SOLS Podcasts

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Peggy Coulombe: Hi, this is Peggy Coulombe with the ASU School of Life Sciences, and welcome to Science Studio. Today we're going to talk about water in Arizona, but not in the usual way. Most of us recognize that water is a precious commodity in a desert, and largely we take it for granted; but how do animals in desert ecosystems cope with the challenges of obtaining water here?

Speaking with us today is John Sabo. He is an assistant professor in the School of Life Sciences, and studies links between water cycle, groundwater aquifers and animals. Basically, he's going to help us understand what happens when you can't simply just turn on a tap. Welcome, John.

John Sabo: Thank you for having me, Peggy.

Peggy: Now before we start to talk about the challenges that happen in nature, let's talk a little bit about water in general. What are the primary sources of fresh water in the world, for example?

John: Well first of all, we can start off with the fact that 97 percent of the water on Earth is in oceans, so it's saltwater and therefore undrinkable. The remaining three percent is freshwater, and three-quarters of that--so 75 percent of that--is frozen, it's ice, in glaciers and snowcaps. And 99 percent of the unfrozen water, that remaining quarter of three percent--so we're getting pretty low here--is fresh water, and 99 percent of that is underground. So a very small fraction is available to us on the surface.

So to answer your question, three percent is fresh water, but less than one percent of it is available to us in visible form on the surface of the Earth.

Peggy: Most of it is underground in aquifers?

John: That's right, in aquifers.

Peggy: And can you tell me what an aquifer is?

John: An aquifer is an underground layer of rock that stores water that we can draw on.

Peggy: Is that where we get most of our drinking water in Arizona?

John: It depends on where you live in Arizona. In Phoenix, the answer is no, we get most of our drinking water from the Central Arizona Project; it's a 300-some-odd-mile-long canal that delivers our allocation of Colorado River water, so it's surface water, to Arizona, through this canal, to the municipalities and farms and reservations that need that water.

Peggy: And why do we get it from CAP rather than tapping into the aquifer?

John: We get it from CAP simply because there isn't enough replenishment of groundwater locally to supply the rate that we need to draw it from the aquifer, so we would drain our aquifers pretty quickly.

Peggy: So we can turn on the tap and get water from more than 300 miles away, but how do the animals get the water?

John: You know, this is a really good question, and it should be an obvious one. One would think that scientists would have a good answer for this question, but as it turns out, it's particularly difficult to measure. We have some, what I think are really good, educated, scientific guesses about where water comes from.

The first is not surprising, it's from surface sources. So many animals drink water from rivers and lakes and other surface sources of water, puddles, what have you.

The other sources are less obvious. The first of these is water that becomes available through precipitation. So we have rain events that make puddles, like I said before. But then, less obvious, there are some animals that can extract water from fog. There are definitely plants that can do this. There are some animals that drink dew, in the morning, on rocks. So these are less-obvious forms of precipitation that are available only at certain times of the day.

Then there are a couple of other sources that we are really interested in, in our lab. The first of these is in food, and in dry habitats like the desert, where there isn't surface water, there isn't fog, there isn't dew in the morning, and there isn't rain a lot, food becomes the primary source of water for most animals in the desert.

Peggy: Are there some other behavioral ways that animals might limit water loss? Does that play a role as well?

John: Yes, so on the whole, for an animal, there are inputs and outputs, and there are ways that we can maximize the amount of water in our bodies, we're animals too. So animals can do that by either taking in more or preventing it from leaving their body. There are many, many different adaptations for conserving water once it's obtained in animals.

So for example, kangaroo rats have very long nostrils so that their nostrils can pull out all the water that is in the air, that they are breathing out back into the dry environment. Gila monsters have urinary bladders that can be used as water storage organs during certain times of the year. These are magnificent adaptations that make the desert life possible. They definitely help, but they are not the whole answer. You still have to get water.

Peggy: So much of your research, then, is in riparian systems. Can you tell me what constitutes a riparian system overall?

John: Mm-hmm. Well, riparian zones have a simple definition. It is just a terrestrial habitat that is adjacent to an aquatic one, typically a freshwater ecosystem, so a river or lake.

In the Sonoran Desert, if you fly over the Sonoran Desert, you'll often see these green ribbons of vegetation in the middle of what you would otherwise think would be desolate desert habitat. Those are the riparian habitats that we study. Some portion of those riparian areas are near surface water. Usually it is right next to the river.

Once you start to move away from the river, many animals don't have the mobility to actually go and get surface water from the rivers. In riparian habitats, much of the water that the animals in higher trophic levels get comes from food, and ultimately from plants, who are getting it from below-ground.

Peggy: So you work in the San Pedro River mostly, and that's south of Tucson. Is the surface water plentiful in the San Pedro River?

John: It actually depends on where you are in the San Pedro River. The San Pedro River is one of the last free-flowing streams in the Western U.S., and it is one of the last perennial rivers in some stretches in Arizona.

At the site where we do most of our research, which is near the Charleston U.S.G.S. Staging Station, it's part of the National Water Information System that the U.S.G.S. runs, that site typically is perennial. In other words, the water runs all year 'round. But in the last two years it has dried up in the dry season, which is at the end of June, early July. It dried up for the first time, in the last two years, in a hundred years.

Peggy: Why did it dry up?

John: Well that's a good question too. There are two working hypotheses for why the river is drying up in places. The first is what we call "groundwater overdraft," and that is simply using the groundwater too much, human use of groundwater. The second is drought, so again, there is a source of water and a depletion of water.

So the two hypotheses are "not enough coming in, and too much going out." Probably both are working at hand, but the evidence is leaning towards the latter--in other words, groundwater overdraft--primarily because we are seeing, in places where wells are being retired, the perennial flow is being restored.

Peggy: So how does groundwater overdraft affect the animals in this watershed area?

John: That's a good question, and it's a complicated answer. It's probably the crux of the next ten years of our research in our lab.

Peggy: [laughs]

John: There are a number of answers, but the first is that the plants in riparian habitats, especially the trees, tend to be dependent on shallow ground water being available. So once that water goes--in other words, when our human use of groundwater becomes so high that it draws the water table below the depth at which the roots of those trees can access it--those trees are going to be gone.

And those trees are, first and foremost, a habitat for all the animals in riparian habitats, and second, but not far behind in importance, the food source, and indirectly, the water source for those animals. So those plants are very important for maintaining the diversity of other plants in the end of story, and also of animals that rely on those trees for habitat, food and water.

Peggy: When we talk about animals, we're talking about a whole series of organisms.

John: That's right. Yeah, we look, in the lab we look at a whole host of different animals that comprise a food web in riparian habitats, from the insects to higher-level carnivores.

Peggy: How do invertebrates on the surface drink groundwater below the surface?

John: That's a good question. They don't have a tap, like you were saying earlier in the program. They can't turn the faucet on and mine the groundwater directly from the aquifer. But they do have this tremendous pump next to them, which is a biotic pump: it's the trees.

The trees are rooted in shallow aquifers in riparian habitats. They are dependent on those sources of water for their persistence, and they are constantly moving that water through their bodies in the process of transpiration to photosynthesize. In doing so, they make water-rich leaves, which are the photosynthetic organs, and those leaves drop onto the ground at a surprisingly high rate, in an area where there are gusty winds, for example. Those leaves that fall to the ground are actually a pretty hot commodity for animals who don't have access to surface water.

Peggy: So you're basically saying that falling leaves are an important part of the water chain that links bugs and predators and birds and lizards.

John: Yeah, it sounds like a kind of farfetched and complex explanation for a pattern we observe; but seriously, we walk through our flood plain habitats at the San Pedro, and we see whole green leaves that used to be whole, but that are chewed almost to the petiole, which is the venation in the leaf. So all the green, water-laden, fleshy part of the leaf is gone, and our immediate inference was that the insects in the system were water-limited, and were getting their water from these green leaves that were falling from up above.

If you are listening to this program and thinking about green fall, the leaves that fall off the trees, you're probably thinking, "Well, there aren't that many leaves that actually fall off the trees, so how could that be a significant source of water?" The answer is, it's true that the amount of water that falls with the green fall is small, it is insignificant compared to, for example, what a cottonwood burns off in a day through transpiration.

But that little water turns out to be very important, because there is no other source of water available for those insects. So the selling point of the research that we do in the lab is that a really small flux in the ecosystem, this trickle of leaves that falls from big trees, is actually profoundly important for the higher levels in the food web; and those leaves would not be available were it not for an aquifer that were close by.

Peggy: Which invertebrates do you work on, and why do you work on these and not something at a higher trophic level?

John: That's a good question. I used to work on lizards. In fact, currently we are focusing primarily on arthropods in the system, and our focal organism is a cricket. It's a detritivore that lives at the bottom of the food chain, so it's the first or primary consumer in a food chain, if you will. It's the one that's probably the most numerically abundant insect in the system, and also likely a very abundant food source for the higher vertebrates.

There are actually two answers to this question. The first is, the crickets are really easy to work with, so we can do physiological experiments in the lab very easily with them. We can use stable isotopes to trace sources of water from aquifers, for example, to their bodies. We can do field experiments with them. They are easier to work with than animals that are larger and at higher trophic levels.

Also, they are that first link between plants who are providing the source of water from the aquifers and those higher trophic levels. So if it isn't important for them, it's not going to matter much up above.

Peggy: How do you think global warming is going to affect the available of groundwater here in our animal communities, and their biodiversity?

John: Another very good question, and another one that is hard to answer. I'm going to answer it from an animal ecologist's perspective. My gut feeling from the literature and from talking to folks who know more about climate change than I do is that the primary way climate change, global warming, is going to affect water resources in this region is through the timing and severity and predictability of rain events.

It's going to change seasonality, it's going to change the intensity of events and make them occur at different times of the year than we expect them to. So from an animal's perspective, that means that things that you are normally used to doing at a certain time may not longer be favorable to do at that time. So reproduction events may be mistimed, critical life history stages may be overspent in times that are too dry now. These are the primary ways that it will affect animals in riparian flood plains, for example.

Peggy: So all those natural selections that have occurred over thousands of years will completely change.

John: That's right, yeah. The little predictability there is in a desert during the monsoon season will be gone.

Peggy: What challenges do you see ahead for Phoenix, with regard to water?

John: Phoenix has a number of big challenges to face in the future, and I think the biggest is that we have an allocation that we get from the CAP system, from the Central Arizona Project. It's an allocation that has an upper threshold, because California, the Rocky Mountain states and Mexico get their allocation too, and there is no more water after that.

So those who have been to the mouth of the Colorado River at the Sea of Cortez, it rarely flows, and that is because the allocations to California, Arizona, and the Rocky Mountain states and then Mexico take that water away through the canal projects or other activities.

Once our population exceeds that allocation we will have to rely on local sources of water, which are scarce, as we know--groundwater which can be depleted quickly. It has been depleted in other cities like Tucson, or what's left from the Salt River Project.

At that point we are going to have to think about water conservation measures, and if those aren't sufficient, think about limiting the amount of growth that we are experiencing in the state currently, because we simply won't have enough water to meet the current trajectory of population growth in the state.

Peggy: John, I want to thank you for joining us today, and I have to say that when I see a tree in the desert I'm going to think very differently about it and everything around it. It's its own little pump for life in the desert here.

This is Peggy Coulombe, and you've been listening to the School of Life Sciences podcast, Science Studio. School of Life Sciences is in the College of Liberal Arts and Sciences on the Tempe campus of Arizona State University. Have a good afternoon.

Nature’s water pump

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SOLS Podcasts 2007