Distant mountains influence river levels 50 years later

05/12/04

Rainfall in the mountains can have a heavy influence on river levels, but the effects are seen 50 years after the rain has fallen, according to a Penn State hydrologist.

"The general thinking has always been that it is 20 kilometers from the mountain to the river, it does not have an impact," says Dr. Christopher J. Duffy, professor of civil engineering. "We are asking what the role of mountain recharge is and how it affects river conditions."

What Duffy is finding is that rainfall and snowfall over the mountains, at least in the basin and range area of New Mexico, play an important part in water table recharge and the level of the Rio Grande.

"This has huge implications for development in the basin," Duffy told attendees at the spring meeting of the American Geophysical Union today (May 18) in Montreal. "The role of the water table is important."

In the Llano de Sandia in New Mexico, the Los Pinos Mountains are about 9,000 feet while the basin where the river runs is at about 4,800 feet. An elevation of about a mile separates the mountains from the river. Precipitation in the mountains does not simply run off down the slope, nor does it all seep into the mountain. Part of the water does run off down the slope, but the rest of the water goes deep into the fractured rock beneath the mountains.

"The time between rain on the mountain and recharge of the riverine water table is about 50 years," says Duffy. "The seven-year, 1950s drought in the area is what is now affecting the Rio Grande and the water table."

Duffy is using a dynamical computer model to investigate ground water in central New Mexico. The terrain is divided into three areas, the mountains, the sloping bajada and the riparian or river area. He looks at environmental variables including rainfall, snowpack, evapotranspiration and altitude. Also, important are the geologic porosity and conductance of the rocks. Much of the data is part of the SAHRA – sustainability of semi-arid hydrology and riparian areas – project funded by the National Science Foundation and based at the University of Arizona. Penn State is a partner in the project.

The model is not meant to predict but to be conceptual, helping to shed light on the interaction of precipitation, the mountains, the river and the river environment. One important factor is the riparian environment where trees are always taking water from the river and the water table and, through evapotranspiration, losing it to the air.

"Each year, the trees always take up the same amount of water per square meter," says Duffy. "When upland recharge decreases, water levels in the river will drop all along its run."

The southwestern U.S. has suffered drought conditions for the past few years and last year's snowpack was below normal. Some areas of the Rio Grande already exhibit very low water levels.

"Developers of the mountains and bajada regions need to consider a longer horizon than a decade when planning to alter the natural environment. It may require a forward view of tens of decades to ensure sustainability," says Duffy. "Even if no obvious year round streams run from the mountains, they are still very important for the recharge of the water table and river."

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