Saturday, January 19, 2008

The effects of the rainstorm

New Mexico Tech doctoral student studies basin where slopes drain water to point in Sevilleta National Wildlife Refuge

Argen Duncan El Defensor Chieftain Reporter

A New Mexico Tech doctoral student is doing collaborative research with implications for climate change, land use and home placement.

Hugo Gutierrez Jurado is studying a basin, where slopes drain any water that falls on them to a particular point, in Sevilleta National Wildlife Refuge. Different types of vegetation grow on the hills, depending on whether they face north and received more moisture or face south and get less moisture.

Jurado's technical paper, which was recently published in the journal Geophysical Research Letters, discussed the effects of a July 2006 rainstorm and flood in the study area and their implications.

The less-arid north-facing slopes had less surface runoff and erosion, as well as more plants, Jurado discovered.

"Vegetation really matters in the way runoff and water are released from the soil," he said.

Position on the slope also effects runoff generation and through flow, or water running down the hill under the surface.

"The major conclusion, I think, is the major assemblage of vegetation and soils on each hill slope leads to different runoff mechanisms," said Jurado's adviser, Assistant Professor of Hydrology Enrique Vivoni.

The north-facing slopes retain water and conserve sediment like sponges, and south-facing slopes act like strainers by letting a lot of water and sediment go through them.

"So you wouldn't want to build your house on a south-facing slope," Vivoni said.

South-facing slopes would have less stability because of moving soil and water.

If the climate became hotter and drier, Jurado's conclusions indicate, ecosystems could shift toward more arid vegetation like the creosote bushes on south-facing slopes. The change would lead to more erosion, which would mean more displaced sediment, and would favor flash flooding.

Too much erosion can damage land and decrease its productivity by reducing the ability of grass to grow. With less grass, the dynamics of the ecosystem change, Jurado said.

Also, large amounts of sediment from erosion can dirty water, and flash flooding can damage houses downstream.

Vivoni said the study has a historic context as well. This region had forests and lakes, but has experienced climate change to become a desert.

The north-facing slopes' ecosystems, with juniper trees and grass, are more like the past vegetation, Vivoni said.

Jurado took his data for the paper after a rainstorm dumped just over three inches of water on the site in 18 hours.

"So that accounts for 1/3 of the total annual rainfall in the area," Jurado said.

For that rainstorm, 78 percent of the precipitation became runoff, compared to the normal 10 percent.

Such rainfall events have a probability of happening once in 150-200 years, and scientists believe they help shape the land. The downpours are minimal in terms of the whole area, Jurado said, but in tens of thousands of years, they can make a significant difference.

After the hard rain, Jurado found the water had dug many rills and gullies in the ground on the south-facing slopes but only began to do so on their north-facing counterparts. The plants on north-facing slopes caused the water to run down in sheets, while on the south-facing slopes, the runoff was concentrated into streams, and so dug into the ground more.

Vegetation led to different soil characteristics on north- and south-facing slopes.

Because of more moisture and plants that required more water, Jurado said, the north-facing slopes built up higher levels of organic matter over the years.

"So this will lead to different water retention characteristics of soils," he said.

Jurado said north-facing slopes have more clay and so release water more slowly.

In addition, water traps airborne calcium particles and deposits them in the soil during rain. The calcium can become calcium carbonate and form an impermeable layer in the ground to change the soil moisture profile.

North-facing slopes have more calcium because it reacts with higher levels of carbon from organic matter to form calcium carbonate. More calcium carbonate means water stays in the soil longer, giving plants more time to absorb the moisture.

In turn, this condition favors plant growth.

Jurado's work is part of a larger collaborative effort with Massachusetts Institute of Technology and the University of Nebraska. NASA is funding the project.

aduncan@dchieftain.com

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