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Hydrology
New water model for Southwest

The Rio Grande is living in the 1950s. According to a new model by a researcher working in the New Mexico water basin, the seven-year drought that affected the area in the 1950s may finally be making its way into the Rio Grande. The model might help regional managers adjust water budgets for the dry river basin, with potential applications to other arid systems in the West.

Christopher Duffy, a hydrologist at Pennsylvania State University, pulled together three disparate blocks that make up a typical mountain-river hydrologic system in the arid western United States: recharge (in the high mountains), transport (through fractured rock, aquifers and surface alluvial fans) and release (the river itself). The model uses data and observations for each section, including the input of rain and snow and the rock types and fracture zones. Duffy also took into account water withdrawals from the Rio Grande by river vegetation, plus the evapotranspiration of plants and evaporation of water on the alluvial slopes (the fan-shaped deposits of loose rocks and sediments from streams coming out of the mountains).

After running the numbers, Duffy calculated that the base-level water flow of the Rio Grande is determined by wet or dry climate conditions that occurred 50 years before. From wet years, he could track “a mound of water that takes a long time to get to the river,” he says, moving through the system like a wave as it hypothetically propagates through the pores and fractures in the rocks. Evapotranspiration at the river corridor level is “a major loss of water,” Duffy wrote in a publication of the model earlier this year in an American Geophysical Union (AGU) monograph. Duffy also presented the model in May at the AGU Joint meeting in Montreal.

“There’s a great deal of transferability in this modeling approach because it can be adapted for other arid western U.S. basins,” says Michelle Walvoord, a hydrologist at the U.S. Geological Survey in Lakewood, Colo. Although the concept of lag-time responses in river systems is not new, Duffy’s “cutting-edge” model is a step forward in incorporating data with “a solid hydrologic conceptual framework,” Walvoord says. The model can respond to a variety of hypothetical environmental changes over fairly complex terrain, and over different scales in time and space.

Aside from the quantitative advances in the model, says Cliff Dahm, an ecosystems ecologist at the University of New Mexico in Albuquerque, Duffy’s connection of the three blocks of the Rio Grande basin is “the most unique part” of his work. “Nobody that I know of has taken those three pieces and linked them into a hydrological model,” Dahm says. The ability to determine lag effects in similar regions has “interesting implications for management” of future water budgets for the Sierra Nevada and off the highlands of the Rocky Mountains, where hydrologic systems feed the Platte, Missouri and other rivers, he says.

Duffy incorporated Dahm’s riparian plant evapotranspiration research, and the scientists are now working together to further perfect the three-part model.

Naomi Lubick

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