Climate Change and Its Impacts on Streamflow: WRF and SCE-Optimized SWAT Models - Data Assimilation for Atmospheric, Oceanic and Hydrologic Applications - page 403

non-climatic drivers, are emerging” ( IPCC 2007a ). Since these climate changes are
likely to alter global surface temperatures, precipitation patterns, sea levels, extreme
events and other aspects of climate on which the natural environment and human
systems depend, substantial impacts are expected on, for example, water resources,
bio-diversity, agriculture and human health. Hence, adaptation to climate change has
become globally very important and especially is of great concern to the developing
countries. Though the impacts of climate change are expected to affect all natural
systems, water resources are likely to be impacted deeply. Current vulnerabilities
to climate are strongly correlated with climate variability, in particular precipitation
variability. Increased changes in precipitation intensity and variability are projected
to increase the risks of flooding and drought in many areas. While temperatures
are expected to increase everywhere over land and during all seasons of the year,
although at different increments, precipitation is expected to increase globally and
in many river basins, but to decrease in many others ( IPCC 2007a ). However,
quantitative projections of changes in precipitation, river flows and water levels at
the river-basin scale remain uncertain ( IPCC 2001 ).
The IPCC also notes that semi-arid and arid areas are particularly exposed to
the impacts of climate change on freshwater and many of these areas such as the
Mediterranean basin, western USA, southern Africa and north-eastern Brazil are
likely to suffer a decrease in water resources due to climate change ( IPCC 2007b ).
The IPCC also reports that current water management practices are insufficient to
counter the possible negative impacts of climate change on water supply, flood risk,
health, energy and aquatic ecosystems. For the development of adaptation policies,
realistic projections of climate change and its impacts on water resources are needed.
Global Climate Models, also known as General Circulation Models or GCMs are
primary tools for prediction of global climate. Precipitation, a principal input signal
to water systems, is not reliably simulated in these global climate models due to
their coarse resolutions ( IPCC 2007b ). GCM projections are subject to substantial
uncertainties in the modelling processes so that climate projections are not easy
to be incorporated into hydrological impact studies ( Mearns et al. 2001 ; Allen
and Ingram 2002 ; Forest et al. 2002 ). It has been noticed that such uncertainties
have produced biases in the simulation of river flows when using direct GCM
outputs for hydrological impact studies. Some studies have found that uncertainties
in climate change impacts on water resources are primarily due to the uncertainty in
precipitation inputs and less due to the uncertainties in greenhouse gas emissions, in
climate sensitivities or in hydrological models themselves ( IPCC 2007b ). Most cli-
mate change impact studies consider only changes in precipitation and temperature,
based on changes in the averages of long-term monthly values. A major problem
in the use of GCM outputs for impact studies is the mismatch of spatial grid scales
between GCMs (typically a few hundred kilometers) and the hydrological processes.
Water is managed at the catchment scale and adaptation is local, while GCMs
work on large spatial grids. Generally, precipitation projections are less consistent
than those of temperature due to its high variability in space and time, with large
inter-model ranges for seasonal mean rainfall responses. These inconsistencies are
explained partly by the inability of GCMs to reproduce the mechanisms responsible