The Longterm Influence of Mountain Rainfall

ARLINGTON, Virginia, May 19, 2004 (ENS) - Rainfall in the mountains has a major influence on nearby river levels, and its effects can be seen as long as 50 years after the rain has fallen, hydrologists say.

Scientists had believed that the downslope distance from a mountain to a river is not significant, and that rain falling on a mountaintop does not have an impact on a river below, according to Christopher Duffy, a civil engineer at Pennsylvania State University.

But Duffy has found that rainfall and snowfall over the mountains, at least in the basin and range area of New Mexico, play an important part in recharge of the water table and the Rio Grande River.

"This has huge implications for development," Duffy told attendees Tuesday at the spring meeting of the American Geophysical Union in Montreal, Canada. "The role of the water table is important."

His study offers a means to guide planning for sustained aquifer management. The United States now pumps some 28 trillion gallons of groundwater every year with little regard for how this affects the hydrological cycle.

Duffy used a computer model to investigate groundwater in central New Mexico, dividing the terrain into three areas - mountains, sloping bajada, and riparian or river area.

His study, funded by the National Science Foundation, examines environmental variables including rainfall, snowpack, evapotranspiration and altitude, in addition to the geologic porosity and conductance of the rocks.

In the Llano de Sandia in New Mexico, the Los Pinos Mountains are about 9,000 feet high while the area below in which the river runs is at an elevation of about 4,800 feet.

About a mile separates the mountains from the river - precipitation in these mountains does not all run downslope, nor does it all seep into the mountains, Duffy explained.

Some of the water goes deep into fractured rocks beneath the mountains.

"The time between rainfall on the mountains and ultimate recharging of the riverine water table is about 50 years," he said. "The seven year, 1950s drought in the area is what is now affecting the Rio Grande and the water table."

The study means developers of New Mexico's mountains and bajada regions "need to consider a longer time horizon than a decade when planning to alter the natural environment," Duffy added. "It may require a forward view of tens of decades to ensure sustainability. 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."