Lake Mead in Crisis! Term Project Update
Brandon Klenzendorf
CE 394K.2 – Surface Water Hydrology March 20, 2008
I have completed a fairly extensive literature review, of which several significant journal articles are listed in the Works Cited section below. For those interested, please see the following link for a summary of my notes and important points in each paper. This is a very informal document and should not be taken as anything more than my personal notes for my use only.
https://webspace.utexas.edu/jbklenz/ce394k/TermPaperNotes.htm
As a result of this literature review, my topic has shifted slightly more towards the possible consequences of global warming on the hydrological cycle, and how this may impact the Colorado River basin specifically. I should make a note that this project will not discuss the causes of global warming, but rather take the warming scenario as an assumption and look at the resulting change to water transport due to an increase in temperature. The current conditions in Lake Mead, as well as the possible future problems will be discussed, in addition to possible causes to the lack of water availability and ways to mediate this problem.
Literature Review Summary and Lake Mead Statistics
A complete summary of my literature review is provided in the link above. However, I will briefly discuss some of the more important findings here. First off, the characteristics of the Colorado River basin have been determined. This includes the total area drained, average annual streamflow values, allocated water demands from the system, the multiple reservoirs in the system, etc. This literature review shows that the majority of the water allocated in the Colorado River basin goes to the Lower Colorado River basin and Mexico. However, the majority of the water generated through precipitation and runoff occurs in the Upper Colorado River basin. The dividing line between the Upper and Lower basins is at Lees Ferry, AZ, as shown in Figure 1.
Figure 1 – Colorado River Basin (source: Barnett and Pierce, 2008)
Lake Mead is in the Lower Basin at the border of Arizona and Nevada, and is formed by the Hoover Dam. I have gathered information about the elevation of Lake Mead as it has changed through the years. This can be seen in Figure 2. Also included are the important elevations at which hydropower can no longer be produced, and the elevation at which water can no longer be extracted by gravity for consumptive use. The dead pool elevation of Lake Mead is 895 ft.
Figure 2 – Lake Mead Elevation (data: U.S. Bureau of Reclamation)
Figure 2 shows that the current level of Lake Mead is the third lowest in its history. However, the minimum flow near 1965 is due in part by the construction and subsequent filling of Lake Powell upstream. Interestingly, the yearly variability in the elevation of Lake Mead has reduced once Lake Powell came into service. The low reservoir levels currently seen is a major concern due to the increased threat of global warming. As described in the literature review, global climate models predict that runoff will decrease due to an increase in temperature. This is due to increased evapotranspiration and snow falling as rain due to the higher temperature. Also, since snow is falling as rain, there is less snow pack in the Colorado River headwaters to provide summer streamflow. Therefore, the timing of the streamflow is expected to occur earlier in the spring. One of the questions I would like to look at is the amount of evaporation that occurs from liquid water as opposed to snow pack. If the same amount of evaporation occurs with liquid water as it does snow, then the decrease in snow should not have a major impact on the influences of global warming.
The main driving force for changes in the hydrologic cycle due to global warming is described in Held and Soden (2006). The major influence is the Clausius-Clapeyron (CC) expression for saturated vapor pressure, es. This is given as:
where T is the temperature, L is the latent heat of vaporization, R is the gas constant, and α is the lapse rate as a function of temperature. α is approximately 0.07 K-1 for temperatures typical in the lower troposphere. It is expected that a doubling of CO2 in the atmosphere will increase the global temperature by about 3 K. This would result in an increase in es of about 20% (Held and Soden, 2006). The result of this is that the atmosphere is able to hold more water vapor as a result of global warming. This change will cause a very complex reaction to many other hydrological processes, such as change in precipitation, mass exchange between the surface boundary layer and atmosphere, horizontal moisture transport, energy transport, etc. The end result that is most applicable to water availability is that currently wet areas will get wetter, and currently dry areas will become drier.
The Colorado River basin is already a semi-arid to arid region with an average annual rainfall of approximately 355 mm. Of this amount of precipitation, roughly 310 mm evaporates, leaving only 45 mm available for runoff and consumptive use (Christensen et al., 2004). Therefore, as a dry area, global warming is expected to possibly decrease the future precipitation, as well as increase the evaporation. Modeling results have found a reduction in runoff of between 10-30% for various warming scenarios. Therefore, the likelihood of keeping the reservoir systems in the Colorado River basin full is decreasing. Barnett and Pierce (2008) looked at the various probabilities of different warming scenarios occurring and how they would impact the probability of the reservoir system running dry. They found, in general, that there is a 50% chance Lakes Mead and Powell will run dry by 2021 (Lakes Mead and Powell form 85% of all the reservoir storage volume in the Colorado River basin).
One last note is to look at the long term streamflows in the Colorado River. Tree ring reconstructions have been conducted by Woodhouse et al. (2006) to estimate the annual streamflow back to the year 1520. These historical flows suggest a lower annual streamflow value than what was typically seen in the 20th century. This is a major concern because water allocations were determined in the Colorado River Compact of 1922 which was during a time of historically high flows. Therefore, the river was effectively over allocated from the start, which further stresses the reservoir system since more water is being used than is actually available, regardless of the impacts of global warming. This seems to be the major problem with the Colorado River basin. Barnett and Pierce show that even without any warming effects, the reservoir system will eventually run dry at the average streamflow values monitored within the past 100 years. Figure 3 shows the 10-year average streamflow values for the tree ring reconstructed streamflows, as well as the monitored streamflow gage data beginning in 1906. The overall average of the entire reconstructed flows is less than the average flows within the past 100 years, and there have been much more severe droughts according to the reconstructed flows.
Figure 3 – Colorado River Streamflows with Tree Ring Reconstructed Flows (data: Woodhouse et al., 2006)
Future Work
The first task I need to complete is to better understand the response of the hydrological cycle to global warming as described in Held and Soden. I also need to organize the many model simulations that have been conducted in order to get an overall picture of what can happen in the Colorado River. I have not done any work on how evaporation changes when compared with liquid water and snow. I think this portion of the project will be the most quantitative. If I can find a good description of the differences in evaporation of snow, I would like to look at how the decreased area of snow pack in the headwaters of the Colorado River could possibly increase evaporation and impact the streamflow of the river.
Works Cited
Barnett, T.P. and D.W. Pierce (2008): “When will Lake Mead go Dry?â€, Water Resources Research, in press.
Christensen, N.S., A.W. Wood, N. Voisin, D.P. Lettenmaier and R.N. Palmer (2004): “The Effects of Climate Change on the Hydrology and Water Resources of the Colorado River Basinâ€, Climate Change, Vol. 62, p. 337-363.
Held, I.M. and B.J. Soden (2006): “Robust Responses of the Hydrological Cycle to Global Warmingâ€, Journal of Climate, Vol. 19, p. 5686-5699.
McCabe, G.J. and D.M. Wolock (2007): “Warming may Create Substantial Water Supply Shortages in the Colorado River Basinâ€, Geophysical Research Letters, Vol. 34, L22708.
Nash, L.L. and P.H. Gleick (1991): “Sensitivity of Streamflow in the Colorado Basin to Climatic Changesâ€, Journal of Hydrology, Vol. 125, p. 221-241.
Woodhouse, C.A., S.T. Gray and D.M. Meko (2006): “Updated Streamflow Reconstructions for the Upper Colorado River Basinâ€, Water Resources Research, Vol. 42, W05415.
