Impacts of Climate Change and Land Use  on the Southwestern United States

Impacts of Climate Change on the Land Surface

Reactivation of Stabilized Sand Dunes on the Colorado Plateau

by
Daniel R. Muhs and Josh M. Been
U.S. Geological Survey


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Figure 1

Distribution of Eolian Sand in the Southwest

Sand dunes and eolian (wind-blown) sheet sands are widely distributed over the southwestern United States, particularly in the southern Great Plains and the southwestern deserts and high plateaus (FIGURE 1). In the driest parts of the southwest, there are areas of active sand dunes, but most parts have dunes that are stabilized by vegetation and the sand is not moving at present.
References used to compile the eolian sand deposits map of the Southwestern U.S.

 

Figure 2

Factors Controlling Sand Mobility

For actively moving sand dunes, there are three requirements (FIGURE 2): a source of sand, winds that are strong enough to move the sand, and a lack of stabilizing vegetation. Depending on the relative balance of these three variables, different dune types may result, including U-shaped dunes (parabolic dunes) with a moderate sand supply but abundant vegetation; linear or longitudinal dunes with a minimal sand supply but strong winds; and ridges with their long axes perpendicular to the wind (transverse dunes) where there is abundant sand and little or no vegetation. Dunes of all three types are found in the southwestern United States. If sand supply is minimal, winds are weak, and/or there is abundant vegetation, no dunes will form.

Sand Deposits and Wind

In the southwestern United States, the largest area of sand dunes is actually not in the deserts, but in the Colorado Plateau region, centered on the four corners area (FIGURES 1 and 3). Sand supplies here are abundant from both sandstone bedrock and dry river channels. In this area winds capable of moving sand are dominantly from the southwest (FIGURE 3). Compared to desert areas around the world where large sand seas are found, the Colorado Plateau has winds capable of moving sand (drift potential) that are very similar (FIGURE 4).

Figure 4

Figure 3

Precipitation/Evapotranspiration Balance

Why aren't the sand dunes on the Colorado Plateau active even though sand supplies are available, and winds are strong enough to move the sand?

The answer is that the balance between precipitation (P) and potential evapotranspiration (PE) is such that vegetation can grow on the dunes and stabilize them. Although the Colorado Plateau has an overall moisture deficit (where the ratio of P to PE is less than 1.0), there is still enough moisture to support plants. In the areas where dunes occur, most P/PE values are around 0.3 to 0.5, which is high enough to support sagebrush and grasses at lower elevations and pinyon pine and juniper at higher elevations (FIGURE 5). Therefore, the reason that dunes on the Colorado Plateau are stable is due to the P/PE balance and its support of stabilizing vegetation.

Figure 5 Is it possible that future climate changes could change the P/PE balance and result in the removal of vegetation and reactivation of the dunes?

Figure 6

This question can best be answered by examining the climatic characteristics of areas with active sand dunes. Studies of stabilized sand dunes in the Great Plains grasslands, mostly active dunes in the Chihuahuan Desert, and fully active dunes in the Sonoran Desert (FIGURE 1) show that there are definite trends toward greater dune activity in drier regions (FIGURE 6). When the P/PE value is low and the percentage of time that wind is capable of moving sand (W) is high, dunes are fully active. The ratio of W to the P/PE value is referred to as the dune mobility index, and has been tested in many regions for its ability to describe the degree of dune activity as a function of climate variables. In fact, it appears that wind, as expressed by W, is actually not as critical as the P/PE value (FIGURE 6). The moisture balance, through its effect on vegetation, seems to be the best indicator of dune activity. Therefore, with a decrease in the P/PE value, we could expect dunes to become more active in the future.

Climate Change and Sand Mobility

Figure 7 Within recent history, has the climate ever been dry enough for dunes on the Colorado Plateau to be active?

Using the dune mobility index and the historic instrumental record, we can examine this possibility. In the past century, the worst drought, which was accompanied by higher-than-average temperatures, occurred in the years 1899-1904, which is shown on the rainfall graph as both a year-by-year and smoothed curve in FIGURE 7. If we calculate the dune mobility index values for W, P, and PE for the Colorado Plateau now (using average values for 1961-1990), we can see that dunes fall into the category of being partly active, but mostly stable, which is what we observe there today (FIGURE 8). If we recalculate the dune mobility index values using data from the 1899-1904 drought, the values are shifted into the category of mostly active dunes. It is possible that some of the dune activity we see today on the Colorado Plateau is actually a remnant of greater activity during that drought, and really doesn't reflect modern conditions at all. In any case, greater dune activity could be expected in such a drought.

Figure 8

What is the impact of active sand dunes on the Colorado Plateau?

The biggest impacts of active sand dunes in the region would be on the Navajo and Hopi people, whose reservation land is either on, or downwind of, the largest areas of sand dunes (FIGURE 3). Many Navajo and Hopi homes are on or near sand dunes; reactivation of dunes would obviously have a negative effect on living conditions. Sheep and cattle are important to the economy of the Navajo and Hopi, and much of the vegetation required for grazing is dune vegetation. In addition, dry farming is practiced in much of the area, some of it on sand dunes. Thus, reactivation of sand dunes in the area would have serious impacts on living conditions, grazing, and farming.

For additional reading:

Breed, C.S., McCauley, J.F., Breed, W.J., McCauley, C.K., and Cotera, A.S., Jr. 1984. Eolian (wind-formed) landscapes. In: Smiley, T.L. et al. (eds.), Landscapes of Arizona--The Geological Story. New York, University Press of America, p. 359-413.

Hack, J.T. 1941. Dunes of the Navajo Country. Geographical Review, v. 31, p. 240-263.

Lancaster, N. 1988. Development of linear dunes in the southwestern Kalahari, southern Africa. Journal of Arid Environments, v. 14, p. 233-244. McKee, E.D. (ed.). 1979. A Study of Global Sand Seas. U.S. Geological Survey Professional Paper 1052, 429 p.

Muhs, D.R., and Holliday, V.T. 1995. Evidence of active dune sand on the Great Plains in the 19th century from accounts of early explorers. Quaternary Research, v. 43, p. 198-208.

Muhs, D.R., Bush, C.A., Cowherd, S.D., and Mahan, S. 1995. Geomorphic and geochemical evidence for the source of sand in the Algodones dunes, Colorado Desert, southeastern California. In Desert aeolian processes. Edited by V.P. Tchakerian. Chapman and Hall, London, p. 37-74.


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