought were similar. The varied main periods of different types of drought in different sub-regions implied that the government should take location and time tailored approaches to cope with droughts. Low correlation between SPI and SSI or SRI was observed at northwestern China, indicating a low connection between meteorological and agricultural/hydrological droughts at short timescales in northwestern arid and semi-arid regions. Additionally, the lags between agricultural and hydrological droughts were longer in northwestern China than in south China, indicating slow propagation of meteorological to agricultural or hydrological droughts in arid- and semi-arid regions. The faster propagation of meteorological droughts in south China implied that the drought adaption should not only be paying attention to meteorological drought, but also to agricultural and hydrological droughts.
The spatiotemporal distribution of droughts, identification of causal factors, the developments and propagation characteristics can only provide critical information necessary to develop strategies for managing and tackling droughts. The information generated in this study will help managers and policymakers to better intervene in the process of drought transmission, weaken or even avoid the impact of drought transmission, and analyze the multi time scale. The trend of drought at multiple time scales and at different regions will help understand the situations and forecast regional future droughts.
These results are useful for policymakers and decision-makers. For example, when meteorological drought onset with continuous dry days, the water resources management department should adjust water allocation ratio and amount or set up a rational reservoir water level. These results could be also integrated with future climate change projections to better forecast droughts by using the newly released data of Global Climate Model in the Coupled Model Intercomparison Project Phase 6.
There are some limitations in this research. First, the modeled data caused errors in drought analysis. Second, the intrinsic mechanics of drought propagation has not been revealed. Future study will focus on revealing the mechanics of drought propagation from one type to another in China.