D. W. Fryrear

Soil Cover and Wind Erosion

ABSTRACT WIND erosion on agricultural lands can be reduced if the soil surface is protected with crop residues. In evaluating the influence of residues on wind erosion, previous research has expressed residues of various crops as an equivalent of flat, small grain. This becomes difficult as the density of the residue changes with weathering, or as crops other than the major cultivated crops are grown. Soil losses due to wind erosion were determined by covering various percentages of the soil surface with simulated flat residues (wood dowels 3.1 to 25.4 mm in diameter). Covering 20% of the soil surface reduced soil losses 57%, and a 50% cover reduced soil losses 95%. The expression SLR = 1.81 e-°°72% scdescribes the relationship between soil loss ratio (SLR) and percent soil cover (% SC) with a correlation coefficient of —0.94 (soil cover limits 8 to 80%). The cover can be any nonerodible material such as large clods, gravel, cotton gin trash, or any diameter stick between 3.1 and 25.4 mm. Percent soil cover is easily measured in the field or can be estimated with a minimum of training and experience.

FIELD WIND EROSION: VERTICAL DISTRIBUTION

The quantity of soil material transported by wind will decrease with height above the surface. Airborne samples were obtained at five levels above four soils. Mass distribution with height differs for materials in saltation or in suspension. A technique was developed to mathematically describe the two modes of transport so the total quantity being transported could be determined by integrating the two equations. Transition height, where the transport mode changes from saltation to suspension, is termed TSS. TSS decreases as surface soil texture changes from a fine sandy loam to a loam, indicating a higher percentage of the eroded material is moving at a greater distance above the soil surface. TSS increases as roughness of the soil surface increases, indicating a decrease in the suspension component and that the majority of eroded material is moving close to the soil surface.

Wind Erosion: Field Measurement And Analysis

ABSTRACT Wind erosion researchers need field equipment and techniques for ascertaining threshold wind velocities and the amount and vertical distribution of the eroded soil particles. To detect moving soil particles and field erosion, sensors and soil samplers to measure surface creep and airborne particles have been developed. A power expression will describe the variation in amounts of suspended material to a 2-m height. The quantity of material (f) and height of material (y) within the saltation zone can be explained by the expression f = fo(l-y/a)P where fo is surface creep, a is height below which 50% of the total mass flow occurs in the saltation process, and p is the slope of the line. With this equipment and the analytical techniques described, the wind erosion process can be studied in the field, and the effectiveness of wind erosion control systems can be evaluated.

Soil Ridges-Clods and Wind Erosion

ABSTRACT A portable wind tunnel was used to evaluate soil losses in 10 minutes from a flat surface, and from ridges 63 to 254 mm high when 0 to 60% of the soil surface was covered with nonerodible clods. Soil losses were reduced 90% with soil ridges 63 to 254 mm high, 89% with nonerodible soil clods covering 60% of the soil surface, and 98% with a combination of the larger ridges and 40% clods. Results of bare soil ridges were very similar to wind tunnel results of three other studies.

Performance of a windblown-particle sampler.

ABSTRACT Asampler was developed to trap windblown particles for a wind erosion field study, and an experimental investigation was undertaken to determine its performance characteristics. A laboratory wind tunnel was employed to ascertain whether the sampler was sampling isokinetically, efficiently, and nonselectively. Experiments have demonstrated that in order for there to be proper flow into the sampler there must be proper flow out of the sampler. This not only requires a vent to exhaust the air but also requires the use of the external wind energy to pull the air through the samplers diffuser. It was found that by adjusting the size of the ventilation screens, located at the rear of the sampler, a variation of the inlet flow could be produced. There appeared to be an optimum ventilation screen size for which the inlet flow was isokinetic. The magnitude of the inlet velocity affected the trapping efficiency of the total mass and the trapping efficiency for a given particle size range.

Wind erosion : Field length

As the wind passes over eroding soil surfaces, the quantity of soil transported by wind between the soil surface and 2 m above the surface increases until the wind stream becomes saturated. The relationship between field length and the mass transported was used to test a transport model that has the form of a sigmoid curve. At the upwind portion of the field, the rate of increase in mass being transported is limited by the emission rate from the soil surface. The mathematics of the sigmoid equation implies that when 63% of the maximum transport capacity of the wind has been satisfied (field length at this point is called critical field length), the rate of increase in transport mass gradually decreases and approaches zero when the wind is saturated with soil particles. From the instrumented field sites, a maximum transport rate of 1231 kg/m width and average soil losses of 7.801 kg/m 2 were measured from a single event on a 2.6-ha field at Elkhart, Kansas. Critical field length varied from 31 to 129 m, depending on wind velocity and field conditions. Field mass transport data supports that a self-balancing mechanism of momentum extraction from the surface wind by accelerating soil particles and the associated shear stress reduction will limit the quantity of material that can be transported between the soil surface and a height of 2 meters.

Using Two Sieves to Characterize Dry Soil Aggregate Size Distribution

ABSTRACT SURFACE soil aggregate size distribution affects many facets of agriculture from wind erosion susceptibility to seedbed suitability. The log-normal distribution generally provides a good description of aggregated soil size distribution. Unfortunately, other measures of aggregation have often been adopted, because they were perceived as easier to apply. In this study, a method to calculate the geometric mean diameter, Dg, and geometric standard deviation from two sieve cuts was developed for log-normal distributions. Results from 10 soil samples using the two-sieve procedure were compared to results from the same samples using multiple seive cuts. The multiple sieve data were analyzed using both a traditional graphical and a linearized least-squares procedure to predict Dg and percentage aggregates greater than 0.84 mm. All the methods gave nearly equal size distribution parameters. The two-sieve procedure is least laborious but does not permit easy detection of samples that deviate from a log-normal distribution.

Modeling the roughness properties of artificial soil clods

Knowledge of the aerodynamic roughness length (Z0) resulting from management practices that include soil clods, vegetation, and ridges is essential to describing their protective roles in wind erosion. The usual method of obtaining Z0 from a logarithmic wind profile has limited accuracy because the