Sand Deposits and WindIn 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).
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.
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?|
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.
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.
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.
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