What are are the implications of long-term drought in New Mexico?

These are great questions to kick off the discussion, but like the best questions, there are no simple answers.

One of the bottom-line messages from the development of the State Water Plan is that we don't have a clear overall picture: "No one State or federal agency currently serves as a clearinghouse for the compilation of all available information needed to inventory the quantity and quality of the State's water resources, population projections, and other water resource demands under a range of conditions."

"Range of conditions" is the key bit there, because one of the key messages from drought science is that societies that don't plan for the dry parts of the drought cycle have great difficulty.

For details on the Middle Rio Grande, where the bulk of the state's population lives, I'd refer you to the Middle Rio Grande Water Plan (the bits that are most responsive to your question are in Appendix B). The the bottom line is that we're "mining" 70,000 acre-feet per year of water from the aquifer.

There's uncertainty about the water used by riparian evapotranspiration (cottonwooods, salt cedar, etc.), but it's a big number, similar in volume to urban use.

But despite the uncertainties in the supply and use data, your final question - What is the limit for a sustainable future? - is the most interesting, but one that science can't answer, because it involves societal values. How impportant is agriculture to us? How important is it to enable urban growth? How important is the river ecosystem? My colleague Tania Soussan is our expert on these issues, and I think captured the debate nicely in a piece she wrote in January:

Agriculture uses most of the water in the West -- 75 percent in New Mexico. State Engineer John D'Antonio estimates that New Mexico could support a doubling of its 1.9 million population with just 10 percent of the water used on farms in the state in a full supply year. "We're going to continue to have lots of movement of water from agriculture to municipal entities," said Denise Fort, a University of New Mexico water law professor. "Water markets are not necessarily a bad thing as long as they respect the public interest."

In general, one of the predictions of greenhouse warming theory is increased variability - wetter wet spells and drier dry spells. Globally, there's strong evidence that the total area in drought in any given year expanded in the last half of the 20th century, probably because of the warmer global temperatures.

The details of how that might play out here, or whether we're already seeing it happen, are a bit murky.

ENSO - the El Nino Southern Oscillation, an ocean-wind pattern out in the Pacific - is a big factor in determining factor in drought patterns here. When we have more or stronger El Nino's, it tends to be wetter here, which is what happened from the mid-70s to late '90s. When the opposite phase kicks in - La Nina - we tend to be dry.

There's a great deal of disagreement on how global warming might effect ENSO. Climate scientists Rasmus Benestad and Raymond Pierrehumbert had a great post on this last week on the RealClimate blog. Their bottom line: "the question about how ENSO will respond to a global warming is still not settled."

One thing we do know is that warming temperatures are affecting our snowpack. I've written several stories about research by Phil Mote, from the University of Washington, on declining snowpack around west. Warmer temperatures mean more precipitation falls as rain rather than snow, and could mean more evaporation before the water gets into the rivers.

El Nino winters tend to be wetter, and La Nina winters tend to be drier.

This is an intriguing suggestion, and some folks have seriously looked at it. More than 100,000 acre feet per year evaporate from Elephant Butte. Jeff Jones did a story back in 2003 on a proposal by Bill Turner to do just what you suggested, but the road blocks at the time were substantial. Folks living around the reservoirs objected, and the state engineer's office balked at giving Turner the rights to the water saved, which was the economic driver for the proposal.

There's a simpler suggestion that's also frequently discussed, which is storing more water in upstream reservoirs rather than Elephant Butte during drought. Currently law prevents this, but there are folks who argue that there would be less evaporation upstream.

Yup, that'll save water. And cactus. We've got a lot of cactus in our yard.

But there's actually a bit more to this that I think is interesting. Here in Albuquerque, our water supply is pumped up from a relatively deep aquifer, and we're just mining it. So every flush depletes the aquifer. But the water ends up going through the sewage treatment plant and out into the Rio Grande once it's cleaned up. Albuquerque's sewage treatment plant is actually one of the Rio's big "tributaries". There are people who joke that extra flushes are a good thing for the river ecosystem: "Flush twice for the minnow.!"

This is a good question, and one to which I don't know the answer. But it's a good excuse to explain what I know and don't know. As the science writer, I know a lot about climate science. My colleague Tania Soussan is our water rights/policy expert. I know some about these issues, but not this one.

Scientists who study this get a little testy when we lay people use the word "cycle," because it implies that drought recurs in a predictable way. It doesn't.

But if you look at the long term drought record, you can see that we've had sustained drought - like a decade or more in duration - once or twice every century, maybe once every 70 years or so on average.

The last really serious drought in New Mexico began in the 1940s, and lingered into the 1960s. (They call it "the drought of the '50s", but actually last longer.) It's reasonable to think that the current drought, starting as it did a bit more than 50 years after the last one, could be another decadal-scale sustained drought.

(For a long look at our drought history, see Henri Grissino-Mayer's web page.)

Lots of different places, depending on where you're from.

Most of the big cities, which get most of their water from groundwater, just keep pumping. This isn't sustainable in the long run, because we're drawing down the aquifers faster than they are being replenished. Some of the agriculture in the state uses groundwater, too - those big center-pivot irrigation systems you see out east.

Cities like Santa Fe and Las Vegas get a lot of their water from mountain snowpack, and don't have multi-year storage, so they have to reduce their use in droughts. And most of the agriculture in the middle Rio Grande Valley depends on mountain snowpack filling reservoirs and ditches. If there's not enough stored up from previous years, they just have to use less. That's what happens in a sustained drought.

It actually comes out pretty clean, though new regulations on arsenic levels are going to require some expensive filtering in the future.

I don't know the answer (my gig being drought science, not water policy). In general, each municipality or water system makes its own decisions based on its own supply and demand situation.

The short answer is "no."

The longer answer depends on two separate problems.

The first is the uncertainty associated with supply. Climate scientists cannot yet predict when droughts will happen, or how many years they will last. What they can say with certainty is that droughts that last a decade or more have happened repeatedly in the past, and there's no reason to think they won't happen in the future.

The second part of the problem is our steadily increasing water demand as population grows. That's also hard to predict, but the trend there is clear. There are more and more us, and we're using more water.

The message from archaeologists is that, in the past, societies that didn't pay attention to the first part - climate's ups and downs - had serious problems if they grew substantially during the wet parts of our inevitable climate cycles and didn't pay attention to the fact that, sooner or later, it's going to get dry again.

There have been a couple of questions on this, so it must be on folks' mind. I don't know a lot about it, but I know that there has been serious consideration in the past (if I remember correctly, Marc Reisner talks about this in his excellent book "Cadillac Desert"). My recollection is that the costs and engineering challenges were too large.

The water we flush, run down the drain, etc., ends up in the Rio Grande, via the sewage treatment plant.

Pretty much all the water we put on our gardens ends up evaporating. There's very little recharge.

Not sure about policies on flood irrigation. As for the second part, a team of economists from NMSU and elsewhere did a study last year on trading schemes, concluding that giving folks the right to sell their water during drought years would make for more efficient water distribution. (But then, they're economists, who think the market can solve anything. :-)

In 2004, PNM used about 400 gallons per megawatt hour. They're trying to reduce it. They're trying to reduce it by 20 percent by 2008. (Thanks to Tania Soussan for hunting down that answer.) The second part's a policy question, but it's worth discussing. Tania says there's a lot of discussion currently about this issue.

Yes, there's a pretty clear relationship between the places that got warmer and the places that saw increased drought. The study was done by Aiguo Dai up at NCAR. You can see more about it here.

That's a political and policy question, not a science one. This has happened elsewhere in the West, but I'm not aware of any governmental jurisdiction in New Mexico that's done it.

This gets to one of the important underlying issues with drought, which is its definition. For my purposes, the best one is, "less water than you need/expect".

In those terms, everyone everywhere is at risk of drought, but the people at greatest risk are the people living closest to the margin of available water supply - which means people in arid climate, like us.

The people at the greatest risk, though, are poor folk in poor countries. Here in New Mexico, drought causes economic stress. It's worth remembering that there are places, like the Sahel along the souther edge of the Sahara, where people die.

I'm a little hazy on the details, but in general water that people put on their gardens down in the valley returns to the aquifer pretty quickly. Up in the heights, in general there's no recharge at all. It doesn't soak down far enough to recharge, except up in the foothills, according to Tania.

As for evaporation contributing to future rain, I'm not the scientist, so I'm not positive. There have been a couple of interesting studies suggesting that there is more rain downwind of big urban areas in the summer (the most famous study was done in St. Louis), but I think it was urban heating rather than evaporation that caused it.

Not sure which water you mean here.

Albuquerque's groundwater isn't filtered. A big new filtration plant is being built for San Juan Chama river water, which we'll start using soon.

We frankly don't do a great job of that. We have some reservoir storage that allows a wet year or a few wet years to be saved up as a buffer.

But the real problem is not saving water during wet years, I think, but expanding our usage during wet periods (such as the mid-70s to mid-90s period). That leaves us vulnerable to the down side of the drought cycle.

Federal taxpayers built the Central Arizona Project for them, a big cement canal so Arizona could use up its share of Colorado River water as allocated by the Colorado River Compact and subsequent agreements back in the 1920s.

Worth noting, back to my ad nauseum theme of the importance of variability, is that the early 20th century, when the Colorado compact was negotiated, was unusually wet. That means that in the long run there is less water in the river than the compact calls for, meaning Phoenix as it now operates is probably unsustainable in the long run.

Thanks, and thanks April and others for some great questions. Apologies to those I didn't get to, but you've given me some good ideas for things I need to learn more about.