Home > Uncategorized > Lake Mead: A Falling Tide

Lake Mead: A Falling Tide

2010 October 4

Introduction

I’ve been watching Lake Mead decline during the 2+ years since Barnett and Pierce published their paper entitled When will Lake Mead go dry?

A water budget analysis shows that under current conditions there is a 10% chance live storage in Lakes Mead and Powell will be gone by about 2013 and a 50% chance it will be gone by 2021 if no changes in water allocation from the Colorado River system are made. This startling result is driven by climate change associated with global warming, the effects of natural climate variability, and the current operating status of the reservoir system. Minimum power pool levels in both Lakes Mead and Powell will be reached under current conditions by 2017 with probability 50%. While these dates are subject to some uncertainty, they all point to a major and immediate water supply problem on the Colorado system. The solutions to this water shortage problem must be ‘time dependent’ to match the time varying, human induced decreases in future river flow

These are pretty alarming claims (and there are some issues with them which I will get into in a later post). But before minimum pool levels or live storage are gone, though, Lake Mead will have to slip through a politically important level: a surface elevation of 1075′. The reservoir may break that first mark as early as next year – although it may take several more years before the US Bureau of Reclamation acknowledges it.

The 1075′ elevation should act as an alarm bell. A drop of just an additional 25′ will cause power generation at Lake Mead to halt.

Interim Guidelines for the Operation of Lake Powell and Lake Mead

Prodded into action by recent drought conditions in the Colorado River Basin, the Federal government facilitated an arrangement between the Colorado River stakeholders which resulted in a management agreeement signed at the end of 2007. This agreement specifies a set of water management rules based on the water levels of Lake Powell and Lake Mead. Lake Powell is located upriver from Lake Mead. There are a couple of dozen smaller reservoirs upriver from Powell. The agreed upon guidelines specify reductions in water for Arizona and Nevada when the Lake Meads’ elevation is projected by the “August 24-Month Study” to slip below 1075 on January 1 of the following year.

Section 2 Determination of Lake Mead Operation During the Interim Period

D. Shortage Conditions

1. Deliveries to the Lower Division States during Shortage Condition Years shall be implemented in the following manner:

a. In years when Lake Mead content is projected to be at or below elevation 1,075 feet and at or above 1,050 feet on January 1, a quantity of 7.167 maf shall be apportioned for consumptive use in the Lower Division States of which 2.48 maf shall be apportioned for use in Arizona and 287,000 af shall be apportioned for use in Nevada in accordance with the Arizona-Nevada Shortage Sharing Agreement dated February 9, 2007, and 4.4 maf shall be apportioned for use in California.

b. In years when Lake Mead content is projected to be below elevation 1,050 feet and at or above 1,025 feet on January 1, a quantity of 7.083 maf shall be apportioned for consumptive use in the Lower Division States of which 2.4 maf shall be apportioned for use in Arizona and 283,000 af shall be apportioned for use in Nevada in accordance with the Arizona-Nevada Shortage Sharing Agreement dated February 9, 2007, and 4.4 maf shall be apportioned for use in California.

c. In years when Lake Mead content is projected to be below elevation 1,025 feet on January 1, a quantity of 7.0 maf shall be apportioned for consumptive use in the Lower Division States of which 2.32 maf shall be apportioned for use in Arizona and 280,000 af shall be apportioned for use in Nevada in accordance with the Arizona-Nevada Shortage Sharing Agreement dated February 9, 2007, and 4.4 maf shall be apportioned for use in California.

August 24-Month Study

Can’t say I know much about how this study is prepared. But the you can find them here:
http://www.usbr.gov/lc/region/g4000/24mo

These studies show 12 months of historical data and projected water surface elevation levels 24 months into the future. Lets take a quick look at the September 2010 projections for the last two years.

From the October 2008 study: a projected September 2010 Lake Mead elevation of 1105′
From the October 2009 study: a projected September 2010 Lake Mead elevation of 1105′
From current USBR elev data: an actual September 2010 Lake Mead elevation of 1083.81′

The August 24-month does much better at projecting the next year January. Of course, this is only a 5 month projection.
August 2007 projection for January 2008:
Projection: 1116.78
Actual 2008: 1116.46

August 2008 projection for January 2009:
Projection: 1112.41
Actual 2009: 1111.78

August 2009 projection for January 2010:
Projection: 1102.24
Actual 2010: 1100.02

Summer Lows

September to January

Water levels tend to be at their lowest in the late summer months (July, August, and September) after which they begin rising again. By January, water levels tend to rise, although the rise is not guaranteed. Over the last 45 years, water levels have fallen 10 years between September and January of the next year. The average rise over these 45 years has been 3.5′ with a max rise of 14.2′ and a max fall of 11.5′. Looking at more recent data, the reservoir has risen 7 times over the last 10 years between September and January with an average gain of 3.3′. In just the last 4 years, it has risen an average of 4.6′.

So let’s look at the 24-month studies:

August 2008
Projected Sep 2008: 1105.00
Projected Jan 2009: 1112.41

Projected Sep 2009: 1105.00
Projected Jan 2010: 1112.19

August 2009
Projected Sep 2009: 1094.05
Projected Jan 2010: 1102.24

Projected Sep 2010: 1105.00
Projected Jan 2011: 1114.31

August 2010
Projected Sep 2010: 1084.07
Projected Jan 2011: 1090.24

These Sep-Jan projections show a rise more than twice as high as historical averages. One wonders which factors the USBR depends on to refill Lake Mead in their projections: climatological or anthropogenic? It might be that the USBR expects management practises or “Intentionally Created Surplus” (ICS) to provide the extra bit.

September to September

What does the year-over-year September change look like?
Over the last 45 years, the mean drop has been a bit less than 1′.
Over the last 10 years, the mean drop has been 11′.
And over the last 4 years, the mean drop has been 11′.

January to January

Likewise, the year-over-year January change?
Over the last 45 years, there has been a mean rise just a hair over 0′.
Over the last 10 years, the mean drop has been 11.5′.
And over the last 4 years, the mean drop has been 7.5′.

1075

Over the last 45 years, Lake Mead has seen a year-over-year drop of more than 9′ in 11 different years (24%). In the last 10 years, a drop of more than 9′ has occurred 7 times. It has dropped more than 9′ in three of the last four years. There seems to be a very good chance that the reservoir, currently at 1083′, will break the 1075 mark next year. But recall that the mark at which the 1075 management rule is invoked is the August projection for the following January. And we have seen how the USBR has been overly optimistic in its projections. So even if 1075 is hit by September next year, its unlikely that the August 2011 24-monthy report will project that January 2012 will be below that mark. Indeed, even assuming a continuous fall in reservoir levels, it could be three years (August 2013) before the USBR is “forced” to print a projection with January below 1075′. There will be great political pressure to not create such a report.

A Climate Factor

All of the above is really just providing background for a recent UCLA study by Tingstad and MacDonald which suggests that the Colorado River will be even drier next year than it has been lately based on the condition of La Nina, the PDO, and the AMO.

… Tingstad and MacDonald found a “striking and significant propensity” for droughts in northeastern Utah when cool sea-surface temperatures in the Pacific associated with La Niña and the negative phase of the PDO were coupled with warm temperatures in the North Atlantic linked to the positive phase of the AMO. During such episodes, snowpack declined on average between 9 percent and 10 percent, and river discharge decreased on average by 18 percent.

The three conditions last converged at least five times between 1945 and 1965, a period that was characterized by generally depressed but variable flows in the river, they said.

The findings are troublesome because not only are all three conditions predicted for 2010–11, but they are expected to be particularly strong, the researchers say. The coming year’s La Niña and AMO are at this point supposed to be the strongest in 10 years, and a strong negative PDO is also building.

The convergence increases the likelihood that Lake Mead, already diminished by 11 years of drought, will fall below 1,075 feet above sea level — a threshold that can result in the reduction of water allocations in Nevada and Arizona, the researchers say. Under a series of agreements among seven U.S. states along the Colorado River and Mexico, California has first rights to the water, so it would not face the same restrictions. Water levels at Lake Mead currently stand just nine feet from the critical threshold, at 1,084 feet above sea level.

“We’re looking at a situation that could pit us against our neighbors,” said MacDonald, who is also a UCLA professor of geography. “We’ve never had to face such a severe decline in Lake Mead and resulting reductions in Colorado River allocations before.”

Under the agreements, the federal authorities have the power, once Lake Mead falls to 1,075 feet, to request reductions of 11.4 percent on Arizona and 4.3 percent on Nevada — cuts of 320,000 and 13,000 acre feet, respectively.

“That means these states might have to find many millions of gallons to make up what seem like relatively small-percentage reductions, and they have to do so at a time when they are already in a drought of considerable duration,” MacDonald said. “That’s going to be a real problem.”

http://newsroom.ucla.edu/portal/ucla/when-three-is-not-a-charm-ocean-172599.aspx

I’ve concentrated on Lake Mead in this post which is like trying to measure someone’s gait by looking at only one foot. Lake Mead is managed in conjunction with Lake Powell and Powell has been refilling in recent years while Mead falls. Back in 2004/2005, managers began making plans to drill beneath the current water intakes for Powell to keep the generators running. An article from 2004. The Interim Guidlines are supposed to help balance the water levels between the two reservoirs while maintaining the water rights of stakeholders. We may soon see it put to the test.

Hmmm… I suppose I should post a picture.

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