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Hello, I'm Glenn Patterson, PhD candidate in watershed science at CSU and developer
and instructor for water courses in CSU's Online Plus program. Before coming to CSU,
I worked for 30 years as a hydrologist with the US Geological Survey. It's hard to talk
about 21st century water issues without mentioning climate change. In this lecture, I would like
to address the effects of climate change on water resources.
Abundant evidence shows that the earth has been warming by several degrees centigrade
during the last few decades. It's highly likely that this trend is related to increasing concentrations
of carbon dioxide and other greenhouse gasses in the atmosphere. Computer models that simulate
the earth's climate tend to agree that the warming trend is likely to continue throughout
the rest of this century. The models show less agreement about projected
changes in precipitation, but they do tend to agree that some places—primarily the
higher latitudes—will be getting wetter and others—primarily the lower latitudes—will
be getting drier. The models also tend to agree that there is likely to be more variation
in precipitation. Both floods and droughts are likely to recur with greater frequency,
duration, and intensity. How are these projected changes likely to
affect the hydrologic cycle? Well, higher temperatures are likely to speed up evaporation
from water and land surfaces and speed up transpiration from plants. No matter what
happens to precipitation, the increased evapotranspiration will dry out the soil and leave less water
available to flow to streams as well as less water available to infiltrate into the ground
and recharge aquifers. Warmer temperatures will also cause a shift in precipitation toward
less snow and more rain and will lead to earlier and more rapid melting of both the seasonal
snowpack and glaciers that have persisted for many years. Melting glaciers might cause
increased runoff in glacier-fed streams in the short run, but in the long run as glaciers
shrink, the melt water runoff will diminish. Along the coasts, rising sea levels are likely
to inundate more low lying land. Impacts on water resources of the United States
will vary. While shift from snow to rain and to earlier snow melt would be widespread,
other effects would vary by region. In areas with increased precipitation, such as the
northeast and certain coastal areas, as illustrated on the right side of this diagram, you would
expect more severe storms, and more flooding. In the interior of the country, illustrated
on the right side of the diagram, we would expect more droughts, less stream flow, and
less groundwater recharge. Water quality is likely to be effected, too, with warmer temperatures,
muddier water from increased erosion and sedimentation, more pollutants from storm water runoff from
heavy rains, and in some places, higher contaminant levels due to less stream flow available due
to diluting waste water discharges. Considering the impacts of both climate change
and population growth, we are likely to see significantly more water scarcity by the year
2025. According to Population Action International based on the United Nations Medium Population
Projections of 1998, more than 2.8 billion people in 48 countries will face water stress
or scarcity conditions by the year 2025. Of these countries, 40 are in West Asia, Africa,
or Sub-Saharan Africa. Over the next two decades, population increases and growing demands,
in addition to changing climate, are projected to push all the West Asian countries into
water scarcity conditions. By 2050, the number of countries facing water stress or scarcity
could rise to 54 with a combined population of 4 billion people, about 40% of the projected
global population of 9.4 billion. Let's look at how these changes are expected
to affect one of our geographic focus areas, the Colorado River Basin. In 2012, the US
Bureau of Reclamation completed a large study on the future of water supply and demand in
this heavily allocated basin and in places outside of the basin, such as Denver and Southern
California, that receive its exported water. On the water supply side of the study, the
effects of climate change were a significant factor in the calculations. In the words of
the report author, "It is widely known that the Colorado River, based on the inflows observed
over the last century, is over allocated and supply and demand imbalances are likely to
occur in the future. Up to this point, this imbalance has been managed and demands have
largely been met as a result of the considerable about of reservoir storage capacity in the
system. The fact that the upper basin states are still developing into their apportionments,
meaning they haven't quite used their allocated water yet, and efforts the basin states have
made to reduce their demand for Colorado River water. Concerns regarding the reliability
of the Colorado River system to meet future demands are even more apparent today. The
basin states include some of the fastest growing urban and industrial areas in the United States.
At the same time, the effects of climate change and variability on the basin water supply
has been the focus of many scientific studies which project a decline on the future yield
of the Colorado River. Increasing demand coupled with decreasing supplies will certainly exacerbate
imbalances throughout the basin." The results of the study are summarized in
this graph which covers the historical period 1919-2008 and the future period 2013-2060.
Supply is defined in terms of the reconstructed natural flow of the Colorado River at Lee's
Ferry in Arizona, which is commonly used as the dividing point between the upper and lower
basin. Analysis of the stream flow records have shown that 92% of the river's flow at
Imperial Dam, the most downstream gauging station on the river in the US, is derived
from tributaries upstream of Lee's Ferry. Natural flow refers to the flow that would
have occurred in the absence of upstream diversions and reservoir operations. The blue line in
the historical part of the graph represents the ten year running average of natural flow
at Lee's Ferry. You can see two sustained periods of relatively high flow in the early
part of the 20th century and in the 1980s. You can also see three periods of sustained
low flow in the 1930s, 1950s, and at the end of the 20th century. The central issue addressed
by the report is illustrated by the convergence of the ten year running average of demand,
represented by the red line, with the ten year running average of supply, represented
by the blue line. Looking to the future, the report takes three
basic approaches to estimating the future supply. Two of these three approaches are
based on the assumption that the future will imitate the past. The first approach, illustrated
in this slide, assumes that the average and the variability of future stream flows will
be similar to flows during the observed period of record, which covers the 107 year period
from 1906 to the present. The second approach assumes that the average
and the variability of future stream flows will be similar to flows during a longer period
as determined from reconstructions based on the width of tree rings. In the Western US,
tree rings have been found to be closely correlated with annual stream flows, with wet years producing
wider tree rings. This reconstruction covers the 1250 year period from the year 762 to
the present, which includes some long periods of both above and below average flows.
The third approach departs from the assumption that the future will imitate the past. This
approach relies on global circulation models to simulate future climate. These models incorporate
the effects of increasing concentrations of greenhouse gasses in the atmosphere and the
changes in climate that are likely to result. The models include various assumptions about
the rates at which greenhouse gasses will be added to or removed from the atmosphere.
These assumptions are known as emissions scenarios. The global models can be regionally downscaled
to provide greater detail about a particular region such as the Colorado River Basin and
they can be linked to hydrologic models to provide projections about future stream flows.
There is very strong agreement among the sixteen models that the southwest will continue to
warm during the next six decades. The warming is projected to occur during all four seasons
with the largest increases during the summer. The crosshatching in the maps represents trends
that are statistically significant. There is also good agreement among the models
that the southwest will experience decreasing precipitation during this period. The amount
of the decrease depends on the emissions scenario with higher emissions scenarios, or more greenhouse
gasses producing greater decreases in precipitation. The result is that the projected future stream
flow, and hence, future water supply based on the climate models as represented in the
right-hand part of this table, is lower than the projected future stream flow based on
the approaches that do not incorporate the effects of climate change in the other three
columns. The difference is on the order of a million acre feet per year.
The projections based on the climate models also exhibit a greater tendency toward extended
droughts and less likelihood of extended water surpluses. The climate models, as indicated
on the right-hand column of this table, project a 48% chance of five years or more in a row
of water deficits compared with 22-30% chances using the other approaches. The climate models
project less than 1% chance of five years or more in a row of water surpluses compared
with 28% chances using the other approaches. Returning to the summary graph, we see that
the blue trace for the future projection of supply is fuzzy, encompassing most of the
range of the area of record. That is because it incorporates all three approaches to estimating
future supply. If we put more emphasis on the approach that incorporates the effects
of climate change, the graph would look more like this and the message about the future
would be that by the year 2060, demand is likely to exceed supply by an average of about
3.2 million acre feet per year. This gap between demand and supply is about the same volume
as the water stored in Flaming Gorge Reservoir in Utah, the largest reservoir on the Green
River, one of the main tributaries of the Colorado.
What is being done about this problem? The study evaluated a large number of possible
actions that could be taken to reduce demand, augment supply, improve our knowledge of the
system, or mitigate some aspects of the problem. By considering the advantages and disadvantages
of each potential action, they narrowed the list down to ten actions listed here that
show promise for helping to resolve the problem. The Bureau of Reclamation and their partners
are taking steps to get some of these actions under way.
In summary, hydrologists and water managers are starting to adjust to the concept that
the occurrence and behavior of our water resources in the future are likely to be different from
the past. Changing climate is likely to bring warmer temperatures, more evaporation, earlier
snowmelt, a shift from snow to rain, less infiltration, less soil moisture, less groundwater
recharge, less stream flow, and more variability in precipitation in stream flow. More people
in the world will be influenced by water scarcity. More innovations will be needed to find ways
to conserve water, enhance supplies, and more efficiently manage the resources we have.
Thank you.
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Effects of Climate Change on Water Resources

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Joseph Hsieh 2020 年 2 月 8 日 に公開
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