Geoscience Reference
In-Depth Information
rise of 0.2°C per decade over the next 100 years.
Even if the concentrations of all greenhouse gases
and aerosols had been kept constant at year 2000
levels, a further warming of about 0.1°C per decade
would be expected. Linked to this prediction are
an increase in sea level of 15-95 cm and changes in
the temporal and spatial patterns of precipitation.
All of these predicted changes will influence the
hydrological cycle in some way, but it is difficult
to pinpoint exactly how. The IPCC predictions
on impacts on water resource matters is shown in
Table 8.3. At the very simple level a temperature
rise would lead to greater evaporation rates, which
in turn puts more water into the atmosphere. This
may lead to higher precipitation rates, or at least
changes in precipitation patterns. How this impacts
the hydrology of an individual river catchment is
very difficult to predict. The most common method
to make predictions is to take the broad-brush
predictions from a global circulation model (often
at scale of 1° latitude and longitude per grid square)
and downscale it to the local river catchment level.
There are several methods used to downscale the
data, and Wilby
et al
. (2000) show that the choice
of method used can influence the modelling
predictions dramatically.
Arora and Boer (2001) have simulated the impacts
of possible future climate change on the hydrology
of twenty-three major river catchments worldwide.
They conclude that in warmer climates there may
be a general reduction in annual mean discharge,
although as some rivers showed an increase this is
not absolute. For mid- to high-latitude rivers Arora
and Boer (2001) concluded that there may be
big changes in the timing of large runoff events that
could be linked to changing seasonal times. This
confirms the findings of Middelkoop
et al
. (2001)
who predict higher winter discharges on the Rhine
(Europe) from 'intensified snow melt and increased
winter precipitation'. In a similar vein Wilby and
Dettinger (2000) predict higher winter flows for
three river basins in the Sierra Nevada (California,
USA). These higher winter flows reflect changes
in the winter snowpack due to a predicted rise in
both precipitation and temperature for the region.
HYDROLOGY AND CHANGE
In water resource management there is a problem
concerning the statistical techniques that we use. In
a frequency analysis technique there is an inherent
assumption that a storm event with similar ante-
cedent conditions, at any time in the streamflow
record, will cause the same size of storm. We assume
that the hydrological regime is stationary with time.
Under conditions of land use or climate change it
is quite possible that these conditions will not be
met. This makes it difficult to put much faith in
a technique such as frequency analysis when it is
known that the hydrological regime has changed
during the period of record. These are the types of
challenges facing water resource management in an
ever-changing world. The following section outlines
some of the changes possible and uses case studies
to demonstrate the possible effects of those changes.
Climate change
At the start of the twenty-first century climate
change is the biggest environmental talking point,
dominating the scientific media and research
agenda. Any unusual weather patterns are linked
to the greenhouse effect and its enhancement by
humans. The summer of 2006 in Northern Europe
was one of the hottest and driest on record and there
was drought. At the same time New Zealand experi-
enced one of the wettest winters on record with
record snowfalls to sea level, followed by a wet
and cold summer. At various times in the media,
both these events were linked to global warming.
The difficulty with trying to verify any real link
to climate change is that hydrological systems
naturally contain a huge amount of variability. The
extreme events we are experiencing now may be
part of that natural variability, or they may be being
pushed to further extremes by climate change. It is
unlikely we will know for sure until it is too late to
try and do anything about it.
Predictions from the Intergovernmental Panel
on Climate Change (IPCC, 2007) suggest that the
earth may experience a global surface temperature
