Wind Tunnel Assessment
After simulated tillage, the soil trays were kept in an oven until we could assess wind erosion. The trays were then transported to a portable wind tunnel which was located inside a non-regulated climate building. The tunnel was powered by a 33-kW engine and had a working section of 7.3 m long, 1.0 m wide, and 1.2 m tall. A complete description of the design and aerodynamic characteristics of the wind tunnel are given by Pietersma et al. (1996).
Wind speed above the experimental trays during the wind tunnel test was measured using Pitot tubes, which were attached to differential pressure transmitters (616W-5 102280-45 differential pressure transmitter, Dwyer Instruments, Michigan City, IN). The inlets (1 cm in diameter) of pitot tubes were mounted at heights of 0.005, 0.01, 0.02, 0.04, 0.06, and 0.10 m above the soil surface immediately downwind of the soil tray inside the tunnel. Data of pressure transmitters were recorded every 1 s by a data logger. Relative humidity, atmospheric pressure, and air temperature were also monitored near the entrance of the wind tunnel to ensure that experiments were run under consistent atmospheric conditions. A Sensit (Model H11-LIN, Sensit Company, Portland, North Dakota) was used to measure saltation activity at a height of 5 cm downwind of the tray.
Horizontal soil loss was measured over two separate. The first sampling period of each test represented limited saltation conditions (no abrader was added to the air stream) while the second period of the test represented with copious saltation conditions. During the second sampling period, abrader (sand 250-500 μm in diameter) was fed into the air steam at the leading edge of the working section of the tunnel; The abrader flux inside the tunnel was maintained at 0.5 g m-1 s-1 which typifies the flux of soil during high winds across the iPNW (Sharratt et al., 2010). During the first sampling period of each test, freestream wind speed was systematically increased inside the tunnel from 2 to 7.5 m s-1 at a rate of 0.6 m s-1 every 15 s. Freestream wind velocity was then abruptly increased to 12 m s-1 and remained at that wind speed for 180 s so as to enhance soil loss. During the second sampling period, freestream wind velocity was maintained at 12 m s-1 for 180 s to generate soil loss with abrader. Horizontal soil loss was measured using a modified Bagnold type slot sampler (Stetler et al., 1997) which trapped sediment in saltation and suspension to a height of 0.75 m (0.225 cm wide) above the soil surface. A collector, made in-house, was attached to the leeside of the tray to trap particulates creeping along the soil surface. After a completed wind tunnel test, the floor and pitot tubes were cleaned to eliminate the influence of residual dust on the next observation prior to placement of the new experimental soil tray.
Aggregate size distribution in the upper 10 mm of the soil profile was determined on 500g soil samples collected from the tray after each test. After the representative samples were air-dried in a green house, samples were processed through a compact rotary sieve (Chepil,1962) equipped with sieves having 0.045, 0.09, 0.42, 0.85, 2.0, 6.3, and 12.0 mm openings. Aggregates, ranging from 12.7–19.0mm size in diameter, were used to determine the aggregate crushing energy (Hagen et al., 1992). Aggregates crushing energy was determined by the mass of the aggregate being crushed and crushing energy imparted to the aggregate, which recently has been measured by a commercial penetrometer (Mohr Digi-Test, hereafter MDT, Mohr and Associates, Inc. Richland, WA) (Pi et al., 2020a). The energy imparted to the aggregate was determined by the force applied to the aggregate and displacement of the force. The energy or work ( W with unit of J ) can be described as:
W \(=\int_{a}^{b}{F(x)}\) (1)
where F (x) is the force imparted to the aggregate and x(m) is the distance over which the force was imparted to the aggregate (displacement) from point a to b. The force imparted to the aggregate was distinguished as initial break force and final force. The force being applied to the crust at the time of fracturing is called the initial break force while the force being applied to crush the crust is called the final force or crushing force (Skidmore and Layton, 1992). However, the final force is often difficult to detect, especially for some weak aggregates (Pi et al., 2019b). The force at 1.5 times the initial break force has been empirically used to estimate final force (Hagen et al.,1995). The typical initial break force and final force as a function of displacement is illustrated in Figure 2, which shows the typical force (N) versus displacement (mm) curves during crushing of aggregates with 4% and 8% clay amendment for four soil types. The average aggregate crushing energy of each treatment was determined for a minimum of 10 aggregates. Crust stability and aggregate stability are equal according to Hagen et al. (1992), because both are controlled by the magnitude of cohesive forces between soil particles (Amézketa,1999; Saleh, 1993).
Soil surface water content and random roughness are key factors that influences soil loss and were assumed to vary across treatments. Both parameters were measured after each wind tunnel test. Soil water content was calculated by the reduction in weight of a soil sample after drying. A portable pin-type profile meter (Allmaras et al., 1966) was used to measure the random roughness. The meter was comprised of a rigid frame with 1 m long and 40 pins that moved vertically through holes in the frame. Forty surface elevations were measured during each test. Random roughness was thus determined based on the standard deviation of height elevations after adjust for surface slope (Zobeck, 2001).
Statistical analysis
A one-way ANOVA was used to examine the effect of clay amendment on soil loss using commercial software (SPSS Statistics 20.0; the SPSS Inc., Chicago, IL). Normality tests were conducted prior to the ANOVA tests. Regression analysis was used to examine the relationship among clay amendment, crust crushing energy, random roughness, aggregate GMD and crushing energy, and soil loss.