Casey Thornbrugh

Between grazing land and shifting sand

 

About the Research

Photograph of Casey Thornbrugh
Casey Thornbrugh, Ph.D. candidate, Department of Atmospheric Science, University of Arizona. Thornbrugh is a UCAR SOARS alumnus. (Photo courtesy University of Arizona.)

 

Ask Casey Thornbrugh about the fragile state of Navajo grazing lands in Arizona and he'll tell you that "the sustainability of the land is essential to the cultural and economic well-being of those who reside on it." This principle fires his work studying sand dune mobility on the Moenkopi Plateau in Arizona, a semiarid region of rolling sandhills on the southwestern side of the Navajo Nation.

In their stable state, the plateau's sandhills are covered with grasses or shrubs, providing sparse grazing for the Navajos' cattle and horses. If that cover is lost, the sandhills may become active sand dunes, barren heaps of sand that offer no feed for livestock and make transportation difficult. Winds blow these active dunes slowly across the plateau, blocking roads and threatening any buildings in their path.

Although active dunes are easily created, replanting and restabilizing them is extremely difficult.

Thornbrugh's main research tool is a widely used index of sand dune mobility, developed by Nicholas Lancaster of the Desert Research Institute. Lancaster applied the index to the Moenkopi Plateau for the years 1986 to 1997, using meteorological data from a weather station installed in 1979 for the Desert Winds Project. (Also see Geometeorological data collected by the USGS Desert Winds Project.)

Under the direction of Margaret Hiza Redsteer (U.S. Geological Survey), Thornbrugh has extended the index—and the meteorological record from the still-functioning weather station—to 2005 to observe the effects of a decade of drought. He has found that, while it correlated well with the earlier years, "the index has some challenges when you apply it from 1997 onward. The reason has to do with the time scale that it's applied to and the nature of precipitation in northern Arizona."

Map in black and white of a part of Arizona
The Moenkopi Plateau in Arizona. The USGS station at Gold Spring provides meteorological data for Thornbrugh's research. (Image courtesy U.S. Geological Survey.)

In a drought, heavy storms alter the index

The last decade has seen almost unrelenting drought in the Southwest. When rain did fall in northern Arizona, though, it sometimes came in extremely heavy individual storms. For example, two record storms in September 2002 provided 45–50% of that year's total precipitation. Applying the mobility index to the 2002 annual data, Thornbrugh obtained relatively low values, supposedly indicating that it was a year with low sand dune growth.

Photos and communication with local residents in 2002, however, demonstrated just the opposite. The parched dirt was too compacted to absorb the heavy rainfall and instead simply washed away, exposing the sand beneath.

Thornbrugh's considering ways to modify the index to account for this precipitation pattern. It might seem logical simply to apply the index to shorter periods of time to avoid dealing with the extreme events, but that doesn't work because in the real world it takes longer amounts of time for vegetation to die back and active dunes to develop.

Future work: The potential for seasonal forecasts

One of Thornbrugh's goals is to use the index in connection with climate forecasts.

Others have combined index data with global model output to forecast dune growth in the coming decades in the U.S. Great Plains, but when it comes to the Southwest, the models disagree greatly about how precipitation will change. He would prefer to use the index with shorter-term forecasts.

"What people would really find useful, especially people who have livestock, is using the index with shorter-term climate data, for example, getting an outlook in the winter of what the coming spring and summer are anticipated to be."

Thornbrugh, who is Native American, hopes that his project will benefit the people of the Moenkopi Plateau and throughout the Navajo Nation. With their deeply ingrained tradition of "belonging to"—not owning—the land, the Navajos will be looking for additional resources and information that they can apply to protect their fragile environment. "Climate change has happened to these people before, and they've dealt with it. In the communities I work with, people are really positive. They will get really upset with you if you just present the problem. The best presentation is, 'This is what might happen, and here are some solutions that have worked in other places.' "


The dune mobility index

"It's very simple, really," is how Nicholas Lancaster explains his dune mobility index, developed in 1988. "Basically, the movement of dunes is directly proportional to the presence of strong winds and inversely proportional to the presence of vegetation." In other words, if there's a lot of wind and not much vegetation, dunes blow away; if there's plenty of groundcover and not much wind, they don't.

The formula is:   M  =    W
P/PET
 

where M is dune mobility, W is the percentage of time that the wind blows about the threshold velocity for sand transport, P is annual rainfall, and PET is potential evapotranspiration (a measure of the atmosphere's ability to remove water from plants).

The wind threshold velocity varies according to the fineness or coarseness of the sand being studied. Solving the equation with data from a stable, plant- or grass-covered area, such as the Nebraska Sandhills, will give a low value—say, around 50. Values of 200 or higher indicate a great danger that the region will degrade into barren, blowing sand. Using northern Arizona data from drought years, Thornbrugh has seen values as high as 400.

Although the index accounts for the major factors affecting sand dune mobility, there are other climatological and land-use factors that would need to be included for a truly complete forecast, such as whether precipitation falls as rain or snow and what kinds of livestock graze on the sandhills.

May 2007