Water worlds

This article originally appeared in the Fall 2014 issue of the U of S alumni magazine, the Green and White

I’m a tropical guy, not a desert guy," Jaivime Evaristo said from Arizona during a recent phone conversation. The U of S PhD candidate’s current location is hardly tropical, but Evaristo has sequestered himself with several other scientists in a basement beneath an imported tropical rainforest shielded by the vast glass pyramid of Biosphere 2, amid the humid breath generated by nearly one hundred species of Amazonian plants. The team is there to examine stable isotopes of water, used as “tracers” to indicate where water comes from, how long it resides in a catchment (regional watershed) system, and the kind of water plants are using.

A 2010 paper published by Evaristo’s supervisor, Jeffrey McDonnell, professor and associate director of the Global Institute for Water Security at the U of S, challenged the current mode of thinking about how water gathers beneath the earth’s surface, a topic dear to the hearts of civic planners and watershed managers who rely on models to make sense of the physical world. Evaristo explained: “The previous view assumed that beneath the soil surface of each catchment area lies one huge tank where all water meets, infiltrates deep into the soil, recharges the water table and becomes the groundwater. At some point, it ends up in streams.”

That is not what Evaristo, McDonnell and their team have observed—in cloud forests in Mexico, in Arizona’s glassed-in rainforest, in Puerto Rico and in Oregon. By studying stable water isotopes, Evaristo and his colleagues are learning that water that ends up in streams is different from water used by plants, most likely as a result of plant-based transpiration and photosynthesis. “We call it ecohydrological separation, a separation between plant water and ground water. Two water worlds—one related to plants, another related to stream flow generation and ground water recharge.”

Seeing and understanding the relevance of scientific research to everyday life is not always an automatic process. “A grandmother should care in her lifespan because this [change in viewpoint] has implications for contaminant transport,” Evaristo said. “Her grandkids should care because this may have to do with the relevance of climate models.”

These global-scale models, used by NASA and universities, are informed by smaller-scale land-surface models that attempt to replicate the oceans, terrestrial parts and the atmosphere. He explained that land surface models are driven by the same assumptions of one large water reservoir. “In the scientific community, the consensus is that climate change is real, driven by humans and naturally as well. It’s not really a question of if the earth will survive, but if we will. That could increase uncertainty in model predictions that are meant to represent the real world. We have to have an understanding of how the world works that is representative of the real world. That we can only assess using water isotopes.

“My mission is to show myself that what we have found in Oregon, Arizona, Mexico and Puerto Rico is something that is ubiquitous in nature.” Evaristo said. He has organized the scientific equivalent of crowd-sourcing, relying on pre-existing scientific sites in diverse parts of the tropical world to determine if the twowater- worlds theory is related to soil or place, or if it is more fundamental, not to do with tropics, but simply occurring in nature. “That can have immense implications on models that drive our understanding of water resources and contaminant transport from mines, a leak, or a contaminant released. We don’t want that to end up in city water sources.”