Drought: the Long, Slow Natural Hazard (Part 1)

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Dave Hebert

Hello and welcome. Or welcome back, depending on your listening habits, to the USGS CoreCast. I'm Dave Hebert.

Today we are giving you the first part of a two-part series on drought. Now we don't usually do two-part episodes, but we've got a couple of great interviews with USGS scientists who studied drought both in the West and in the Southeast, and we wanted to make sure that you've got all the goods.

For instance, did you know that drought is the one of the most expensive natural hazards to address? Or that hurricanes in the Gulf and the Atlantic and drought in the West might be related? You're going to get that and a lot more over these next two episodes.

Up first, we have Julio Betancourt, a research hydrologist in Tucson, Arizona, and Greg McCabe, a physical scientist in Denver, Colorado, and they are going to walk us through the drought situation in the Western United States and a lot more.

Julio, Greg, thank you very much for joining me. First of all, I just wanted to talk about drought in general, but it seems like in the news media and may be even the public perception of what drought is, there's this temptation to say, you know, "Well we haven't had rain in a while, so we're in a drought," or "Well it rained a lot last week so the drought is over."

Could you clear up some of those . . . some misconceptions, maybe, by briefly explaining what actually constitutes a drought?

Julio Betancourt

Well this is Julio, Dave. And, I mean, there's lots and lots of definition of drought and it depends on your perspective. Certainly, you know, a farmer may have a completely different perspective than a water manager on what constitutes drought so drought is kind of in the eyes of the beholder.

And we have to be careful about the definitions. Let me put it in a different way and that is that both the instrumental and tree-ring record of the Western United States is characterized by alternating and widespread dry periods and wet periods.

And these are times, in my mind . . . these are times when the mean precipitation is either exceeded for a few years to maybe even a decade or more or when wet conditions are exceeded for one year or for more than one year or up to a decade or more.

And so you have these alternating dry and wet periods that are often times widespread, meaning, you know, several basins—large scale basins—are affected at the same time, and that's the kind of variability, precipitation variability that we're really worried about.

In other words, when it's widespread in persistence, meaning running from one year to the next, and affecting multiple basins at the same time. And so that's kind of a qualitative definition of drought, which would be periods when the precipitation is actually less than the mean persistently for a few years, and it's broad scale, meaning affecting, you know, more than one basin at a time. And so I think that that, in essence, is what we're talking about when we refer to drought in the Western U.S.

Greg McCabe

Dave, this is Greg, and I would just add, as Julio mentions, there, there isn't really a hard and fast definition of drought. And so in research, people will usually define their own definition in order to, for example, look at the temporal and spatial variability.

But one of the things you will notice is that in a lot of these long persistent droughts, there are occasionally wet periods of short duration. And as you mentioned, sometimes people think that just because it rained during a specific season, they start to think that drought is over, and that often isn't the case. So you will have these long drought periods that might have small, short wet periods inside of them.

Julio

A good example is 2005, which kind of came at the tail end of a drought that had started back in 1996 and sort of persisted to about 2004, and then we have this El Nino event in 2005, and for example, Phoenix got really, really wet. And everybody was of course dancing in the rain, thinking that the drought was over, but that's not necessarily the case. We've gone back again to relatively dry times after 2005. So there's a tendency to celebrate much too early when there's darkening clouds and a sign of rain.

Dave

Right. Could drought be rightly considered a long, slow natural hazard?

Greg

This is Greg. I believe it can be because it does occur slowly, often people don't pay attention to it until they're in a severe drought. But I think if you talk to water managers in the Western U.S., they consider it a hazard and something they worry about.

Julio

Ah, this is Julio again. And I've done a little bit of work on fire ecology in the Western U.S., and what's interesting is ecologists describe ecological disturbances in a way that doesn't . . . that traditionally has not included persistent drought. You know, it has had to do with fire, it has to do with insect outbreaks and things like that, but we now know—and we're painfully aware of it in the last few years—that you can have drought-induced disturbances like the bark beetle outbreak that killed literally millions of pinyon pines and ponderosas in 2002 and 2003. These droughts are in fact natural disturbances and hazards that have very, very wide impacts.

Dave

Right. So what parts of the Western United States are seeing dire drought conditions right now?

Greg

Things aren't as severe right now as a couple of years ago, but there is drought right now in Southern California, most of Nevada, a good part of Utah and Western Wyoming, and also in Montana, and it now even extends over into the Central Plains.

Dave

And what is . . . what is the long term outlook for those locations?

Greg

Well, it's difficult to say. But it looks like if you look at what we know of connections between drought and sea surface temperatures it looks like this drought should persist for a while. We still have sea surface temperature patterns that are consistent with drought in the Western U.S.

Julio

This is Julio, and as probably the viewers, or I mean, the listeners know, this year we're in a La Niña year. It has been kind of an unusual La Niña year, meaning cooling temperatures in the tropical Pacific, which are usually associated with drought specifically in the Southwestern United States.

And so the seasonal drought outlook for the next few months of the springtime, which is usually very highly correlated with tropical Pacific sea surface temperatures, is actually for drought to persist in Southern California and to develop and persist in the southern High Plains down into south Texas and also in places like Florida.

Dave

That sort of leads me to my next question. What are some of the spots that look like could be developing into a drought pattern next?

Julio

You know, it's really hard to tell. One of the more important questions right now in modern climatology is whether or not there's much predictability beyond just a few months. And the predictability within a few months is primarily based on what we know about the tropical Pacific and the teleconnections that arise from cooling or warming in the central and eastern Pacific.

You know, I think most climatologist would argue there's not much predictability beyond just a few months, and so it's kind of hard to say. It depends really on what the tropical Pacific is doing along with the other oceans.

Dave

Are any of the places in the West that have been seeing drought for long periods of time 

. . . are any of those places seeing recovery?

Julio

Actually the expectation is that some of the intermountain West may improve, and by intermountain West, I would mean southern Idaho, western Wyoming, northwestern Utah, and Nevada—in other words, the Great Basin. So the expectation is there may be some improvement over the next few months in the Great Basin, but some other critical areas don't see that. In fact, I think South Dakota and North Dakota are supposed to continue to see drought . . . maybe some improvement on the edges. And certainly the southern High Plains and Southern California and southwestern Arizona look like they're going to continue to see drought.

Dave

Okay. So what sort of things is the USGS doing to address drought?

Greg

This is Greg. There are a number of research projects where we're trying to . . . for example some of the work that Julio and I are involved with is we're trying to explain or identify what are the connections between sea surface temperatures and atmospheric circulation in the occurrence of drought. So we're trying to find climate information that will tell us first of all, what causes these droughts, and then second of all, is there some predictability to it. So there is that type of work going on.

Julio

I would like to add that, you know, all of us are very concerned about the rise of temperatures, particularly in winter and spring in the Western U.S. and the impact that warming—that regional warming—has in terms of exacerbating drought effects.

And you've probably seen in the newspaper recently that there was a paper that came out in Science by Tim Barnett and others, including some of our USGS colleagues, that takes a close look at this warming that seemed to occur almost in step-like fashion, beginning in the mid- 1980s.

And we wondered about the warming because it was such a . . . such an abrupt warming, almost a step change, in the last 20 years or so, that we wondered whether it was connected to changes in upper air circulation that may or may not have any relation to greenhouse gases. But now the Tim Barnett paper indicates through modeling that a large portion of the temperature change that we've all been talking about since the mid-1980s may be due to the greenhouse gases alone.

And the reason that we're concerned from a hydrological standpoint, particularly in terms of drought, is that the tendency for warmer temperatures is actually to exacerbate the impact of the drought.

In fact, one of the drought indices that we all use, the Palmer Drought Severity Index, is very strongly affected by temperature, and temperature also has a tremendous impact on the water budget. And I'll let Greg address this because he just published a paper recently on exactly this topic, where he looked at the impact of warming on the water budget for the Colorado River.

Dave

Take it away, Greg.

Greg

Sure. One of the concerns that I have in the upper Colorado, and in the Colorado Basin as a whole, is that consumptive use of water has grown to a point that we . . . we actually need pretty close to average or wetter than average flow in order to meet the needs in the Basin.

And so one of the experiments we did is we simply took a water balance model, which simulated flows of the upper Colorado, and a simple reservoir model, and we looked at what would be the impact of just some small warmings on the Basin. We didn't change precipitation; we just said what if it warms up a little bit?

And one of the warmings was close to a degree Celsius, which was close to what the trend had been over the past century. And then we did a 2 degree Celsius warming, and what was surprising was that just with these modest warmings, the models of the upper Colorado reached a level where we are entering a period, or could enter a period, where the risk of failing to meet the requirements of the Colorado Compact . . . we might be in the position where we won't meet those, and it could happen quite often.

Dave

Sorry to interrupt you: What is the Colorado Compact?

Greg

Well, just in simple terms, the Colorado Compact is this set of rules that were set up to divide up the water supply from the Colorado River amongst several of the Western States. And it was developed in the early 1900s based on information from a very wet period in the instrumental record.

And since then, of course, populations and agriculture and so forth has increased in the West, so the demands on the water has increased. And just with the slight warming there's risks . . . definitely increased risks of failing to meet the requirements of that agreement. And if you look at some of the work that Julio and others had done with climate reconstructions, something that concerns me is that the 20th century appears to be one of the wettest centuries in the past 500 to 1,000 years.

So if we've been living in a wet period and we are already taxing the upper Colorado River Basin water supply, regardless of global warming, I'm concerned of just a movement back towards drier conditions, which we've seen in the past. And then, what that will mean for western water supply. And that's without considering the warming, and if you put the warming on top of that, there could be trouble.

Dave

Right.

Julio

I wanted to talk a little bit about a paper that I was involved in recently with two other USGS scientists: Chris Milly and Bob Hirsch, as well as another scientists from the U.S., Dennis Lettenmaier, as well as a couple of scientists from Europe. And the title of the paper was "Stationarity is Dead: Whither Water Management?"

I don't want to go into a lot of details about stationarity—let's just say that it's the assumptions that nature will behave in the future in the same way it's behaved in the past. So imagine the Colorado River, you know, if the allocations were made on a . . . based on a period from 1905 to 1920 that, as Greg said, was relatively wet.

As it turns out, it was so wet that the Colorado River Compact arguably was over allocated. More water was . . . more water was split up than actually existed on the average over the long term. Now, we have a different problem, which is with global warming and the climate change associated with it, we have a situation at which the climate may already have exited the envelope of natural variability.

And, you know, it's unclear whether or not we can predict exactly what's going to happen in the future, but models have already shown that it's going to be warmer, and they're all in agreement . . . and now we have a set of models that are in agreement that it's also going to be drier, particularly in the Southwest.

So now we can't really assume that, you know, the future is going to behave like the past, and we've got a real problem because practically all our water systems are designed and operated based on this assumption that there is an envelope of variability that's very well defined, it's relatively flat, and that we can manage for the mean, for example. And that assumption is now dead.

So we've got a problem in that most of our water systems are actually managed on, you know, this stationary assumption that, you know, things vary but they don't really change over the long term. And now it's clear that things are changing over the long term.

And we may be in some cases over allocated and in some cases under allocating, and we may not be assessing risks such as floods very well now. We've got a moving target. This is actually a big challenge for hydrology and engineering in the U.S. as we start to consider, for example, replacing America's decaying infrastructure.

Dave

Right. Is there anything else that we haven't touched on that you think the public needs to know about drought in the West, or about drought in general?"

ulio

Well, I think . . . I think people have to understand that drought is not a local thing. You know, it's a regional thing. And that sometimes we're unaware of connections that occur across the country. You know, it could very well be that if Greg and I are right about the influence of the North Atlantic, that there is a tendency for drought across the West and over much of the United States to actually be correlated with hurricane activity in the Atlantic. And so, you know, we may have situations where drought relief and hurricane relief are simultaneous things.

Greg

I would just mention one other thing, and Julio alluded to this earlier. You know, right now we see . . . we define drought based on what we've experienced over the 20th century, which we know has been a wet century. I think what the public needs to know, and Julio mentioned that, it looks as though we're moving, our actual climate . . . changing. And as we move into these new conditions, what we now consider drought may actually become what is the new climate condition and not an anomaly, but what we end up having to live with.

Dave

Right. That's certainly a lot to digest. Gentlemen . . . Greg, Julio, thank you very much for your time. I really appreciate it.

Greg

Great.

Julio

Dave, thank you.

Dave

Well that about wraps things up from out West. Stay tuned for the second part of this droughtcast coming tomorrow, which will cover the drought situation in the Southeastern U.S.

To get up-to-date drought and other water resource info in the United States, go water.usgs.gov/waterwatch. That's one word: waterwatch.

CoreCast, of course, is a product of the U.S. Geological Survey, Department of the Interior. This is Dave Hebert; thanks for listening, and I will talk to you tomorrow.

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Music credit:

"Old Hen Cackle" by Coleman & Harper

 

 

Mentioned in this episode and other resources:

 

• Stationarity Is Dead: Whither Water Management? (a paper by Julio Betancourt and others)
• Pacific and Atlantic Ocean influences on multidecadal drought frequency in the United States (a paper by Julio Betancourt, Greg McCabe, and Michael Palecki)
• Climatic fluctuations, drought, and flow of the Colorado River (a USGS fact sheet by Greg McCabe and others)
• USGS WaterWatch
• The Palmer Drought Severity Index
• CoreCast Episode 34—Drought: the Long, Slow Natural Hazard (Part 2)