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4.1. Introduction and Scope
This chapter assesses our understanding of the implications of climate change
for the hydrological cycle, water resources, and their management. Since the beginnings
of concern over the possible consequences of global warming, it has been widely
recognized that changes in the cycling of water between land, sea, and air could
have very significant impacts across many sectors of the economy, society, and
the environment. The characteristics of many terrestrial ecosystems, for example,
are heavily influenced by water availability and, in the case of instream ecosystems
and wetlands, by the quantity and quality of water in rivers and aquifers. Water
is fundamental to human life and many activities—most obviously agriculture
but also industry, power generation, transportation, and waste management—and
the availability of clean water often is a constraint on economic development.
Consequently, there have been a great many studies into the potential effects
of climate change on hydrology (focusing on cycling of water) and water resources
(focusing on human and environmental use of water). The majority of these studies
have concentrated on possible changes in the water balance; they have looked,
for example, at changes in streamflow through the year. A smaller number of studies
have looked at the impacts of these changes for water resources—such as the
reliability of a water supply reservoir or the risk of flooding—and even
fewer explicitly have considered possible adaptation strategies. This chapter
summarizes key findings of research that has been conducted and published, but
it concentrates on assessing opportunities and constraints on adaptation to climate
change within the water sector. This assessment is based not only on the few studies
that have looked explicitly at climate change but also on considerable experience
within different parts of the water sector in adapting to changing circumstances
in general.
This chapter first summarizes the state of knowledge of climate change impacts
on hydrology and water resources (Section 4.2), before
assessing effects on the hydrological cycle and water balance on the land (Section
4.3). Section 4.4 examines potential changes in water
use resulting from climate change, and Section 4.5 assesses
published work on the impacts of climate change for some water resource management
systems. Section 4.6 explores the potential for adaptation
within the water sector. The final two sections (Sections
4.7 and 4.8) consider several integrative issues as
well as science and information requirements. The implications of climate change
on freshwater ecosystems are reviewed in Chapter 5, although
it is important to emphasize here that water management is increasingly concerned
with reconciling human and environmental demands on the water resource. The
hydrological system also affects climate, of course. This is covered in the
Working Group I contribution to the Third Assessment Report
(TAR); the present chapter concentrates on the impact of climate on hydrology
and water resources.
At the outset, it is important to emphasize that climate change is just one
of many pressures facing the hydrological system and water resources. Changing
land-use and land-management practices (such as the use of agrochemicals) are
altering the hydrological system, often leading to deterioration in the resource
baseline. Changing demands generally are increasing pressures on available resources,
although per capita demand is falling in some countries. The objectives and
procedures of water management are changing too: In many countries, there is
an increasing move toward “sustainable” water management and increasing
concern for the needs of the water environment. For example, the Dublin Statement,
agreed at the International Conference on Water and the Environment in 1992,
urges sustainable use of water resources, aimed at ensuring that neither the
quantity nor the quality of available resources are degraded. Key water resources
stresses now and over the next few decades (Falkenmark, 1999) relate to access
to safe drinking water, water for growing food, overexploitation of water resources
and consequent environmental degradation, and deterioriation in water quality.
The magnitude and significance of these stresses varies between countries. The
late 1990s saw the development of several global initiatives to tackle water-related
problems: The UN Commission on Sustainable Development published the “Comprehensive
Assessment of the Freshwater Resources of the World” (WMO, 1997), and the
World Water Council asked the World Commission for Water to produce a vision
for a “water-secure world” (Cosgrove and Rijbersman, 2000). A series
of periodical reports on global water issues was initiated (Gleick, 1998). The
impacts of climate change, and adaptation to climate change, must be considered
in the context of these other pressures and changes in the water sector.
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