This map portrays two different types of dust data and is assembled from several different sources. The colored contour lines are based on predicted rates of dust emission, whereas the colored dots represent measured rates of dust deposition.
The predicted rates of dust emission were calculated by combining two sets of model values for dust in the <20 µm size range provided by D.A. Gillette (U.S.EPA). One set of model values was derived from a dust-production model primarily using data and parameters from agricultural soils and regions (Gillette and Hanson, 1989). This set was combined with data from another model that estimated dust emission caused by dust devils (Gillette and Sinclair, 1990). The model values were calculated at a grid spacing of 81.5 km for the entire United States; we cropped the areal coverage to points west of 100° longitude and south of 42° latitude, and re-contoured the data with GIS techniques to produce the colored contours on our map. The 100th meridian is generally accepted as the eastern limit of the arid regions of the United States.
The measured rates of dust deposition (flux) were obtained from several different published and unpublished sources. These studies (Table 1) used different types of collectors, the collectors were at different heights above the ground, and in some cases the sources gave only the deposition rate for silt and clay combined (i.e., <50 µm size range). No effort was made to adjust for different collector heights, on the assumption that proximity to the ground surface would only seriously affect measurements of sand flux and would have little effect on the <20 µm flux. Comparison of the flux values from 61 sites in southern Nevada and California (Reheis and Kihl, 1995) showed that on average, the <20 µm flux was 65 percent (standard deviation=12 percent) of the <50 µm flux. This conversion factor was used to estimate the <20 µm flux for data sets reporting only the <50 µm flux (all other sites excepting those in central and southern Texas). Four dust traps in the data set of Rabenhorst et al. (1984) and R. Drees were located slightly east of the 100th meridian and are plotted on the map even though they fall outside the eastern limit of the dust-emission contours.
|Study area||Collector type||Height||Length of record||No. of sites||<20 µm flux measured?||Data source|
|Southern Nevada and California||Marbles in pan||2 m||11 years||61||yes||Reheis and Kihl (1995)|
|Owens Valley (east central Calif.)||Marbles in pan||2 m||4 years||7||yes||Reheis (in press)|
|Northern Nevada||Marbles in pan||2 m||1 year||1||yes||Reheis, unpub. data|
|Channel Islands||Marbles in pan||0 m||1 year||2||no||Muhs (1983)|
|Tempe, Arizona||House roof||3-5 m||1 year||1||no||Péwé et al. (1981)|
|Las Cruces, New Mexico||Marbles in pan||0.9 m||5-10 years||7||no||Gile et al. (1981)|
|Texas Panhandle||Glycerol in glass jars||1.4 m||6 months||5||no||Machenberg (1987)|
|Central and southern Texas||Plastic spheres in buckets||1.5 m||3 years||13||yes||Rabenhorst et al. (1984); R. Drees, unpub. data|
The predicted emissions and measured fluxes correspond in many places on the map. However, the contour lines represent predicted dust emission whereas the points represent measured dust deposition. Thus, a non-correspondence of line and dot colors may mean that either (1) the actual rates of dust emission are different (usually higher) than the predicted rates, or (2) dust emitted from a source area is deposited downwind in areas where less dust is emitted. At several sites in Texas, for example, dust flux (deposition rate) is much higher than the predicted emission rates. The high flux rates in central Texas are probably due to downwind transport of dust derived from the western part of the Panhandle and adjacent areas. The high flux rates in southern Texas, however, could be due either to southerly transport of dust from the Panhandle or to an under-estimation of dust emissions in this area.
Under-estimation of dust emissions is the likely explanation for large mismatches of predicted emissions and measured fluxes in southern California. The modeled dust emissions do not adequately represent wind erosion and dust production in natural (uncultivated) desert areas, because the equations are primarily based on data from agricultural regions, the majority of which are not in desert areas (D. A. Gillette, oral commun., 1997). In addition, local factors cause some of the anomalies. For example, the red dots labeled 25.0 and 16.3 in south-central California are in the Transverse Ranges and reflect dust derived from the Mojave Desert to the east that is funneled through passes by the Santa Ana winds (Reheis and Kihl, 1995). This dust is transported westward at least as far as the Channel Islands (Muhs, 1983). The extremely high value in Owens Valley (note the red dot labeled 198.3) is produced by dust storms from the artificially desiccated Owens (dry) Lake.
Drees, Richard, 1997, written communication: Texas A&M University, Department of Soil and Crop Sciences.
Gile, L. H., J. W. Hawley, and R. B. Grossman, 1981, Soils and geomorphology in the Basin and Range area of southern New MexicoGuidebook to the Desert Project, New Mexico Bureau of Mines and Mineral Resources Memoir 39, 222 p.
Gillette, D. A., and K. J. Hanson, 1989, Spatial and temporal variability of dust production caused by wind erosion in the United States, Journal of Geophysical Research, v. 94, p. 2197-2206.
Gillette, D. A., and P. C. Sinclair, 1990, Estimation of suspension of alkaline material by dust devils in the United States, Atmospheric Environment, v. 24(A), p. 1135-1142.
Machenberg, M. D., 1987, Analysis of dust-trap samples collected on the southern High Plains and adjacent areas, in Gustavson, T. C., ed., Geology and geohydrology of the Palo Duro Basin, Texas Panhandle: Report on the Progress of Nuclear Waste Isolation Feasibility Studies: Texas Bureau of Economic Geology OF-WTWI-1985-49, p. 337-343.
Muhs, D.R., 1983, Airborne dustfall on the California Channel Islands, U.S.A: Journal of Arid Environments, v. 6, p. 223-238.
Péwé, T. L., E. A. Péwé, R. H. Péwé, A. Journaux, and R. M. Slatt, 1981, Desert dust: Characteristics and rates of deposition in central Arizona, U.S.A., in Péwé, T. L., ed., Desert Dust: Origin, Characteristics, and Effect on Man: Geological Society of America Special Paper 186, p. 169-190.
Rabenhorst, M.C., Wilding, L.P., and Girdner, C.L., 1984, Airborne dusts in the Edwards Plateau region of Texas: Soil Science Society of America Journal, v. 48, p. 621-627.
Reheis, M.C., and Kihl, R., 1995, Dust deposition in southern Nevada and California, 1984-1989: Relations to climate, source area, and source lithology: Journal of Geophysical Research, v.100, p. 8893-8918.
Reheis, M.C., in press, Dust deposition downwind of Owens (dry) Lake, 1991-1994: Preliminary findings: Journal of Geophysical Research.