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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 placesprimarily the

  • higher latitudeswill be getting wetter and othersprimarily the lower latitudeswill

  • 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.

Hello, I'm Glenn Patterson, PhD candidate in watershed science at CSU and developer

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B1 中級 美國腔

氣候變化對水資源的影響 (Effects of Climate Change on Water Resources)

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    Joseph Hsieh 發佈於 2021 年 01 月 14 日
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