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K. Ikazaki, H. Shinjo, U. Tanaka, S. Tobita, T. Kosaki
ABSTRACT. Wind can erode fertile topsoil and reduce soil fertility. Evaluating the effect of wind erosion on soil fertility is
crucial to achieve sustainable agriculture in areas suffering from desertification caused by wind erosion. To estimate soil loss and associated soil nutrient loss by wind erosion, flux of coarse organic matter (COM) (defined here as free organic debris larger than 200 mm) and soil particles (defined as the other soil components) must be measured separately. This is because their modes of transport are different, and COM plays a prominent role in soil nutrient dynamics in some semiarid zones where COM accounts for a large percentage of the total soil carbon. Because the Big Spring Number Eight (BSNE) sampler can trap both COM and soil particles 0.05 m above the surface, we designed a sediment catcher, the Aeolian Materials Sampler (AMS), to trap these components below 0.05 m. This device can be manufactured easily at low cost. AMS performance was tested by wind tunnel experiments over a range of wind velocities typically observed in erosive storms and with six incident wind angles because the AMS is a buried‐type sampler that is unable to rotate toward the wind. The trapping efficiency of the AMS for COM and soil particles was not 100%, but it can be calibrated easily using wind data. Therefore, we can estimate the mass flux of COM and soil particles and evaluate the effect of wind erosion on soil fertility using the AMS with the BSNE sampler.
Keywords. Coarse organic matter (COM), Sediment catcher, Semi‐arid, Soil fertility, Wind erosion.
ind erosion is a major contributor to deserti‐ fication (UNEP, 1997). Because wind de‐ taches and removes the topsoil, which is usually more fertile than the subsoil, from ar‐ able land, wind erosion affects soil fertility (Daniel and Lang‐ ham, 1936; Zobeck and Fryrear, 1986; Sterk et al., 1996; Larney et al., 1998; Bielders et al., 2002) and crop productiv‐ ity (Lyles, 1975; Larney et al. 1998). Therefore, it is crucial to evaluate the effect of wind erosion on soil fertility and crop productivity to achieve sustainable agriculture in areas suf‐ fering from desertification caused by wind erosion.
To estimate soil loss by wind erosion, free organic debris should be considered separately from soil particles because its mode of transport by wind is different from that of soil par‐ ticles owing to higher sensitivity to airflow. In this article, we define coarse organic matter (COM) as free organic debris
>200 m in diameter that consists mainly of plant residue,
Submitted for review in April 2008 as manuscript number SW 7455; approved for publication by the Soil & Water Division of ASABE in January 2009.
The authors are Kenta Ikazaki, ASABE Member, Graduate Student, Laboratory of Soil Science, Graduate School of Agriculture, Kyoto University, Kyoto, Japan, and Research Fellow, Japan Society for the Promotion of Science, Tokyo, Japan; Hitoshi Shinjo, Assistant Professor, Graduate School of Agriculture, Kyoto University, Kyoto, Japan; Ueru Tanaka, Associate Professor, Graduate School of Global Environmental Studies, Kyoto University, Kyoto, Japan; Satoshi Tobita, Senior Researcher, Japan International Research Center for Agricultural Sciences (JIRCAS), Tsukuba, Japan; and Takashi Kosaki, Professor, Graduate School of Global Environmental Studies, Kyoto University, Kyoto, Japan. Corresponding author: Kenta Ikazaki , Laboratory of Soil Science, Graduate School of Agriculture, Kyoto University, Kitashirakawa Oiwakecyo Sakyo‐ku, Kyoto 606‐8502, Japan, phone: +81‐75‐753‐6101; fax: +81‐75‐753‐6103; e‐mail: [email protected]‐u.ac.jp.
whereas we define soil particles as the remaining soil compo‐
nents that are composed of free organic debris <200 m and
soil particles. We determined the lower limit of COM to be
200 m because the amount of organic debris <200 m was
small in the test soil of this study. This COM is conceptually similar to the free light fraction (free LF) defined by Six et al. (1998) as particulate organic matter existing between aggre‐ gates. The difference between COM and free LF is that COM includes organic debris >2.0 mm in diameter because it con‐ tains a considerable amount of plant nutrients in some envi‐ ronments.
Because COM and free LF account for a large percentage of the total soil carbon in some semiarid zones (Shang and Tiessen, 1998; Steffens et al., 2007) and thus play a promi‐ nent role in soil nutrient dynamics, it is essential to measure COM flux to elucidate the effect of wind erosion on soil nutri‐ ent status. In the Sahel region of West Africa where deserti‐ fication by wind erosion is severe (Lal, 1993), the potential soil loss by wind from a cultivated field was estimated to be
5.0 mm year-1 (Bielders et al., 2000). In this topsoil with a
thickness of 5.0 mm, we found that COM accounted for 35% to 52% of the total carbon and 22% to 34% of the total nitro‐ gen (Ikazaki, not published).
The Big Spring Number Eight (BSNE) sampler (Fryrear, 1986) is the most‐used sediment catcher for measuring soil flux 0.05 m above the surface in field conditions. Owing to its near‐isokinetic characteristic (Fryrear, 1986; Shao et al., 1993; Goossens et al., 2000), the BSNE sampler can trap both COM and soil particles with high accuracy. Because large sediment flux occurs near the soil surface (Stout and Zobeck, 1996; Rasmussen and Mikkelsen, 1998) and mass flux calcu‐ lated using only the BSNE sampler would bear uncertainty (Zobeck et al., 2003), Stout's near‐surface passive creep/ saltation sampler (Stout and Fryrear, 1989) is recommended
for measuring the flux below 0.05 m. However, Stout's sam‐ pler is expensive and requires certain expertise to manufac‐ ture, which is not available in some developing countries. Therefore, in this study, we designed a non‐rotating sediment catcher that can be manufactured easily at low cost to mea‐ sure COM flux and soil particle flux below 0.05 m. We veri‐ fied its performance using a wind tunnel experiment. This sampler should be used in combination with the BSNE sam‐ pler to measure the flux from the ground to 1.0 m above the surface.
M
ATERIALS ANDM
ETHODSSAMPLER DESIGN
We designed a wedge‐shaped (20° internal inlet angle)
passive creep/saltation sediment catcher (fig. 1), following earlier designs by Bagnold (1941), Greeley et al. (1982), Fry‐ rear (1986), Stout and Fryrear (1989), Nickling and McKen‐ na Neuman (1997), Bauer and Namikas (1998), and Rasmussen and Mikkelsen (1998), and named it the Aeolian Materials Sampler (AMS). It is a buried‐type sampler such as the Guelph trap (Bauer and Namikas, 1998) and consists of a container, collection pan, and lid (fig. 1). To use the AMS: (1) the container is installed in the ground up to the dotted lines in figures 1c and 1d, (2) the collection pan is packed into the container (fig. 1b), and (3) the lid is put on the container (figs. 1c and 1d). When collecting the sample, we first took off the lid and then removed the collection pan from the con‐ tainer. Because the container remained in the soil, we did not need to dig another hole for every collection.
The AMS can easily be manufactured of galvanized sheet metal at low cost (e.g., it cost 31 euros to manufacture one
AMS in Niger in 2007). These characteristics are important for measuring wind erosion in developing countries under field conditions, where the variability of mass transport over space requires many samplers as replications (Sterk and Stein, 1997; Visser et al., 2004). In addition, manufacturing a sampler near the study site will enable us to save transporta‐ tion cost.
The AMS depends on gravitational settling to trap aeolian materials. After air laden with COM and soil particles enters the inlet (100 mm wide, 50 mm high), COM and soil particles settle out in the collection pan and the air discharges through the outlet (322 mm wide, 30 mm high).
To increase the trapping efficiency of the AMS, we de‐ signed it with three features. First, its wedge shape draws the air and sediments into the sampler and minimizes the effects of flow stagnation, which is called the Venturi effect (Nick‐ ling and McKenna Neuman, 1997; Rasmussen and Mikkel‐ sen, 1998). Second, an apron (200 mm long, 50 mm wide) in front of the inlet prevents scouring effects at the inlet, as pro‐ posed by Illenberger and Rust (1986). If scouring is allowed to occur at the front of the inlet, the level of the soil surface will drop so that the base of the inlet is no longer level with the soil surface and the sampler cannot suitably collect creep‐ ing materials (Jones and Willetts, 1979; Rasmussen and Mik‐ kelsen, 1998). Third, the collection pan is long enough (500 mm) to ensure that most soil particles entering the inlet will settle out in the collection pan.
The length of a saltating particle trajectory is 11 to 14 times its height (Nalpanis et al., 1993). Therefore, most soil particles entering the inlet are expected to settle out in the collection pan because (1) more than 95% of soil flux within 0.05 m above the surface is found below a height of 0.04 m (Rasmussen and Mikkelsen, 1998) and (2) the length of the
Figure 1. Photographs and two‐dimensional diagrams of the AMS: (a) the three parts of the AMS, (b) collection pan packed into container, (c) front of the AMS with lid on container, and (d) back of the AMS (outlet is located 20 mm above the soil surface), and (e) sampler dimensions. Thickness of the sheet metal is ignored in the figure.
collection pan is 10 times the height of the inlet. However, some COM entering the inlet could pass through the outlet, as the exact trajectory of windblown COM has not been ex‐ amined.
Wedge‐shaped passive samplers generally have fine stain‐ less steel wire mesh at the outlet to trap fine soil particles more effectively, but the AMS does not. This is because the mesh not only adds to the cost but makes it technically diffi‐ cult to manufacture the AMS in some developing countries.
WIND TUNNEL EXPERIMENT Methodology
Because the AMS is a buried‐type sampler, it is unable to rotate toward the wind and thus will not always face into the wind and the direction of arriving sediment. Therefore, it is important to measure the trapping efficiency at different inci‐ dent wind angles to the AMS. In this study, we used a full‐ scale AMS and also a 1:2 scale model for two reasons: (1) the
full‐scale model was too large to rotate more than 10° in the
trials, whereas the 1:2 scale model can rotate up to 45°, and
(2) the 1:2 scale model could provide the trapping efficiency at low wind velocity, as fully discussed later.
According to the logarithmic law for the mean velocity, wind velocity at height z (m), u(z), is written using the follow‐ ing formula:
u(z) = (u*/k)ln(z/z0) (1)
where u* is the friction velocity (m s-1), k is the von Karman
constant, a dimensionless number empirically determined to
be 0.40 (with an uncertainty of about 5%), and z0 is the sur‐
face roughness length (m) (see Leys and Raupach, 1991, for more information). To make the airflow around the 1:2 scale model dynamically similar to that around the full‐scale mod‐ el, it is necessary to conform the wind velocity at a similar or homologous height of the 1:2 scale model to that of the full‐
scale model, u1:2(z/2) = ufull(z). Therefore, z0 in the trials with
the 1:2 scale model should be adjusted to half of that in the trials with the full‐scale model.
When airflows around the full‐scale and 1:2 models have the same Reynolds number at a similar height, we can consid‐ er them dynamically similar (Pankhurst and Holder, 1965). The Reynolds number is a dimensionless number used to pro‐ vide a criterion for determining dynamic similitude and is de‐ fined as:
Re = ul/ (2)
where Re is the Reynolds number, l is the characteristic
length (m), and is the kinematic fluid viscosity (m2 s-1).
Now l in the trials with the 1:2 scale model is half of that in
the trials with the full‐scale model, l1:2 = 1/2lfull. Under the
condition that the temperature and pressure in both trials are
the same, we can assume that values for in the two trials are
equal. Thus, the results from the trials with the 1:2 model at
u1:2(z/2) are presumed to be the same as those with the full‐
scale model at 1/2ufull(z).
Generally, the trapping efficiency is calculated using the following formula:
E = mact/mpred× 100 (3)
where E is the trapping efficiency (%), mact is the mass actual‐
ly trapped by the sampler (g), and mpred is the mass predicted
to be trapped by a perfectly efficient sampler (g). There are
two major methods to estimate mpred: the tray method adopted
by Fryrear (1986) and Stout and Fryrear (1989), and the non‐ tray method adopted by Shao et al. (1993) and Goossens et al. (2000). In the tray method (Stout and Fryrear, 1989), a known quantity of the soil was first placed uniformly on the tray across its width. The width was almost the same as that of the wind tunnel. Second, the tray with the soil was posi‐ tioned on the windward side of the sampler. Third, the height of the tray was regulated to adjust the depth of the saltation curtain to the inlet height of the sampler. If the soil is set at the ground level, then the depth of saltation curtain will be lower than the inlet height, resulting in overestimation of the trapping efficiency. Fourth, a wind of fixed velocity was gen‐ erated until the soil on the tray was completely blown off.
Then mpred was calculated by multiplying the mass of the soil/
unit‐length of the strip by the width of the sampler inlet. In the non‐tray method (Shao et al., 1993; Goossens et al.,
2000), mpred was calculated by measuring a flux density of the
soil with height using a suction‐type isokinetic sampler. As Goossens et al. (2000) mentioned, the tray method may not be the best because the depth of the saltation curtain is ad‐ justed, which does not happen in natural conditions, and the non‐tray method provides more precise information than the tray method. However, we adopted the tray method in this study because: (1) the trapping efficiency of the BSNE sam‐ pler measured using the non‐tray method of Shao (1993) (85.5% average) was consistent with that measured using the tray method of Fryrear (1986) (89.5% average), and (2) it was not realistic to prepare enough COM for the non‐tray method because the concentration of COM in the test soil was low (1% by weight), and separating COM from the soil by sieving and dry panning required a heavy workload.
Estimation of Trapping Efficiency
We tested the trapping efficiency of the AMS using a porta‐ ble suction‐type wind tunnel with a working section (4.50 m long, 0.60 m wide, 0.70 m high) and a maximum flow speed of
approximately 16 m s-1 at the center of the tunnel. A honey‐
comb stabilizer was placed 1 m upwind of the working section to reduce the turbulence and produce a uniform airflow. During the experiment, the tunnel was set up on bare soil covered by sheet metal, except for where the AMS was installed.
In the trials with the full‐scale model, the z0 was adjusted
to 0.0012 m, which was the average z0 in the pearl millet
(Pennisetum glaucum) field in the International Crops Re‐ search Institute for the Semi‐Arid Tropics (ICRISAT) Sahe‐ lian Center (ISC) in Niger from the beginning of March to the beginning of July, the period during which wind erosion was
severe in 2006. On the other hand, the z0 was adjusted to
0.0006 m in the trials with the 1:2 scale model. These adjust‐ ments were made by covering the sheet metal with an artifi‐ cial grass mat of which the grass height was 0.003 m, and by installing artificial blocks (0.60 m long, 0.010 m wide, 0.015 m high) on the grass mat at a right angle to the airflow from the start of the working section to 0.50 m upwind of the AMS inlet. We measured wind velocity at heights of 0.025, 0.050, 0.150, and 0.300 m in the trials with the full‐scale model and at 0.012, 0.025, 0.150 and 0.300 m in the trials with the 1:2 scale model using Pitot tubes (454 modular system and 0638 1345/0638 1445 differential pressure probes, Testo AG, Lenzkirch, Germany). One example of the wind velocity pro‐ files at the position of the sampler is shown in figure 2. From
0 5 10 15 20 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 Full scale 1:2 scale Height (m) Wind Velocity (m s-1)
Figure 2. Wind velocity profiles in the trials with the full‐scale model and 1:2 scale model. The solid line and dashed line represent the regression curves in the trials with the full‐scale model and 1:2 scale model, respec‐ tively.
The test COM and soil particles were collected from the topsoil approximately 5 to 10 mm deep in the cultivated field before the first erosive storm arrived at the ISC, and were sep‐ arated by sieving and dry panning. The particle size distribu‐ tion of COM was measured with flat sieves, whereas that of soil particles was measured using a laser diffraction particle size analyzer (Partica LA‐950, Horiba, Ltd., Kyoto, Japan) with minimal dispersion (sonication at 20 kHz and 26 W for less than 1 min) (fig. 3).
The tray (0.60 m long, 0.10 mm wide, 0.003 m deep) was positioned 0.30 m upwind from the AMS inlet in the trials with COM and 0.40 m upwind in the trials with soil particles.
A uniform strip of COM (67 g m-1) or soil particle (133 g m-1)
was placed on the tray across its width, as per Stout and Fry‐ rear (1989).
In the trials, we chose four different wind velocities, 6.0 (only for the trials with the 1:2 scale model), 6.7, 7.6, and 8.4
m s-1 at the center of the inlet, which cover the normal range
1 10 100 1000 10000 Mass Percent (%) 0 10 20 30 40 1 10 100 1000 10000 Mass Percent (%) 0 2 4 6 8 (a) 50 D = 220 (b) Particle Size (mm) Particle Size (mm) μm
Figure 3. Particle size distribution of the test (a) COM and (b) soil par‐ ticles used to estimate the trapping efficiency. D50 is the median diameter.
Duration (min) 0 2 4 6 8 10 12 Holding Capacity (%) 94 96 98 100 102 104 106
Wind velocity at 2 m height* 7.7 m s-1
9.9 m s-1
17.2 m s-1
21.5 m s-1
Figure 4. Relationship between holding capacity and duration at different wind velocities. *Wind velocity at 2 m height was calculated by extrapo‐ lating the wind velocity profiles in figure 2 because wind velocity is often measured at that height in the field experiments.
of wind velocities observed in erosive storms. We also chose
six different incident wind angles to the AMS: 0°, 5°, 10°,
20°, 30°, and 45°. For each combination of wind velocity and
incident wind angle, at least four independent test runs for COM and three runs for soil particles were carried out.
The tunnel was operated until 10 s after all COM or soil particles were removed from the tray to account for the hold‐ ing capacity proposed by Fryrear (1986). In a preliminary in‐ vestigation, we measured the holding capacity of the AMS for three durations (10 s, 1 min, and 10 min) and four wind velocities with three replications, and found that the holding capacity was about 100% and did not change within the range of durations used (fig. 4).
The trapping efficiency was calculated using equation 3
and averaged. The value of mpred was calculated by multiply‐
ing the mass of the COM or soil particles/unit‐length of the strip on the tray by the width of the sampler inlet, as per Stout
and Fryrear (1989): mpred = 67 × 0.1 × cos() for COM and
mpred = 133× 0.1× cos() for soil particles in the trials with
the full-scale model, where is the incident wind angle to the
AMS (°).
R
ESULTS ANDD
ISCUSSIONTRAPPING EFFICIENCY
Within the range of wind velocity used, the trapping effi‐ ciency for COM was more than 60% in 31 of 33 combinations of wind velocity and incident wind angle (fig. 5). The trap‐ ping efficiency was constant with wind velocity but de‐ creased slightly as incident wind angle to the AMS increased. Because the effect of wind velocity was statistically rejected in developing the regression equation to predict the trapping efficiency, the trapping efficiency was averaged for each
angle (fig. 6), resulting in a well‐fitted regression curve (R2
= 0.94, P < 0.01):
Ecom = 61.0 + 18.0exp(-0.0614)(0 < < 45) (4)
where Ecom is the trapping efficiency for COM (%). Although
the trapping efficiency of the AMS was not 100%, this equa‐ tion suggests that we can easily calibrate the trapping effi‐ ciency for COM using wind data.
The trapping efficiency for soil particles was more than 50% (fig. 7). It decreased with increasing wind velocity as
re-4 8 12 16 20 24 T rapping Efficiency (%) 0 20 40 60 80 100 0° 5° 10° 20° 30° 45° Angle
Wind Velocity at 2 m Height* (m s-1)
Figure 5. Effect of wind velocity and incident wind angle to the AMS on the trapping efficiency for COM. *Refer to figure 4.
0 10 20 30 40 50 A verage T rapping Efficiency (%) 0 20 40 60 80 100
Incident Wind Angle to the AMS (°) R = 0.94 (P < 0.01)
2
Figure 6. Relationship between average trapping efficiency for each angle and incident wind angle to the AMS. The regression curve was calculated using equation 4.
ported by Fryrear (1986) and Goossens et al. (2000) for BSNE, POLCA, and SUSTRA samplers and with increasing incident wind angle as reported by Nickling and McKenna Neuman (1997). In the trials, we observed that some soil par‐ ticles entering the AMS were ejected from the inlet. There‐ fore, this relatively low trapping efficiency may be due to: (1) stagnation pressure caused by the non‐isokinetic conditions at the inlet (Goossens et al. 2000), and (2) counter‐flow caused by the difference in the wind velocities between the top and bottom of the inlet (Stout and Fryrear, 1989; Rasmus‐ sen and Mikkelsen, 1998). According to Stout and Fryrear (1989) and Rasmussen and Mikkelsen (1998), some soil par‐ ticles entering through the inlet can be forced out from the bottom of the inlet by the counter‐flow. Because the AMS is installed in the ground, it is sensitive to the effect of the counter‐flow. Rasmussen and Mikkelsen (1998) found that the trapping efficiency of an Ames trap installed at a height of <15 mm was only 50%, although the efficiencies at heights of 15 to 35 mm, >35 mm, and 100 mm were 87%, 96% (Ras‐ mussen and Mikkelsen, 1998), and 83% (Shao et al. 1993), respectively. This difference can be explained by the fact that when a sampler is placed in a lower position, the difference in the wind velocities between the top and bottom of the inlet is greater and consequently the counter‐flow effect is larger. The trapping efficiency for soil particles can be fit to the
following formula (fig. 8, R2 = 0.84, P < 0.01):
Esoil = -2.40u2m + [96.1 + 12.7exp(-0.0523)](0 < < 45) (5) 4 8 12 16 20 24 T rapping Efficiency (%) 0 20 40 60 80 100 0° 5° 10° 20° 30° 45° Angle
Wind Velocity at 2 m Height* (m s-1)
Figure 7. Effect of wind velocity and incident wind angle to the AMS on the trapping efficiency for soil particles. *Refer to figure 4.
Trapping Efficiency Calculated (%)
40 50 60 70 80 90 100 Measured T rapping Efficiency (%) 40 50 60 70 80 90 100 1:1 R = 0.84 (P < 0.01) 2
Figure 8. Relationship between measured trapping efficiency for soil par‐ ticles and that calculated. “Trapping efficiency calculated” was derived using equation 5.
where Esoil is the trapping efficiency for soil particles (%),
and u2m is the wind velocity at 2 m height (m s-1). Although
the trapping efficiency was not 100%, this equation suggests that we can calibrate the trapping efficiency for soil particles easily using wind data.
Because the AMS is effective only in limited incident
wind angles (±45° range), at least four AMS units should be
used to cover all wind directions. Under field conditions, wind direction is not stable, but fluctuates. Therefore, we have to measure the wind direction and wind velocity at short intervals to calculate the vector mean of the wind direction when calibrating the trapping efficiency.
C
ONCLUSIONSWe developed a new sediment catcher, the AMS, that can trap both COM and soil particles moving below 0.05 m and can be manufactured easily at low cost. Although the trap‐ ping efficiency of the AMS was not 100%, it can be calibrated easily with wind data using equations 4 and 5. Therefore, the AMS allows us to estimate COM flux and soil particle flux when used with the BSNE sampler, which can measure both fluxes 0.05 m above the surface. This provides vital informa‐ tion for achieving sustainable agriculture in areas suffering from desertification caused by wind erosion. However, prior to application of the AMS to other soil conditions, calibration experiments are necessary to determine the relationship be‐ tween the trapping efficiency and wind data under those con‐
ditions. In addition, to improve the AMS, further studies are necessary to find out more about the inlet conditions and de‐ fine the causes of the reduced trapping efficiency of the AMS.
ACKNOWLEDGEMENTS
This study was performed under the JIRCAS‐ICRISAT collaborative project, “Improvement of Fertility of Sandy Soils in the Semi‐Arid Zone of West Africa through Organic Matter Management”. Part of this study was financially sup‐ ported by the Japan Society for the Promotion of Science (JSPS). We acknowledge Ludger Herrmann, who allowed us to use the wind tunnel, and Dirk Goossens and John E. Stout for helpful advice on the wind tunnel experiment. We are grateful to Katsuhiko Itami for help in carrying out particle size analysis with a laser diffraction particle size analyzer.
R
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