# Responses of Seasonal Indicators to Extreme Droughs in Southwest China
## 1 Introduction  
Extreme weather events, for example, droughts and floods, have occurred more frequently in the world over the past decades, with serious impacts on human society and ecosystem [1]. In particular, several heavy drought events have hit Southwest China (SWC) in the summer of 2006, from the autumn of 2009 to the spring of 2010 and the late summer of 2011, being a record-breaking drought event during the last 50 years [2; 3]. Frequent droughts have caused serious damage to water resources [4], ecosystems [5], agriculture [6; 7] and the whole society [8] in SWC and beyond. Especially, droughts can greatly dry up rivers and ponds, reduce drinking water and affect vegetation, with the consequence of reducing vegetation productivity, increasing tree mortality and causing a massive agricultural reduction and even famine [4; 9; 10; 11; 12]. The above three droughts cumulatively impacted the drinking water for more than 46 million people and the total economic losses exceeded 40 billion RMB [13; 14; 15]. High-frequency drought events and catastrophic damage have increasingly attracted the attention from the government and academia. Considerable researchers explored the characteristics as well as the physical causes and significant influence of droughts in SWC [8; 16]. Characteristics of drought are commonly measured in terms of duration, intensity, severity and spatial extent [17; 18]. Drought indices play an important role in identifying and quantifying drought and are recognized as the best approach for obtaining a measure of relative wetness or dryness [19; 20; 21]. The commonly used drought indices include the Palmer Drought Severity Index (PDSI), the Standardized Precipitation Evapotranspiration Index (SPEI), the Standardized Precipitation Index (SPI), the Vegetation Health Index (VHI) and the Temperature Vegetation Drought Index (TDVI) [11; 21; 22; 23; 24; 25]. With the advantage of taking account of the cumulative effect of moisture supply and demand, PDSI has been widely used in drought studies all over the world [26]. In SWC, drought indices including PDSI, SPI, SPEI and others have been successfully applied in drought studies [27; 28]. Extreme droughts are often caused by serious rainfall deficit and sustained high temperature, which were related to the anomaly of Western Pacific Subtropical High (WPSH) in SWC [28; 29; 30; 31]. Lack of moisture and warmer temperature in troposphere made air hard to be saturated and form rain [31; 32]. Additionally, a positive correlation between the Arctic Oscillation (AO) index and precipitation in SWC indicates that a negative phase of the AO may bring drought to the region [13; 31]. Regarding the influence of drought, response of vegetation to drought is one of hot topics. Some studies have documented the influence or recovery of drought to different types of vegetation, different periods of growth time, different stages of drought and different places [33; 34; 35; 36]. For example, savannas, croplands and mixed forests were more vulnerable to drought than deciduous forests and grasslands, while evergreen forests were more resistant to droughts and even were not influenced in the early period of drought [10; 36]. When it comes to recovery of drought, studies in the United States found that drought recovery duration indicates a positive correlation with drought severity and duration, but a negative relationship with gross primary productivity (GPP) [37; 38]. Moreover, vegetation greenness is more susceptible to drought than productivity, and more than 65% of vegetation can be restored within three months in SWC [12]. In terms of the response of surface water to drought, ecosystem water-use efficiency (WUE) was found different trends in response to severe drought episodes with a pattern of a decrease in the Northern Hemisphere and an increase in the Southern Hemisphere [37]. Another study found that terrestrial water storage in drought years deficit up to hundreds of gig tons in SWC [4]. Very few studies paid attention to changes of surface water during drought.