Droughts

Meteorological droughts are normally defined as abnormal periods of moisture deficiency relative to the long-term average over a given region (Heddinghaus et al., 1991). It is well known that droughts are one of the most damaging climate-related hazards to impact societies (Woodhouse and Overpeck, 1998). The Palmer Drought Severity Index (PDSI) (Dai et al., 2004) show the global very dry areas (PDSI<-3.0) increased remarkably since the late 1970s due to decreased precipitation and increased temperature over some regions under global warming background. It is suggested that warmer climate would result in longer lasting and more severe drought due to enhanced evaporation (Gregory et al., 1997) if without enough precipitation at the same time. For China as a whole, during the past half century annual precipitation has no significant secular trends, but decreasing trends in the number of rainy days and the longest durations of consecutive rainfall have been detected (Zhai et al., 1999a; 1999b). Also, the spatial distribution of precipitation trends across China is uneven (Zhai et al., 2005a). Decreasing trend has been observed over southern Northeast China, North China, and over the Sichuan Basin, whereas increases occurred in western China, the Yangtze River valley, and southeastern coast.

Using the observations as well as proxy data, Ye et al. (1996) found there were several significant dry or wet phases and abrupt changes of precipitation in the Yangtze River and Yellow River Valleys during recent 100 years. In the 1920s and mid-1960s, there were two climatic jumps from wet to dry in China. The 1960s' drying jump initiated the persistent and aggravating dry conditions in the Yellow River valley and was a part of some larger-scale climate changes involving the well-known drying process in Sahel, North Africa, since the late 1960s (Yan et al., 1990).

The devastating drought conditions in 1997, 1999 to 2002 in many areas of northern China have been a growing concern of both the Chinese government and general public (Zhang et al., 2003; Zou et al., 2005). During 1972-1997, there were 20 years during which the Yellow River experienced seasonal drying-up (zero streamflow) episodes. After the early 1990s, the drying-up happened more frequently than before, with an earlier start time and longer periods. The severe drought of 1997 in northern China resulted in a period of 226 days with no streamflow in the Yellow River, the longest drying-up duration on record. Many studies have examined changes of humid or dry conditions in China during the recent decades. An expanding trend of drought areas in northern China (except western Northwest China) in the latter half of the 20th century has been well documented in many studies (Zou et al., 2005; Wang and Zhai, 2003; Ma and Fu, 2003b) (Figure 8.12). The successive large increases of dry areas (PDSI<-1.0) since the late 1990s in Northeast, North and eastern Northwest China were unprecedented during the past half century. No obvious trends in drought areas have been found in other regions. There exist large multi-year to decadal variations in drought areas. For example, in the regions of the Yangtze River, quite extensive droughts occurred in the 1960s and late 1970s, while drought stress became relatively small after 1980 (Zhang et al., 2001; Zou et al., 2005). In western part of Northwest China, drought areas have decreased since the late 1980s (Wang et al., 2002; Shi et al., 2002).

Compared with the trend in temperatures over different regions in China, some researchers (Ma et al., 2003c; Xie et al., 2003) attributed the increase of severe droughts over north and northeast China to the regional warming climate. It was noted that risk of droughts had been increased since the late 1970s, as the regional warming not only led to higher temperatures but enhanced drought development (Dai et al., 2004). However, Zhai and Zou (2005b) suggested that variations in regional precipitation were still one of the most influential factor for drought development.

Dry/wet variations in eastern China are mainly governed by the strength of the East Asian monsoon circulation (Guo, 2003; Shi, 1996). In general, weaker summer monsoons lead to less summer precipitation in North China but more in the Middle and Lower Reaches of the Yangtze River (Guo et al, 2003). Guo et al. (2003) and Lu et al. (2004) found that strong summer monsoons existed during the period from 1951 to the mid-1970s, followed by a systematic reduction in the late 1970s, in association with strengthening and southwestward expansion of the WNP subtropical high. This change resulted in more frequent droughts in North China and floods in the middle and lower reaches of the Yangtze River after the 1980s (Gong and He, 2002a; Lu et al., 2004). Gong et al. (2002b) found that a strengthened spring Arctic Oscillation (AO) could trigger a northward movement of rain belt in China, i.e. dryness in the Middle and Lower Reaches of Yangtze River and above-average precipitation in northern China.

The anomalies of precipitation, temperature and other climate variables in many areas might be related to ENSO as well. Gong and Wang (1999) revealed that more precipitation occurred in East China and less in northern China in autumn and winter during El Niño years, while the opposite cases were found during La Niña years. In past El Niño years, northern China often experienced droughts in summer. The time series of ENSO since 1880, established by Wang (1999), showed that in recent decades, the frequency of La Niña events decreased dramatically and that of El Niño events increased. These changes might partly cause the southward move ment of rain belt in eastern China since the 1980s (Zhao, 1996). Huang and Yan (1999) found that southerly monsoon currents over eastern China had weakened since the 1960s, leading to the strengthening of droughts in North China and floods in the middle and lower reaches of the Yangtze River in recent decades.

Fig. 8.12 Time series of annual percentage areas (bars) in drought conditions (PDSI<-1.0) over different regions within China during 1951-2003. The smooth curves are generated using an 11-point binomial filter. Due to too few stations in early years, its calculation of drought areas is started from 1954 in Tibet region. (NE: Northeast; N: North; ENW: eastern Northwest; WNW: western Northwest; E: east; S: South China; Tibet: Tibet Plateau) (Zou et al., 2005)

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Fig. 8.12 Time series of annual percentage areas (bars) in drought conditions (PDSI<-1.0) over different regions within China during 1951-2003. The smooth curves are generated using an 11-point binomial filter. Due to too few stations in early years, its calculation of drought areas is started from 1954 in Tibet region. (NE: Northeast; N: North; ENW: eastern Northwest; WNW: western Northwest; E: east; S: South China; Tibet: Tibet Plateau) (Zou et al., 2005)

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