More than 95% of the rain that falls in Arizona is lost to evapotranspiration

A research project involving Arizona’s three public universities aims to get a better sense of how much water is in Arizona’s groundwater basins — and figure out how to get more water into aquifers, rather than being lost to evaporation.

The Arizona Tri-University Recharge and Water Reliability Project finds, among other things, that more than 95% of the precipitation that falls around the state is lost to evapotranspiration. In the Phoenix Active Management Area groundwater basin, the amount of evapotranspiration is about equal to the amount of precipitation.

Kathy Jacobs is the former director of the Center for Climate Adaptation Science and Solutions at the University of Arizona; she recently retired. She says the project’s purpose is to help the Arizona Department of Water Resources identify ways to capture water that’s currently being lost and get it underground as recharge.

Full conversation

MARK BRODIE: Kathy, you found there is a lot of water that falls from the sky that we're losing, right?

KATHY JACOBS: It’s an enormous amount of water that we lose. In fact, it’s more than 95% of the water that comes down as snow or rain is lost statewide. But there are actually basins in the state where evaporation exceeds the amount of water that actually falls as rain or snow, which, of course, is only possible if water is being pulled up out of the aquifer or the soil, which is in fact happening.

MARK BRODIE: I mean, for a state that is grappling with a very long-term drought, that feels like a pretty big problem.

KATHY JACOBS: It’s a very serious problem, and it is, unfortunately, going to get worse because of climate change. So as the air temperature increases, it basically sucks more water out of plants and out of the soil, which is not a good thing for the long term.

MARK BRODIE: Yeah. So what parts of the state is this more of a problem than others?

KATHY JACOBS: Well, all of the state, of course, has been facing a drought. I would say different parts of the state have different portfolios of water supplies, and those water supplies are more or less affected by this, this problem.

At the large scale, of course, the Colorado River is being affected by this problem. On smaller scales, not every basin even has any surface water or water flowing across the surface that they can capture. So much of this really is about how can we manage land differently in order to really change the amount of water that gets recharged into the aquifer.

MARK BRODIE: So what are the kinds of things that can be done to try to reduce the amount of precipitation that’s lost?

KATHY JACOBS: Well, the main thing that we are proposing is that we get the water as quickly as possible to a place where it can recharge. And it turns out that along the Mogollon Rim, in particular, there are lots of areas that are made of very porous rock, karst topography, it’s called. And that rock is very easy to recharge through, as well as places across the state that have fissures or other kinds of cracks in the surface. Those are easy to to recharge through. And in addition, of course, along the stream beds, we have very porous material, and that helps recharge as well.

So the idea is, how can you more quickly get water into the ground, rather than letting it pond on the surface or basically just sheet flow across the surface until it’s all evaporated?

MARK BRODIE: So is the idea then to get more of the the rain and and snow fall to the Mogollon Rim? Like do would you be getting it somehow from, for example, the Phoenix area or the Tucson area up to the Mogollon Rim so it can go into the ground?

KATHY JACOBS: No, kind of the opposite, actually. Most of the snowfall and rain actually is already on the Mogollon Rim because the mountains actually act as a barrier and and cause the air to rise and then that causes snow and rain to fall. So with that is fortunately actually where we have a larger proportion of our rain and snow.

The idea is to manage the snow pack more effectively, to make it last longer, to make more of it actually melt and get into the soil or into the aquifer. One of the ways you can do that is by being really smart about the way you do forest thinning. There is forest thinning going on across the state in many places already as a fire management technique, and our suggestion is that we actually change the way, the pattern that we use for doing that kind of forest management to try to get the water into areas where it can recharge and also to protect the snowpack.

MARK BRODIE: So, OK, so then what do you do when it, for example, rains, you know, we’re predicted to have a maybe a wetter than average monsoon season in the Phoenix area, what what can be done with all that water or if anything at all?

KATHY JACOBS: Actually, urban areas are a very major potential source of new water because of course, we are increasing the runoff every time we pave a surface, more water runs off than with in a natural environment. And so we can be much smarter than we are now about capturing that water quickly and getting it recharged more quickly.

One of the things that we identified as a possibility is more effectively working between the flood control districts and the water management districts and and utilities to see if there’s a way to deliberately plan for recharge rather than just planning for flood, for flood reduction.

So there is actually a very large quantity of water that falls in urban areas, much of which just runs off and evaporates or sits in a pond and evaporates. If it gets into a dry well, which is a very frequently used technique in the Phoenix area, that’s a great thing.

MARK BRODIE: Yeah, I mean, how big of a change would this have to be? Like what kinds of efforts would have to be made to really capture as much water as possible? It sounds like what you’re saying in the Phoenix area, at least some of that infrastructure already exists.

KATHY JACOBS: Yes, that’s right. However, I think each new subdivision or large-scale development can actually be tweaked in ways that can enhance capture and recharge. That being said, we don’t think even this very large list of ideas is a solution in and of itself. We think there’s going to have to be a portfolio of solutions, and the Central Arizona Project will continue to be part of that portfolio.

Conservation is, of course, incredibly important. That’s probably the easiest kind of water to generate is to reduce demand. We also have, of course, municipal effluent, which is a growing resource. So it’s not that we have to depend entirely on this new capture and recharge thought process. It’s just an enhancement, and it’s particularly important, I think, in both urban areas and rural areas where there aren’t many other options.

MARK BRODIE: Sure. I’m curious if you have an opinion about sort of the the way that water is stored and gets to where it’s going. Like when you’re talking about evaporation, like is this a problem, for example, for the CAP canal system? Is this a problem for the reservoir system? Like should we be thinking about storing and transporting water in in ways that maybe don’t lead to so much evaporation?

KATHY JACOBS: That is a great question. It turns out that there is very significant amount of loss from open bodies of water, particularly lakes. That being said, our entire system is built on reservoirs and canals, and so changing that would is a pretty major concept.

In terms of covering the Central Arizona Project canal, obviously there have been some new efforts to do that, to shade it using solar panels, etc. on the Gila River. I don’t think that the total volume of losses in the CAP canal is very large, I’ve heard it’s something like 2%, which is, is not a big deal. But when you look at the volumes we’re talking about across the entire state, suppose it could be considered. It is an expensive option for sure.

MARK BRODIE: All right. Kathy, thank you so much. I really appreciate it.

KATHY JACOBS: All right. Thanks.

MARK BRODIE: Kathy Jacobs is the former director of the Center for Climate Adaptation Science and Solutions at the University of Arizona.