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The Habitable Planet: A Systems Approach to Environmental Science 

Water Resources Interview with Tom Maddock

Interviewer: Can you tell us what you do?

TOM: I am a Professor of Hydrology and Water Resources at the University of Arizona where we have a Department of Hydrology and Water Resources in the College of Engineering.

Interviewer: What is hydrology?

TOM: Hydrology is the study of water. We have four disciplines on the campus that study water. These are essentially the discipline of surface water studies, ground water, water quality (which would be water chemistry), and water systems (which would be things like modeling processes). My particular area is in the field of ground water/surface water interactions and how they might affect the ecology.

Interviewer: What got you interested in science?

TOM: I was interested in science at a relatively early age, but never worked hard at it. I was interested in mathematics first and then mathematics led me into the sciences. I was a student at American University and had a Professor by the name L.G. Johnson who later became President of Howard University. He taught me an introductory math course that just lit up the world for me in terms of mathematics. During my undergraduate work at the University of Houston, I worked primarily in math and physics. Then when I went to Harvard for my graduate work, I studied under the Committee of Applied Math and ultimately did a mathematical oriented thesis in environmental systems. So, I actually went into several areas of science. I started off in in what’s known as plasma physics, and then went into combustion and finally ended up in hydrology, or environmental issues.

Interviewer: What is the fundamental research question you’re investigating?

TOM: I work in the area of ground water/surface water interactions. In particular if you withdraw from the ground water system, if you’re close to a stream, you may intercept water that would have arrived at the stream or draw water directly out of the stream. This reduces the amount of water available downstream to a farmer, to a critter, to a tree. The other problem that you have with depletions of surface – like ground – water pumping is that there’s a riparian unit, which is essentially trees, bushes, grasses that grow next to the stream, even though these may be out in the absolute desert away from it. When you drop the water table, you can destroy those ecological units. That is the area that I’m trying to understand: how much, or how little water can these systems survive with and how they work.

Interviewer: How does your research fit into the big picture of hydrological research?

TOM: Studies of ground water/surface water interactions fit into the big picture because there are surface water sources throughout the Southwest that are now being affected by this ground water pumping. As a matter of fact, nearly all the water cases that are heard by the Supreme Court involve issues between ground water pumpers and surface water. It’s an international problem. Almost every country in the world that uses ground water as a resource is having troubles with it affecting surface water systems. And, it’s an area which, generally in the past, has been avoided or hasn’t been studied. The laws involving water have evolved in such a manner that they are considered as two distinct sources of water that aren’t related, or interrelated. Now, there are some states that don’t do that. But it’s a physical problem in that you’re taking water out of the stream. It’s an ecological problem in that you’re reducing habitat, not only the plants’ habitat, but also the birds and animals that may live in that habitat. And it’s a social-political problem because you have certain people using surface water, and certain people using ground water, and they’re interfering with each other.

Interviewer: Could you explain the difference between ground water and surface water?

TOM: The difference between ground water and surface water is that you can float a stick in surface water. And with ground water, you really can’t do that except maybe looking down a well and dropping the stick down the well. In essence, surface water flows on the surface, in streams, rivers, and lakes. Ground water flows in systems, which are called aquifers; it’s water moving through pores that occur in the rock material that makes up the aquifer. It’s called a porous media.

Interviewer: What are the different kinds of aquifer?

TOM: One way of classifying an aquifer is by looking at the material that it’s made of. The most common is porous material, porous rock, which is sands and gravels deposited over the years. Another is fractured rock, and this is more hard rocks, the salts, that have become fractured through ages; water flows through them. There’s still another way of classifying aquifers, and that is whether an aquifer is a free surface aquifer—which means that if you were to put in a well, the water would flow up to that point where the water table is in the well—and a confined aquifer, where if you put in a well, the water is essentially under pressure and would be forced upward. If it actually comes to the surface, a confined aquifer is sometimes called an Artesian aquifer.

Interviewer: What are the interactions between ground water and surface water?

TOM: There’s a two-way interaction between ground water systems, and the aquifer, and the surface water system. First of all, the surface water system can recharge the aquifer. When it does this, the river or stream is called a losing stream. Or the aquifer can supply water to the stream. In this case, it’s called a gaining stream. The amount of water coming in is referred to as a base flow. So if you pump ground water, the thing that you’re generally interfering with is base flow.

Interviewer: Why do we need to learn more about this?

TOM: We are losing habitat along the southwestern rivers. And the primary reason that we’re losing that is because of ground water pumping, which is reducing the surface water, which is then affecting the habitats. The principle concern of most people is to try to understand how the raising and lowering of water tables near a riparian habitat affects the habitat. And my research group does that.

What we are finding is that the riparian habitat along many of our stream systems is disappearing. Now, let me explain what I mean by a riparian system. Basically, very close to the stream, maybe within a maximum of one to two hundred meters away from the stream, you’re going to find an abundance of trees, like cottonwoods and willows, bushes, grasses, that eventually just disappear as you move further away. And, this unit of plant life is a riparian system. Because all the plants are there, the animals’ and birds’ habitat that area too. So, if you reduce that habitat, either by dropping a water table or reducing surface water or a supply, essentially what you’re doing is destroying not only plant life, but probably animal life too.

Interviewer: What techniques do you use to answer your questions?

TOM: We have a research team that I oversee in the Department of Hydrology which is involved in several facets of the study of these riparian systems. We do mathematical modeling of these systems. There are certain issues and certain processes that can only be described mathematically. For example, the very process of surface water being depleted by ground water pumping is something that can only be established by mathematical modeling. You cannot go out and physically measure this. You can measure things that indicate how this behavior might occur, and this allows you to model it, but there are specifics you can’t model. Then we have to have experiments in the field.

We conducted a very famous experiment that National Public Radio, NPR, called “The Tree Torture Experiment.” The idea was to put in a series of wells in a cluster of cottonwood and willows that we had instrumented, so that we could tell how much water they were using. Then we pumped the wells and dropped the water table and watched how the trees responded to it. That’s a field study.

We do laboratory work too, where under a very controlled environment, we study how certain interactions can occur between ground water and surface water systems and amongst the plant systems. I had a graduate student who actually did a study in a greenhouse to determine if you reduced the amount of water to the plant, how the plant responded in its transpiration. Plants transpire. It’s a form of evaporation, but it’s called transpiration.

Interviewer: What are the results of this research?

TOM: What we’ve been able to do is to introduce what are called ground water flow models, which are mathematical models of the aquifer system. We found a way of modeling the process of how plants reduce their uptake of water as the water table is dropped. And we’ve done this in a mathematical fashion that allows us to introduce this process of transpiration into large scale mathematical models of the ground water flow system, i.e., the flow and the aquifers.

Interviewer: How much water is used by riparian systems?

TOM: Some of the trees in the riparian systems can use quite an astounding amount of water. It’s been estimated that under perfect conditions, a healthy cottonwood, a healthy adult cottonwood can take as much as 90 gallons of water per day. So we’re talking about a fairly substantial amount of water that’s being transpired by the trees. If you have a cottonwood taking up this amount of water, then a group of cottonwoods in the area of the river draws a substantial quantity of water from that river.

Interviewer: Would it help to cut down trees?

TOM: One thing that had been suggested to save water in rivers was perhaps to cut down all the trees along it. The problem is, when you do that, regular evaporation takes over. You can end up actually losing more water with regular evaporation than you would from transpiration from the trees. There were several studies done by the Bureau of Reclamation and one by the U.S. Geological Survey that established this. There really wasn’t much gained by chopping down the riparian units, or the riparian habitat of the areas and saving water, because it let the sun in. The sun evaporated more water. They ended up losing more water than if they’d just left the plants there.

Interviewer: If you take away the trees, then I assume the riverbanks erode.

TOM: That’s right, as was demonstrated in [Hurricane] Katrina. You can see what happens when you do away with wild habitat. One of the biggest flooding problems that occurred during Katrina was because the wetlands had been removed. In our area of Arizona, if you remove the trees, the grasses, the bushes, nothing may happen for a long period of time because of the lack of rainfall. But when we do get rainfall, it can be very intense and we get major flooding. If we get major flooding and this material is gone, these plants are gone, and we get huge problems with erosion. During the 1983 flood, down by First Avenue, a building crashed into the river because of erosion.

Interviewer: Why is studying water interactions of particular interest?

TOM: The problem in the Southwest is that we are having continued growth and in many places the only place that you can get water to sustain that growth is from ground water development. Aquifers are a limited source of water. They’re like a bathtub filled with sand in which you put water and then you’re taking the water out of the bathtub. Several things happen. Of course it dries up and as it dries up, it shrinks, so the sand starts collapsing. This is called subsidence. This is what we see in many of the major cities that are using ground water as a resource. Tucson is no exception to this. We’re seeing subsidence in many areas. Subsidence can be quite, quite, large. It can be as much as several meters producing fissures in the Earth.

There are two problems with using large scale pumping of ground water resources. As you drop the water table, you essentially increase the cost associated with extraction of that water. The second problem is as you go deeper into the aquifer, that water has been there a much longer period of time. It’s had time to dissolve the solids of the material that it’s sitting in, and with that occurring, you can get some pretty bad materials. For example I think there’s been as many as eight wells that had to be shut down in the Tucson area because of arsenic. This is not somebody contaminating the aquifer—it’s nature itself. So the deeper that you have to extract the ground water, the more likely you are to end up with some type of quality issue, such as arsenic, or sulfates, and certain nitrogen compounds.

Interviewer: What issues are related to depletion of water as you move downstream?

TOM: Most of the rivers, particularly in the Southwest now, have some type of dam on them. And whenever you have a dam, you release water. Water flows down and as it flows down, it is diverted. This is usually done by small diversion dams. Quantities of water are taken from the river and they go out onto the fields. Some of that water actually returns to the river through what are called drains. But, in essence, what is happening is that as you move downstream, you’re taking more and more water out. That water is lost; it’s what’s called consumptive use.

So, as you move downstream and you have more and more of this water loss, you end up at the end with a smaller and smaller quantity of water available. If you add on to this ground water pumping, which is either intercepting water that would have arrived at this stream, or pulling water out of the stream, you’re putting even greater stress on it.
There’s also a process called a run of the river. That is, certain irrigators or certain water rights have the capability of taking every drop of water out of the river because in many cases, leaving water in the stream does not constitute a beneficial use of water. So the idea is you use it or you lose it. Because, if you’re not using all of the water, somebody else will get the right for it. It’s only been in the last ten years or so that we started to understand that there is an inherent right of the fish, and the riparian system itself, to have a quantity of water in the river. But in the past, any water that was left in the river was considered wasteful so it was all appropriated. Whenever you have a water right, you usually have an appropriable right, which means you can take water out.

With an appropriable right’s system, the first person with the right gets the water, his amount of water. And what’s left over, goes to the second. And if there’s any left over from that, it goes to the third. It’s quite possible that only the first gets it. It’s also possible that there’s enough water in the system that everybody gets water.

Interviewer: How has the growth of cities affected appropriation rights?

TOM: One of the things to keep in mind is that in many cases the early water rights for surface water systems were associated with farms and mines. What has happened is that over a period of time, agriculture has been replaced by growth of houses. So now what you have is an urban demand, or a rural demand for water that is not connected with the farm system, nor connected with the mines. So you have a conflict of the use of the water. And so the issue of appropriation comes into question because, if a farmer has a higher right than the urban user, then in a period of drought, the farmer gets his water, but the urban system doesn’t. There’s a conflict there. And this is a conflict that the courts are going to have to face. It’s going to be an issue in the future, because there is only a limited amount of water, particularly in the Southwest.

Interviewer: Cities get a lot of their water from aquifers. How does that affect the surface water system?

TOM: What has happened since the 1940s is the development of the high lift pump, and the completion of rural rectification. These are the two big processes that made ground water extraction easier than it had been in the past. You have a tremendous amount of development of ground water occurring. Unfortunately, ground water is more available near streams than it is in other places. It’s what we call the Willie Sutton Principle of Water. Willie Sutton was a bank robber who was once asked why he robbed banks. And, he said, “Because that’s where the money is.” In the same instance, the wells go in near the river because that’s where the water is. So what you have is this growth of ground water pumping, but of course, the ground water pumping interferes with the surface water systems, and essentially depletes surface water. And you end up with a conflict.

Interviewer: Is the ground water extraction system likely to break down?

TOM: Some aquifer systems have had such a high extraction rate that they have become uneconomical to pump. It costs too much money to lift the water to the surface. That’s one problem. The other problem is that as the extractions go deeper, the water quality can change. We saw that here in Tucson with an introduction of arsenic into wells. The other problem we have with our ground water system is that many times we contaminate it ourselves with leaky underground storage tanks from gasoline or spills of various contaminant materials. In Tucson, we had a big problem with TCE, trichloroethylene, which polluted a number of wells in the Southwest causing a number of health problems. This is a very, very limited quantity of water that you’re adding a lot more straws into. Ultimately what happens is, the milkshake runs dry. You run out of water.

Interviewer: Has this happened in the past?

TOM: If you look at cities in ancient cities in the Southwest or in Mediterranean countries, a lot of those cities rose and fell on the availability of water, particularly in places like Mesopotamia. Even in our own country whole cultures disappeared because of water problems—either lack of water, or too much water. A lot of people don’t realize that too much water can be a problem, too. If you raise the water table high enough in a system, you get what’s called water logging and salts build up around the plant roots. And the plants will die. This is particularly important in agricultural regions. We’re now suffering mainly from too little water because we have too many straws in the system.

Interviewer: How does your research contribute to a bigger picture?

TOM: One of the things that we’re trying to understand is how drought processes work. One of the biggest research areas occurring nowadays is the effect of climate change on ground water and surface water systems. We can maybe make some prognosis about what our future is going to be like, whether we’re going to get more water or less water. These are very active research areas, not only at this University, but other Universities across the country and across the world. The research that we’re doing on surface water/ground water systems is tied into the management of a regional water supply, which in turn, is connected to a state or a country water supply, which in turn, is connected to international issues involving water supply. The things that we study in our area ultimately lead to producing water management capabilities used in other places throughout the world.

Interviewer: Can we go back and undo damage already been done?

TOM: One of the interesting questions that I’m sometimes asked is, what can we do to go back the way we were. The answer to that is that there isn’t any way. Population levels in this country have grown to 300 million people. Each one drinks water. As the population levels increase, demands for water are going to increase also. So there really isn’t any way to go back.

But there are things that we can do. First of all, the problem right now is not so much lack of water. There is enough water for the entire population if you are able to move it around. The trouble is that the cost of moving water around the country is very expensive, very prohibitive. The last major undertaking of that kind was the Central Arizona Project, which was just astronomical in price. Unfortunately, what would be nice is to be able to have a pipeline that you could turn on and off; when it rained too much in New England, you could pipe that to the Southwest or when it rained too much in Seattle, you could move some of that water around. But there are too many environmental issues associated with that. And it’s a rather simplistic idea. In essence, we are just going to have to live within our means and right now, we haven’t been able to do that. In the future, if we can’t do that, then we’re going to have big problems.

The primary issue that cities, particularly in the Southwest, are having to look at is the idea of sustainability. In other words, how much water does it take to sustain a certain lifestyle, a certain quality of life? This is a question that really is not that well understood. If you’re lucky enough to have a mixture of surface water and ground water systems that you can extract water from to get your water supply, you’re in better shape than if you have one of these and not the other. The reason for that is that in wet years, when you have lots of water, you can maybe recharge the aquifer. In dry years, when you don’t have a lot of surface water, you can extract water from the groundwater systems. There’s kind of a sustainable quantity. The trouble is that most people don’t understand what the sustainability is in their particular area. And that’s a real problem.

You asked me how we might solve this problem. Perhaps the way that you could go about solving it is to determine for every piece of ground in the United States, what its sustainable quantity of water could be. Then, based on our population levels, go to those areas that have a higher sustainable level than those that are less sustainable, which would kind of empty the Southwest, which is very popular because of the sun.

Interviewer: Can you tell me how trees and plants interact with ground and surface water systems?

TOM: We would like to understand how much water a plant is extracting from the ground water system, and, indirectly, from the surface water system. If you extract water from the ground water system, you may pull water from the surface water system into the ground water system. We need to understand how water moves through the plant. It comes in through the roots, travels up the trunk of the tree, and expires through the leaves, in evaporation, which we refer to as transpiration. The amount of water that comes up through the roots in certain plants is dependent on the water table. We study plants that like to keep their “toes,” so to speak, in the water table. They extract water directly from the water table or from the capillary fringe, which lies on top of the water table. We’ve found some remarkable things about the way these plants behave.

Interviewer: Can you integrate that into your models?

TOM: This is above our modeling capabilities at the moment. Ultimately, we’ll have to do that. Right now, what we do, in terms of models, is have a curve for what are called plant functional groups. A plant functional group can be groups of plants that have the same water use processes, like cottonwoods and willows.

Interviewer: What are the curves that you develop?

TOM: We develop curves for water use of plants by different plant groups. Certain groups use the same amount of water. They may be different species. We classify those as a particular group. What we do for these groups is develop a curve that relates the water level to how well the plant transpires, how well it uses the water. We have three critical points on a curve. The first point is called the extinction depth. That is, if the water table drops below that point, the plant will expire. Then if the water table rises, there will be a point where the plant uses the maximum amount of water. In other words, this is its most efficient point. And then, as the water table rises even further, you can actually end up at the point where you drown the plant because it has too much water, and this is anoxia. In essence it goes back to zero again. One of the things that our researchers do is try and develop curves for these plants for various plant groups, rather than single species. We may have five thousand species, but they may be able to be grouped in as little as four or five different types of groups. We refer to these as plant functional groups.

Interviewer: Can you explain how the ground and surface water can become contaminated by humans?

TOM: Both groundwater and surface water systems can get contaminated by actions of mankind. For example, surface water systems can be contaminated by releases of contaminants from factories, from overfertilization in agriculture, and also from natural processes in the soil, which normally would not affect the surface water system, but happens because you spread water on the land surface and the water infiltrates down and then flows to the stream again underground. Salts are a problem. The other thing is, that in many cases people discharge treated effluent into a stream system. That treated effluent may not have bacteria or viruses, but it does have fairly high nitrate levels, or nitrogen levels. And that again ends up in the ecosystem. And it can end up in the ground water system and can be spread in the surface water system.
Excess nitrates, say from agriculture, when they enters into a stream, can produce clogging layers that reduce the amount of water that’s being recharged to the aquifer system. These clogging layers are very difficult to get rid of. What you have to have is a major flood event that scours them out. They grow, they are biological in nature, and they feed off the nitrogen and compounds that are in the fluid, in the water.

Interviewer: What are your predictions in terms of the environment?

TOM: The southwestern United States has one of the largest population growths of anywhere in the United States. Because of that, you’re putting an extreme amount of demand on the water resources that are available, both surface water and ground water. With that continued growth and the possibilities of droughts, particularly in the highland areas of the United States that supply the waters for the rivers like the Colorado, the Rio Grande, the Arkansas, this can lead to a rather bleak outlook. Cities like Tucson and Phoenix live on both ground water and surface water and have to have both to survive. In Tucson, we have what’s called the Central Arizona Project, which is water that comes out of the Colorado River. With prolonged periods of drought in the highlands of Colorado, we may not be able to get that surface water because the appropriation system gives California a higher right than Arizona. So California gets their water first and then comes Arizona. And you have Nevada clamoring in the background wanting their share of water, too. That can lend itself to some real political infighting in terms of water. We’re in a period of what is considered to be a prolonged drought in the Southwest. But if you look at historical records based on things such as tree rings and ice cores, what you find is that in the past there have been far worse droughts than what we’re involved with now. In fact, in the last two thousand years, the climate variation has been relatively small compared to what it was in previous periods of time. If we get to the high variation coming back again, we could have some real problems. We could have a drought, like we see now in the Southwest lasting literally for fifteen, twenty years, quite possibly even longer. We’re trying to promote this growth that’s occurring without an adequate water supply to maintain it.

The real problem that we have, particularly in the Southwest, but I think every place in the world, is that with increasing populations and either shortages of water, or water in the wrong place, we are becoming very vulnerable. The Southwest has a very unique vulnerability from climate change simply because, where do we get the water if there is no water? I mean, we can’t be dependent on the rainfall. And if we continue to extract water from the ground water system, we’re going to dry up the aquifers. If the drought affects the surface water system, then we’re not going to get any water, for instance, from diversions into the aqueducts like the CAP, the Central Arizona Project. There’s a problem.

Series Directory

The Habitable Planet: A Systems Approach to Environmental Science 


Harvard Smithsonian Center for Astrophysics in association with the Harvard University Center for the Environment. 2007.
  • ISBN: 1-57680-883-1