Current and future environmental change - Fundamentals of the Physical Environment - page 706

Plate 28.3 The Merced River in full spring flood in May 2005, generated by rapid snow-melt of above average winter snowfall
in its headwaters in the Sierra Nevada, California. Huge boulders in the central bar were moved and re-deposited during the event.
Photo: Ken Addison

In addition to regional variations identified above,
the many positive or negative feedbacks in terrestrial
hydrometeorological and hydrogeological systems further
complicate the changing patterns and regimes of run-off
and river flow. In particular, existing anthropogenic
changes in land use and climate-driven vegetation
changes, agriculture and industrial practices re-route
water and alter water budgets. However, to the extent that
we can make a global prediction of river discharge trends,
it is highly probable that most land surfaces will experience
general disturbance of river regimes, with higher water
(and sediment) discharges and/or more flash flooding and
channel destabilization. The impact on human societies
will be far-reaching, strongly influenced by the regional
adaptive capacity or vulnerability of water and channel
management, the extent of population growth and stage
of economic and technological development.
Two IPCC 2007 forecasts give particular cause for
concern. Despite increasing monsoon precipitation, the
availability of fresh water in areas of high population

growth and economic expectations in the large river
basins of central and south-eastern Asia is very likely to
decrease , adversely affecting 1 billion people by mid-
twenty-first century. In addition, the number of people
living in 'severely-stressed river basins' worldwide is likely
to rise from 1.4-1.6 billion in 1995 to 4.6-6.9 B by 2050,
under the A2 scenario of continually increasing global
population and economic growth in regions like southern
Asia. This illustrates the complex nature of hydrological
processes and water resource management during climate
change, in which water demand may outstrip supply even
where precipitation increases. Higher and more intense
rainfall, fuelled for example by intensification of monsoon
storms in the Ganges-Brahmaputra basin, will drive faster
run-off and higher flood peaks. River management in such
regions will have to balance more efficient drainage and
routing for flood protection with water retention for
agricultural, industrial and domestic use.
Drought, of course, is the other side of the water stress
coin, with the most serious falling precipitation levels, of