11.1.2.2.2. Precipitation
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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