Blow Sand Areas
We have known for sometime that large-scale erosion in this part of the Mojave Desert emanates from blow sand areas. Because these areas do not crust over time, they remain unstable and erodible. During certain meteorological periods, such as occurred during the severe drought years from 1985 to 1992, and more recently from 1998 to 2002, these areas, as in a domino effect, seriously erode downwind stable but non-vegetated areas. Eventually, large areas of the disturbed desert begin blowing.
Quantifying Emissions from Blow Sand Areas
During 2002-2003, we manually identified two potential blow sand areas for additional research studies. The objective is to determine how much material is being eroded during high wind events, as a function of wind speed and fetch. The temporal and areal extent of sand migration is also being measured, together with the reduction of sand transport across naturally vegetated areas downwind. In addition, the penetration of the downwind sand fetch edge into the adjacent stable vegetative area is being investigated.
Using the expertise of MRI and borrowing from the knowledge of the Owens Lake study, we have instrumented one 25-acre site and are in the process of instrumenting the second site. Each site has a sand fetch with dimensions approximately 500 feet parallel to the prevailing wind (west) and 1200 feet perpendicular to the prevailing wind (Figure 2). Adjacent to this sand fetch in the prevailing downwind direction is an area of equivalent size which is stabilized with native vegetation and/or mulch/woodchips. At each site, we have deployed an array of 18 Cox sand catchers, three immediately upwind of the sand fetch, 6 inside the sand fetch (two rows of three) and the remaining 9 downwind of the sand fetch in the stable land (three rows of three). The distance between the samplers within each row is determined empirically from the spread in wind direction from due west or 270° which is typically the direction of high eroding winds. At each blow sand site we have installed a continuously operating 10-meter anemometer to provide wind speed and direction.
The sand masses collected by the Cox sand catchers are periodically retrieved and correlated with the measurements obtained from a collocated Sensit. The recording surface of the solar powered Sensit is at the same 15 cm height above the surface as the Cox sand catchers. The Sensit provides a temporal history of the sand flux, both in saltating particle counts/min and kinetic energy. It is also used to determine the threshold wind velocity that initiates movement of saltating sand. This value will probably change seasonally because of the soil moisture content. A minimum two windy season study will help determine the intensity and frequency of saltation and erosion from the two representative blow sand areas.
Figure 3 displays preliminary sand catcher results for a 6 week period from May through June 2003. The units are in inches of sand observed in the transparent collection tube. Approximately 68 g of sand is correlated with one inch of sand observed in the tube. Ultimately these sand masses will be converted to sand flux values. The prevailing wind direction is from the west. The blow sand research site collected data in an orderly manner. Only traces of sand were captured by the sand catchers upwind of the sand flux area, while increasing amounts were measure as the samplers were located further into the sand flux area. The sand amounts are also dependent on the height of the sand catcher inlet. Originally set at 15 cm above the ground, scouring around the collection tubes tends to increase this height.
After only about a year, the one foot thick blow sand was eroded down to the consolidated soil beneath. As the sand fetch area was eroded over this period, the sand transported by saltation accumulated along the border of the vegetated area immediately downwind of the sand fetch area (Figure 3). A relatively stable row of sand dunes (about 1-2 feet high) with a downwind distance of less than 100 feet was formed (denoted by “Dunes in Figure 3). These dunes were stabilized by native vegetation so that only relatively small sand fluxes were observed downwind of the row of dunes (Figure 3).