In defense of “vapor pressure deficit”

Posted by on

If you follow weather forecasts, you’ve heard about “relative humidity” (RH). But it’s one of those maddeningly less-than-useful measures of our weather that probably needs to be just retired. That’s wishful thinking, of course. But in an interesting introduction to their latest research into the increasing dryness of the air and the risk of fire that attends thereto, Richard Seager and his colleagues make another plea. Paraphrasing a 1936 paper by D.B. Anderson, they write:

Anderson (1936) points out that RH is not an absolute measure but merely a ratio of two known quantities expressed as a percentage.

If you can do the math quickly in your head, you can keep an intuitive grasp of the meaning of RH in a given situation. But using a measure that requires your audience to do math in their head to make sense of what you’re telling them is a bad communication strategy. Riffing off of Anderson, Seager and colleagues argue for the importance of a different measure that requires no such math – “vapor pressure deficit”. I’ll skip their equations for this:

VPD gives an absolute measure of the atmospheric moisture state independent of temperature. For example, for a given wind speed and atmospheric stability, above a surface that is not water-limited, a specific VPD leads to the same rate of evaporation, regardless of temperature.

Why should we care? Because vapor pressure deficits are rising in the southwestern United States, and are closely linked to wildfire risk. The public communication element of “VPD” vs. “RH” is really just a sidelight to an important new paper about rising fire risk as the southwest warms. (In particular they look in detail at VPD and the Rodeo-Chediski and Hayman fires.) But I found it intriguing. I’d love to have it added to my daily forecast page.

 

Creeping toward shortage: Lake Mead now headed for bigger drop next year than we thought

Posted by on

Folks worried that Lake Mead might drop below elevation 1,075 and trigger a first-ever Lower Colorado River Basin shortage now have more to worry about. The latest monthly model runs from the U.S. Bureau of Reclamation (pdf) have increased the odds, and suggest that Mead (currently at 1,077.03) could drop all the way to 1,054 by the end of the 2016 “water year” – 18 feet lower than projected just one month ago.

This is all a result of the interaction of two important water management rules – one that calls for holding more water upstream in Lake Powell to keep that reservoir from dropping too far, and a second that calls for curtailing water deliveries to Arizona, holding the water back in Lake Mead, outside Las Vegas.

The latest monthly Bureau model runs, released this afternoon, point toward a likely need to cut releases from Lake Powell, on the Colorado River along the Arizona-Utah border, beginning Oct. 1. That would result in less water being delivered to Lake Mead, downstream, increasing the chances of a first-ever shortage declaration and cascading shortfalls in Arizona as early as Jan. 1. So far odds are against that second eventuality – Arizona cuts beginning Jan. 1. But it’s very close right now, with projected Jan. 1 levels of 1,075.92, just 11 inches from the trigger point.

Regardless of whether we hit the 1,075 trigger this time around, however, reduced releases from Lake Powell would ensure huge drops in Lake Mead next year.

This is all part of the complicated operating rules adopted in 2007 intended to balance shortfalls in Powell, the main upper basin storage reservoir, and Mead, the main reservoir downstream for use by Nevada, Arizona, and California. Powell has been hovering right around a trigger point (elevation 3,575 feet above sea level on Jan. 1) that would require cutbacks to keep more water in Powell. There’s a lot of “if this then that” rules stumbling over one another here, but if the August monthly modeling run shows Powell is too low, that automatically invokes a rule to hold more water in Powell and release less water to Mead. And less water in Mead translates to increased risk of triggering a second rule that would keep more water in Mead next year by cutting deliveries to Arizona.

Background:

The federal role in western drought

Posted by on

The federal government, through its water agencies (and the funding providing via taxpayers in other places) used to be a major player in the development of the West. This Michael Doyle story, in describing a Congress up to its axles in California drought and unable to move an inch, suggests that is no longer the case:

Five months into a new Congress, and deep into a lasting drought, California water legislation still stymies and splits the state’s lawmakers.

Clearly Californians have not passed the first important hurdle for federal action on state-level water issues – unanimity within the state about what should be done. Traditionally a state gets its act together and presents a united position to Congress as a necessary precondition to federal action. But one of the reasons such unanimity is harder today than it used to be is a reduced federal ability to throw vast sums of money at internal state water conflicts. Used to be that part of a deal was federal dollars to build a big canal or dam or something that all the state players could get behind. We’re done with that.

Second is a deeper values division about the best way to deal with our water problems. California has always had a north versus south problem, but now…. Water for the environment? Ag versus urban? This may be greatest hurdle.

The remarkable U.S. water conservation success story

Posted by on

The U.S. economy and population are growing. Our water use is not. New research by Peter Debaere and Amanda Kurzendoerfer of the University of Virginia helps further disentangle the reasons behind this remarkable U.S. water conservation success since the 1980s.

The break between population, economic growth, and water use is something Peter Gleick has pointed out before: “The assumption that demand for water must inevitably grow is false.”

Debaere and Kurzendoerfer have taken our understanding  a step further by disentangling the roles of sectoral changes in our economy (has our core economy changed in a way that simply offshores our water use while we shift to doing other tasks that require less water?) versus underlying efficiency gains in the economy left behind:

Debaere and Kurzendoerfer, 2015

Debaere and Kurzendoerfer, 2015

The top line is their model of water use simply tracked GDP growth. The middle line is a “what if” it was simply “sectoral changes” in our economy – essentially offshoring our water use. The bottom line is what actually happened:

Technological improvements are responsible for the remainder of the water productivity gains–at least 50%. Since actual water is influenced by technological changes as well as structural shifts in the economy, the difference between the lowest line in Figure 1 (actual water use) and the second-highest one marks the contributions of technological improvements. The sizeable impact of technological improvement is a welcome finding if one considers technology an opportunity for replication abroad. Indeed, transferring technology can be a more actionable way of bringing about less water use, especially compared to the slow-moving process of structural shifts toward a less water-intensive service economy.

The virtues of alfalfa in drought

Posted by on

Alfalfa, which recently handed over its”Demon Crop” title to almonds, is really a far better crop in drought than common wisdom suggests, according to U.C. Davis’s Dan Putnam:

Contrary to popular belief, alfalfa has several unique positive biological properties and advantages when it comes to water. Due to these properties, alfalfa is remarkably resilient when it comes to severe drought conditions.

Among other things, Putnam argues that “deficit irrigation” – the ability to cut way back on water during the heat of a summer drought – provides important water management flexibility. The full piece is worth reading.

Water Ranch, Gilbert, Ariz.

Posted by on
Black-necked stilt, Gilbert Water Ranch

Black-necked stilt, Gilbert Water Ranch

In Phoenix for a meeting, I had a couple of hours’ hole in my schedule and skipped out to the “Gilbert Water Ranch”. It’s an interesting example of what the water management solution space looks like in arid central Arizona.

It’s a series of groundwater recharge basins in the Phoenix suburb – both treated municipal sewage and imported Colorado River water. It’s also a lovely municipal park (lots of walkers on a weekday morning). And the birds seem to enjoy it as well.

Mapping the Colorado River Delta – a recognition that Mexico is a part of the basin

Posted by on

It’s a nuance, but of such subtleties are progress often made.

In the Bureau of Reclamation’s new “Moving Forward” report on the future of the Colorado River Basin, a subtle change to the bureau’s canonical map:

Colorado River Basin map, courtesy USBR

Colorado River Basin map, courtesy USBR

There, at the bottom. See that little bit poking down into Mexico? That’s the Colorado River Delta. The Bureau’s official map used to cut off its depiction of the Colorado River Basin at the U.S.-Mexico border. Here, for example, is the map that appeared in the predecessor to the “Moving Forward” report, the 2012 Basin Study:

2012 Basin Map

2012 Basin Map

On the heels of the enormously successful Minute 319 environmental pulse flow down through that Mexican Delta, a quiet recognition that the Colorado River Basin is a thing changed.

On the Gila River in New Mexico, a “field of dreams” argument

Posted by on

From Brett Walton’s piece on a proposal to divert water from the Gila River in New Mexico (in the Lower Colorado River Basin), an old school argument for building western water infrastruccture:

Gutierrez does not believe that the unit cost for water — what a city or a farmer will pay for an acre-foot — is required information. He believes that contracts for the water before construction begins are unnecessary as well. “There’s already a need,” he said. “Once the water is available people will use it.”

Is this Field of Dreams assumption — that if the project is built, someone will pay for the water — sufficient for the commission to continue pushing for construction?

“That assumption is enough for me,” replied Gutierrez, who talks about investing the AWSA money and using the returns to help finance the project. “It may not be enough for the environmental community.”

The piece is interesting throughout.