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Rainfall in boreal Asia is highly variable on seasonal and interannual as well as spatial scales. The time series of annual mean precipitation in Russia suggests a decreasing trend; these tendencies have amplified during 1951-1995, especially in warm years (Rankova, 1998). In long-term mean precipitation, a decreasing trend of about -4.1 mm/month/100 years has been reported in boreal Asia. During the past 10-15 years, however, precipitation has increased, mostly during the summer-autumn period (Izrael et al., 1997b). As a result of this increase in precipitation, water storage in a 1-m soil layer has grown by 10-30 mm (Robock et al., 2000). The large upward trends in soil moisture (of more than 1 cm/10 years) have created favorable conditions for infiltration into groundwater. The levels of major aquifers have risen by 50-100 cm; the growth of groundwater storage has resulted in increasing ground river recharge and considerable low-water runoff.
Annual mean rainfall is considerably low in most parts of the arid and semi-arid region of Asia. Moreover, temporal variability is quite high: Occasionally, as much as 90% of the annual total is recorded in just 2 months of the year at a few places in the region. Rainfall observations during the past 50 years in some countries in the northern parts of this region have shown an increasing trend on annual mean basis. A decreasing trend in annual precipitation for the period 1894-1997 has been observed in Kazakhstan. The precipitation in spring, summer, and autumn, however, has shown slight increasing trends. In Pakistan, seven of 10 stations have shown a tendency toward increasing rainfall during monsoon season (Chaudhari, 1994).
In temperate Asia, the East Asian monsoon greatly influences temporal and spatial variations in rainfall. Annual mean rainfall in Mongolia is 100-400 mm and is confined mainly to summer. Summer rainfall seems to have declined over the period 1970-1990 in Gobi; the number of days with relatively heavy rainfall events has dropped significantly (Rankova, 1998). In China, annual precipitation has been decreasing continuously since 1965; this decrease has become serious since the 1980s (Chen et al., 1992). The summer monsoon is reported to be stronger in northern China during globally warmer years (Ren et al., 2000). On the other hand, drier conditions have prevailed over most of the monsoon-affected area during globally colder years (Yu and Neil, 1991).
In tropical Asia, hills and mountain ranges cause striking spatial variations in rainfall. Approximately 70% of the total annual rainfall over the Indian subcontinent is confined to the southwest monsoon season (June-September). The western Himalayas get more snowfall than the eastern Himalayas during winter. There is more rainfall in the eastern Himalayas and Nepal than in the western Himalayas during the monsoon season (Kripalani et al., 1996). The annual mean rainfall in Sri Lanka is practically trendless; positive trends in February and negative trends in June have been reported, however (Chandrapala and Fernando, 1995). In India, long-term time series of summer monsoon rainfall have no discernible trends, but decadal departures are found above and below the long time averages alternatively for 3 consecutive decades (Kothyari and Singh, 1996). Recent decades have exhibited an increase in extreme rainfall events over northwest India during the summer monsoon (Singh and Sontakke, 2001). Moreover, the number of rainy days during the monsoon along east coastal stations has declined in the past decade. A long-term decreasing trend in rainfall in Thailand is reported (OEPP, 1996). In Bangladesh, decadal departures were below long-term averages until 1960; thereafter they have been much above normal (Mirza and Dixit, 1997).
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