Name: 氣候變化對水資源的影響 (Effects of Climate Change on Water Resources)
Uploaded: 2021-01-14T08:40:09.000Z
Duration: 14 min 41 s
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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