4.3.6.2.1. Cold and cool temperate climates
These areas are characterized by precipitation during winter falling as snow
and include mountainous and low-lying regions. A major proportion of annual
streamflow is formed by snow melting in spring. These areas include large parts
of North America, northern and eastern Europe, most of Russia, northern China,
and much of central Asia. The most important climate change effect in these
regions is a change in the timing of streamflow through the year. A smaller
proportion of precipitation during winter falls as snow, so there is proportionately
more runoff in winter and, as there is less snow to melt, less runoff during
spring. Increased temperatures, in effect, reduce the size of the natural reservoir
storing water during winter. In very cold climates (such as in Siberia and northern
Russia), there is little change in the timing of streamflow because winter precipitation
continues to fall as snow with higher temperatures. The largest effects are
in the most “marginal” snow-dominated regime areas.
The effects of climate change on the magnitude of annual runoff and flows through
the year are much less consistent than the effect on streamflow timing because
they depend not on the temperature increase but on the change in precipitation.
In general, precipitation increases in high-latitude areas under most scenarios,
but in lower latitudes precipitation may decrease. Kazcmarek et al. (1997),
for example, show a decrease in annual runoff in Poland under a Geophysical
Fluid Dynamics Laboratory (GFDL)-based scenario (by around 20% by the 2050s)
but an increase under a Goddard Institute for Space Studies (GISS) scenario
(by as much as 20%); in both cases, the season of maximum flow shifts from spring
to winter.
Similar patterns are found for rivers in mountainous regions or draining from
mountains. The Rhine and Danube, for example, would both see a reduction in
spring flows and an increase in winter runoff (Grabs, 1997; Starosolszky and
Gauzer, 1998), as would rivers draining east and west from the Rocky Mountains
in North America.
4.3.6.2.2. Mild temperate climates
Hydrological regimes in these regions are dominated by the seasonal cycles
of rainfall and evaporation; snowfall and snowmelt are not important. Here,
climate change tends to affect the magnitude of flows in different seasons—by
an amount that depends on the change in rainfall—and may lead to an exaggerated
seasonal cycle, but it generally does not affect the timing of flows through
the year. In the UK, for example, most scenarios result in an increase in winter
runoff and, particularly in the south, a decrease in summer runoff (Arnell and
Reynard, 1996); similar patterns are found across most of western Europe under
most scenarios (Arnell, 1999a). Low flows tend to occur during summer, and changes
in low-flow frequency are closely related to changes in the balance between
summer rainfall and summer evaporation. Across most mid-latitude temperate regions,
summer rainfall would decline with global warming, leading to a reduction in
low flows.
The detailed effect of a given change in climate, however, depends to a large
extent on the geological characteristics of the catchment. Studies in the UK
(Arnell and Reynard, 1996) and Belgium (Gellens and Roulin, 1998) have indicated
that in catchments with considerable groundwater, changes in summer flows are
largely a function of the change not in summer rainfall but in rainfall during
the winter recharge season.
4.3.6.2.3. Arid and semi-arid regions
River flows in arid and semi-arid regions are very sensitive to changes in
rainfall: A given percentage change in rainfall can produce a considerably larger
percentage change in runoff. There have been relatively few studies in such
regions since the SAR, but work has been done in southern Africa (Schulze, 1997),
Australia (Bates et al., 1996), northern China (Ying and Huang, 1996), and southern
Russia (Georgiyevsky et al., 1996; Shiklomanov, 1998).
4.3.6.2.4. Humid tropical regions
Runoff regimes in these regions are very much influenced by the timing and
duration of the rainy season or seasons. Climate change therefore may affect
river flows not only through a change in the magnitude of rainfall but also
through possible changes in the onset or duration of rainy seasons (such as
those caused by monsoon).
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