More Than 95% of the Rain that Falls in Arizona is Lost to Evapotranspiration
Rain falls on backyard pavers in central Phoenix.
Christina Estes / KJZZ
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?