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thaw and soil moisture effects on wind erosion
Geomorphology 207 (2014) 141–148
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Freeze/thaw and soil moisture effects on wind erosion L. Wang a,b, Z.H. Shi a,c,⁎, G.L. Wu a, N.F. Fang a a b c
State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, CAS and MWR, Yangling, Shaanxi Province 712100, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, PR China
a r t i c l e
i n f o
Article history: Received 4 July 2013 Received in revised form 29 October 2013 Accepted 30 October 2013 Available online 6 November 2013 Keywords: Freeze/thaw Soil moisture Wind erosion Effective particle size
a b s t r a c t Wind erosion is very pronounced in semiarid regions during late winter–early spring and has major impacts on regional desertification and agriculture. In order to identify the effects of freeze/thaw and soil moisture on wind erosion, wind tunnel experiments were conducted to compare wind erosion effects under various soil moisture gradients in frozen and thawed soil. The variation of surface soil moisture after wind erosion and the effective soil particle size distribution was tested to explain the differences. The results showed that surface soil moisture content decreased in thawed soil and increased in frozen soil after wind erosion. The mean weight diameter, which increased with increasing soil moisture, was smaller in thawed soil than in frozen soil. The wind-driven sediment flux of frozen and thawed soil both decreased with increasing moisture, owing to the heavier soil particle weight and stronger interparticle bonding forces. The critical soil moisture content for suppressing wind erosion was around 2.34% for frozen soil and around 2.61% for thawed soil. The wind-driven sediment flux of thawed soil was always larger than that of frozen soil at the same moisture content, but this difference became negligible at moisture contents above 3.38%. We may speculate that wind erosion will be more severe in the future because of the lower soil moisture content and fewer soil freezing days as a result of global warming. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Wind erosion is defined as the entrainment, transportation, and deposition of soil particles by the air steam. It has been considered as a serious environmental threat that leads to changes in global biochemical cycles, agricultural productivity decline, property damage, and human health hazards and contributes to climate change (Okin et al., 2004; Ravi et al., 2007; Foltz and McPhaden, 2008; Huang et al., 2010). Wind erosion is widespread in arid and semiarid regions around the world (Ravi et al., 2006). The seasonal fluctuation of climate in semiarid regions results in a variation of vegetation cover and soil properties; therefore soil erodibility is a time varying rather than a static characteristic (Chepil, 1954; Bajracharya et al., 1998). Wind erosion is very pronounced during late winter–early spring when frozen soil thaws, largely because of dry, windy weather and bare, loose surface soil (Li et al., 2004). The process of freeze–thaw can effectively alter soil structure (Pawluk, 1988; Layton et al., 1993; Bullock et al., 2001), which plays an important role in determining soil susceptibility to erosion (Chepil, 1954; Layton et al., 1993; Li et al., 2004). The destructive strength of the freeze–thaw impact is governed by the freezing temperature, the number of freeze–thaw cycles, and the moisture content at freezing (Logsdail and Webber, 1959; Bullock et al., 1988; Bajracharya et al., 1998; Oztas and Fayetorbay, 2003; ⁎ Corresponding author at: College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, PR China. Tel.: +86 27 87288249; fax: +86 27 87671035. E-mail address: [email protected] (Z.H. Shi). 0169-555X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geomorph.2013.10.032
Ferrick and Gatto, 2005; Kværnø and Øygarden, 2006). The stability of soil aggregates is considered a good indicator of soil structure and declines with decreasing freezing temperature (Oztas and Fayetorbay, 2003) and increasing number of freeze–thaw cycles (Bullock et al., 1988; Bajracharya et al., 1998; Kværnø and Øygarden, 2006). However, the effect of the number of freeze–thaw cycles on wet aggregate stability is not consistent: aggregate stability increased as the number of cycles increased from three to six but decreased afterward (Oztas and Fayetorbay, 2003). Additionally, the stability of soil aggregates is negatively correlated with soil moisture content at the time of freezing (Oztas and Fayetorbay, 2003). Soil with high moisture will form more ice with stronger expending forces, which probably breaks interparticle bonds, while at low moisture content ice crystals just complete their growth in the soil pores (Nickling and Bennett, 1984; Bullock et al., 1988; Ferrick and Gatto, 2005). Seemingly, freeze–thaw may cause destruction of the bonds holding aggregates together (Bullock et al., 1988) leading to variation of soil aggregate size distribution. Generally, soil aggregate size has an overall decreasing trend after freeze–thaw impact (Yang and Wander, 1998; Wang et al., 2012), which may influence wind erosion profoundly (Colazo and Buschiazzo, 2010). Many studies focus on the variation of soil erodibility before or after freeze–thaw impact, neglecting the erodibility of soil that stays frozen (Chepil, 1954; Pawluk, 1988; Layton et al., 1993; Bajracharya et al., 1998; Oztas and Fayetorbay, 2003). The sensitivity of soil to erosion after freeze–thaw impact is well-known, while the difference of soil erodibility between frozen soil and thawed soil is still unclear. Some research has shown that the ability of soil to resist the shear forces, which
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greatly influences soil erodibility, declines when frozen soil thaws (Ting et al., 1983). Moreover, stronger wind speeds are needed for dust uplift on frozen grounds (15.7 m s−1) than on thawed grounds (12.6 m s−1) (Han et al., 2011). Seemingly, wind erosion appears more frequently on thawed soil than on the frozen soil, but the exact reason is ambiguous. Investigating the changes in wind erosion owing to frozen soil thaws in advance from global warming is meaningful for better predicting wind erosion in the future. Soil moisture may also effectively influence the freeze–thaw impact, so soil moisture should also be taken into account. Within the last few decades, the soil surface conditions have become an important aspect of studies of geomorphological processes related to wind erosion (McKenna-Neuman and Nickling, 1989; Fécan et al., 1999; Wiggs et al., 2004; Han et al., 2009, 2011). The purpose of this paper was to identify the effects of freeze/thaw and soil moisture on wind erosion, providing a basis for research on wind erosion in cold seasons of semiarid regions under global warming. The differences of wind erosion under various soil moisture gradients in frozen and thawed soil were compared. The Liudaogou catchment of the Loess Plateau was selected as the sampling area because it is within a semiarid region suffering from serious wind erosion and because the ground is commonly frozen from late autumn to early spring. The variation of surface soil moisture, effective soil particle size distribution, and wind-driven sediment flux was measured to investigate wind erosion variation when frozen soil thaws at different moisture levels. 2. Materials and methods
Fig. 1. Time series of (A) monthly average precipitation, evaporation, and temperature; (B) monthly average wind speed and days of gale (NBeaufort force 8).
2.1. Soil samples The sampling area is in the Liudaogou catchment (38°49′ N., 110°23′ E.) of Shenmu County in the northern part of the Loess Plateau, China. It is characterized as a continental semiarid and seasonal wind climate with an average temperature of 8.4 °C. Monthly mean temperatures range from − 9.7 °C in January to 23.7 °C in July, so that part of the ground suffers from freezing during the winter and early spring. The mean annual precipitation is 437 mm with 77.4% occurring from June to September with evaporation also in this period. The mean annual gale days (N Beaufort force 8) are 16.2, and gales occur mainly in spring. Detailed meteorological data are shown in Fig. 1. Loess soil with poor ability to resist erosion is widely distributed in the study area, thus the local soil erosion rate approximates 10,890 t km− 2 y− 1 (Wang et al., 1993). The particle size distribution (with dispersion treatment) and other soil properties are shown in Table 1. 2.2. Sample tray preparation Soil samples were crushed and passed through a 2-mm sieve, wetted with a known amount of distilled water by a sprayer in the form of a fine mist, then covered with a Perspex sheet for 24 h to make sure the water was evenly distributed. The specific soil moisture contents were 1.27 ± 0.03%, 2.02 ± 0.09%, 2.34 ± 0.05%, 2.61 ± 0.12%, and 3.38 ± 0.05% (gravimetric moisture content) according to the varying surface soil moisture contents of different positions of the slopes in the study area (Wang et al., 2004). The prepared soils in the sample trays (79.5 × 50.0 × 9.7 cm3) were packed with a known quantity based on field bulk density obtained from the sampling area. To minimize differences in bulk density, the unit was divided horizontally into half and each packed separately. Afterward, each sample tray was covered with a Perspex sheet before wind tunnel tests to keep the moisture constant. In order to prevent disturbance of the soil surface, the Perspex sheet was supported with foam and wooden sticks to maintain a specific distance from the soil surface. Six sample trays were prepared for each moisture content; three frozen and three thawed. Three of the sample trays were put into a freezer set at −11 °C for 24 h, and put into the
wind tunnel immediately. The other three sample trays were put into the freezer set at −11 °C for 24 h, but thawed at room temperature for 24 h before wind tunnel experiments. The room temperature during all experiments was 10.5–12.0 °C.
2.3. Effective soil particle size distribution tests Pre-wetted soil was put into aluminum boxes (85 cm3) following the same procedure as for the sample trays. Soil samples in the aluminum boxes that stayed frozen or thawed were all passed through a 2-mm sieve. Subsamples (b2 mm) were separated by moving a 1-mm sieve (by hand) right and left by 3 cm with 50 repetitions for 2 min. The N1-mm particles were collected, and the sieving was repeated for the b1-mm factions with the next smaller sized sieve. This procedure was repeated for every sieve size (0.5, 0.25, 0.15, and 0.1 mm). All fractions were oven dried at 65 °C and weighed for calculating effective soil particle size distribution. Eroded sediments in the field consist of both primary particles and soil aggregates (Mitchell et al., 1983), which constitute what can be termed the ‘effective particle size distribution’ (Martinez-Mena et al., 2000). By contrast, ‘ultimate particle size distribution’ data are commonly evaluated after sediment has been fully dispersed into its primary particles (Slattery and Burt, 1997; MartinezMena et al., 2002). Therefore, considering the effective particle size of
Table 1 Selected physical and chemical properties of the study soila. Bulk density (g cm−3)
Mean values Standard deviations
1.35 0.07
pH
8.96 0.21
SOC (g kg−1)
1.23 0.08
Particle-size distribution (%) Sand
Silt
Clay
65.57 2.11
31.24 1.87
3.19 0.24
a Sand (2–0.05 mm), silt (0.05–0.002 mm), clay (b0.002 mm). SOC = soil organic carbon.
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sediment is crucial because this may govern the actual behavior of the transported sediment. 2.4. Wind tunnel tests The wind tunnel experiment was carried out at the Institute of Soil and Water Conservation, Chinese Academy of Science. The total length of the wind tunnel was 24 m, including a power section (3.55 m), a speed regulation section (1.5 m), a rectification section (10 m), a test section (1.28 m), a sediment collection section (3.02 m) and a diversion section, following the design criteria for a low speed wind tunnel. The 1.2-m-wide and 1-m-high cross section can produce free turbulent airflow and a stable airflow field. Its airflow velocity uniformity is b1%, and the static pressure gradient in axial direction is b0.005. Wind velocity was measured by a pitot tube installed 30 cm above the tunnel floor and can be changed continuously from 0 to 15 m s−1 via the control panel. A prepared sampling tray was placed into the wind tunnel and the soil surface was exactly paralleled to the test section's floor. A wind speed of 14 m s−1 was selected as it is close to the maximum monthly average wind speed at 30 cm above the soil surface. Blowing duration was 7 min according to pre-experiment tests. The wind-driven sediment of each sample was trapped in the deposition chamber at the end of the wind tunnel with a capture efficiency of ~90%. It was then collected and weighed after oven drying at 105 °C with an electronic balance to calculate the wind-driven sediment flux (g m−2 s−1). The gravimetric soil moisture content was measured in three replicates before and after each test on small samples taken down to 2 mm by scraping with a sharp knife. 2.5. Data analyses All data were analyzed using SPSS 11.0 (SPSS Inc., 2001). The effects of freeze/thaw and soil moisture on wind erosion were tested by twoway ANOVA. Mean values were compared using S-N-K (Student Newman Keuls). Data were transformed through ‘compute procedure’ prior to statistical analysis to make its distribution more normal when necessary. 3. Results and discussion 3.1. Variation of surface soil moisture contents after wind erosion The ratio of surface soil (0–2 mm) moisture contents after wind erosion (w1) to before wind erosion (w2) were all b 1 in thawed soil and N 1
Fig. 2. Ratios of surface soil (0–2 mm) moisture contents after wind erosion (w1) to before wind erosion (w2) at different soil moisture contents. Bars indicate standard deviations.
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in frozen soil (Fig. 2) at each moisture content. This meant that, after wind erosion, surface soil moisture decreased in the thawed soil but increased in the frozen soil although the blowing duration was only 7 min. The surface soil moisture of the frozen soil increased slightly with wind blowing probably owing to the condensation of the atmospheric moisture on the topsoil. Additionally, during the freezing process, water in the liquid and the vapor phases within the soil may significantly redistribute (Hoeckstra, 1966; Hansson et al., 2004) owing to the pressure-induced gradients and temperature gradients (Burt and Williams, 1976). The water was moved from the subsoil to the freezing zone nearer the surface (Ferguson et al., 1964; Cary and Mayland, 1972), leading to an increase of surface soil moisture content as well. The amount of movement probably depends on available soil water, temperatures of the frozen zone, length of frozen period, and physical properties of the soil (Ferguson et al., 1964). Cold soil particles absorb the atmospheric moisture as long as they creep on the soil surface so that the wet particles bond together (Fig. 3). Some ice may be sublimated by the strong wind (van Dijk and Law, 2003), and then the wind-dry soil particles are blown away and the wet particles are retained on the topsoil. The surface soil moisture of thawed soil decreased immediately with wind blowing until it was reduced to a threshold (which depended on the soil physicochemical properties and particle distribution) when entrainment would occur (Wiggs et al., 2004). As the dry surface soil was blown away, a ‘new’ surface layer with higher moisture appears and this too will be removed once it is dry enough. This phenomenon circulates during the whole wind erosion process.
3.2. Effective particle size distribution of frozen/thawed soil Fig. 4 shows the effective particle size distribution of frozen and thawed soil at different moisture contents. Effective particles in the size of 2–1 and b 0.1 mm showed an obvious change with soil moisture while the variation of other effective particle sizes was negligible in frozen soil and in thawed soil (Fig. 4). The 2–1 mm particle content increased with increasing moisture while the b0.1 mm particle content decreased, indicating that fine particles (b0.1 mm) probably gathered into large particles (2–1 mm) through soil moisture. Much research has found that the bonding forces among soil particles are, besides the electrostatic and van der Waals forces, a result of wet bonding forces owing to the presence of water, which increase with increasing soil moisture in a certain range (Yao and Cheng, 1986; McKenna-Neuman and Nickling, 1989; Saleh and Fryrear, 1995; Fécan et al., 1999; Cornelis et al., 2004). When soil water is frozen, ice in the soil has a tensile strength and is therefore responsible for the cementing of solid particles. With increasing volumetric ice content, this effect becomes more important and the cohesion increase too (Arenson et al., 2004). A decrease in temperature below the freezing point and/or an increase in soil initial moisture content may lead to an increase in volumetric ice content (Yang and Wu, 1980; Müller-Lupp and Bölter, 2003). Thus the mean weight diameter (MWD) of thawed soil and frozen soil increased with each successive addition of moisture (Table 2). Soil particles (without any dispersion treatment) of b 0.25 mm can be considered as fractions highly erodible by wind (Liu et al., 2003). Fig. 5 shows that b0.25 mm soil particles decreased with increasing moisture in frozen and in thawed soil, leading to the reduction of erodible fractions. Additionally, the phenomenon of fine particles (b0.1 mm) gathering into large particles (2–1 mm) was more obvious in frozen soil than that of thawed soil (Fig. 4). The bonding forces of liquid water are lower than that of ice, because the activation energy of the hydrogen atom reduces when the temperature decreases, which consequently strengthens the bonding forces of molecules (Yang and Wu, 1980). As a result, the MWD of thawed soil was smaller than that of frozen soil at the same moisture (Table 2). Thawed soil contained more highly erodible
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A
B
C
D
Fig. 3. Soil surfaces at the moisture content of 2.61%. (A) Frozen soil before wind erosion; (B) frozen soil after wind erosion; (C) thawed soil before wind erosion; (D) thawed soil after wind erosion. Note the gathering soil particles after wind erosion on the frozen soil surface.
fractions (b0.25 mm) than frozen soil, and the difference increased with increasing moisture (Fig. 5). 3.3. Influence of freeze/thaw and soil moisture on wind-driven sediment flux 3.3.1. Wind-driven sediment flux of frozen soil varied with soil moisture The two-way ANOVA revealed freeze/thaw and soil moisture can interactively influence wind-driven sediment flux (Table 3). Fig. 6 shows that the wind-driven sediment flux of frozen soil decreased with increasing moisture. For instance, the wind-driven sediment flux
at 1.27% moisture content is about six times higher than at 3.38%. Ice around the individual soil particles could increase the weight of the particles, making them heavier and more difficult to drag and lift from the soil surface by wind. As previously stated, more large soil particles are present in frozen soil at high moisture content because of stronger ice-bonding forces (Figs. 4, 5 and Table 2), which resulted in less erodible fractions and suppression of wind erosion. Simple correlation coefficients between wind-driven sediment flux and indices of effective soil particle size distribution were calculated (Table 4). The result showed that wind-driven sediment flux was found to be negatively correlated with MWD and 2–1 mm particle content and positively correlated
Fig. 4. Effective particle size distribution of frozen and thawed soil at different soil moisture contents. Bars indicate standard deviations.
L. Wang et al. / Geomorphology 207 (2014) 141–148 Table 2 Mean weight diameter (μm) of frozen/thawed soil at different soil moisture contentsa. Freeze/thaw impact
Frozen Standard deviations Thawed Standard deviations
Soil moisture content 1.27%
2.02%
2.34%
2.61%
3.38%
155.54Aa 1.95 145.18Ba 1.08
166.99Aab 0.96 151.27Bb 2.87
178.76Ab 2.31 158.75Bc 1.39
268.88Ac 12.4 175.40Bd 0.44
309.13Ad 12.5 185.81Be 5.66
a Values of different moisture contents followed by the same lowercase letter are not significantly different, and values for frozen/thawed soil followed by the same uppercase letter are not significantly different at p b 0.05.
with b0.25 and b 0.1 mm particle content, suggesting that large particles are effective in controlling wind erosion, which is consistent with the finding of Colazo and Buschiazzo (2010). Additionally, the shear resistance of frozen soil, which affects wind erosion profoundly, increases with increasing ice content until it reaches a threshold (Nickling and Bennett, 1984). This threshold is seldom reached in wind-eroded regions owing to very low soil moisture contents. All the above factors will influence the reduction in wind-driven sediment flux with increasing soil moisture. The difference in wind-driven sediment flux was negligible between 1.27% and 2.02% moisture content and between 2.61% and 3.38% moisture content (Fig. 6). We can speculate that when soil moisture is low, small increases cannot change the antierodibility of frozen soil significantly. At high moisture contents, the antierodibility of frozen soil is sufficiently strong to suppress wind erosion, so the increase in moisture has little effect on wind-driven sediment flux reduction. Soil moisture content around 2.34% is critical for frozen soil to resist wind erosion. This critical soil moisture level was probably influenced by soil particle distribution. Soil containing more fine particles has a larger specific surface area, which strongly adsorbs water around the particles leading to more unfrozen water in the frozen soil compared to soil with less fine particles; thus the mechanical strength of the former soil is weaker than the latter soil at the same soil moisture content and negative temperature (Yao and Cheng, 1986).
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3.3.2. Wind-driven sediment flux of thawed soil varied with soil moisture The wind-driven sediment flux of thawed soil decreased with increasing moisture (Fig. 6). For instance, wind-driven sediment flux at 1.27% moisture is ~200 times more than that at 3.38% moisture. Similarly to frozen soil, water films around the soil particles in the thawed soil may increase the weight of the particles, and more large soil particles are present in thawed soil at high moisture content because of stronger wet bonding forces (Figs. 4, 5 and Table 2), leading to suppression of wind erosion. Simple correlation coefficients between wind-driven sediment flux of thawed soil and indices of effective soil particle size distribution are shown in Table 4. In Fig. 6, significant differences in wind-driven sediment flux of thawed soil at moisture contents of 1.27%, 2.02%, and 2.34% are shown, while at 2.61% and 3.38% moisture levels, the difference was not significant, suggesting that the impact of moisture on wind erosion is greater at low moisture contents. Apparently, moisture content around 2.61% is critical to prevent wind erosion of thawed soil. The critical moisture content varies in soil types. Nickling (1978) found 3–4% (gravimetric) was the critical moisture content to hamper wind erosion of surface soil consisting of fine sand and silt. Chen et al. (1996) found that the soil moisture content of 4–6% (gravimetric) could prevent wind erosion of loessial sandy loam, and Han et al. (2009) found that moisture contents above 0.802% (gravimetric) could resist wind erosion of sand. The differences in these results are likely owing to differences in physicochemical properties of the soils, especially the primary soil particle distribution. Together with the results of the present study, we can say that finer soil particles need more moisture to suppress wind erosion. Wet bonding forces consist of capillary forces and adhesive forces of adsorbed water films surrounding the particles (Yao and Cheng, 1986; McKenna-Neuman and Nickling, 1989; Saleh and Fryrear, 1995; Fécan et al., 1999; Cornelis et al., 2004). The former is the result of the tension exerted by the air–water interface and by the pressure difference inside and out of the water wedge at the contact point of particles (Fisher, 1926), while the latter is the result of wet layer bonding because of the electronic, van der Waals and hydration forces (Tuller et al., 1999). Capillary and adhesion forces cannot be easily separated, since the capillary ‘wedges’ are at a state of internal equilibrium with the adsorption
Fig. 5. Cumulative percentage content of effective particles of frozen and thawed soil at different moisture contents.
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Table 3 Effects of soil moisture and freeze/thaw on wind-driven sediment flux (g m−2 s−1)a. Freeze/thaw impact
Frozen Thawed
Soil moisture content 1.27%
2.02%
2.34%
2.61%
3.38%
0.80 ± 0.17 42.32 ± 1.73
0.71 ± 0.13 7.55 ± 1.15
0.30 ± 0.02 0.67 ± 0.08
0.20 ± 0.05 0.31 ± 0.02
0.15 ± 0.03 0.19 ± 0.02
Factors
F-ratio
P-value
Soil moisture content (A) Freeze/thaw impact (B) Interaction (A × B)
646.63 446.71 160.63
0.000 0.000 0.000
a
Two-way ANOVA results: F-ratio and P-value for main effects and interaction.
‘films,’ and the ones cannot be changed without affecting the others (Hillel, 1982). However, Fécan et al. (1999) pointed out the existence of a minimum moisture content, corresponding to the maximum amount of water that the adsorption molecular can trap (Yao and Cheng, 1986), below which the equilibrium shifted into adsorbed water and the capillary water does not induce strong wet bonding forces. The critical moisture value mentioned above can be considered as the minimum moisture according to Han et al. (2009). He proposed that a slight increase in moisture can dramatically enhance the wet bonding forces resulting from adsorbed water films before the critical moisture value. While exceeding the critical moisture value, wet bonding forces mainly resulted from capillary forces that did not vary intrinsically with slight increases in moisture content, leading to the different degrees of wind erosion variations among different soil moisture contents. Fig. 6 reveals that a slight moisture increase of b 2.61% can dramatically reduce wind erosion modulus thus supporting the findings of Han et al. (2009). 3.3.3. Difference of wind-driven sediment flux between frozen soil and thawed soil Wind-driven sediment flux of thawed soil is more than that of frozen soil at the same moisture content (Table 3, Fig. 7), probably owing to the weaker shear resistance of thawed soil than frozen soil (Yang and Wu, 1980; Ting et al., 1983). Frozen soil is made of mineral solid particles,
ice, unfrozen water, and gases of various chemical compositions (Razbegin et al., 1996). During freezing, solutes are excluded from the ice phase and concentrated within films of unfrozen water, leading to the formation of inorganic precipitates as a consequence of exceeding their solubility limit, which may enhance soil structural stability (Sletten, 1988). Furthermore, ‘soil strengthening’ may occur after freezing because of the increased dilatancy, structural hindrance, and tension in the unfrozen water film, and consequently the strength of frozen soil generally exceeds the sum of ice strength plus soil strength (Ting et al., 1983). After wind erosion, surface soil moisture decreased in the thawed soil while it increased in the frozen soil (Fig. 2). Moisture attached to the surface of frozen soil during the wind erosion process seems to create a protective film to prevent soil particles being transported, although the film is shallow. Additionally, more large soil particles are present in frozen soil than thawed soil at the same moisture content (Figs. 4, 5 and Table 2), leading to a relatively higher erodibility of thawed soil. The critical soil moisture value for suppressing wind erosion is around 2.34% for frozen soil and around 2.61% for thawed soil, indicating that thawed soil needed more moisture to resist wind erosion. Fig. 7 shows that the difference in wind driven sediment flux between frozen soil and thawed soil decreased with increasing soil moisture. For instance, a fiftyfold increase in wind-driven sediment flux was found when frozen soil thawed at moisture content of 1.27%, but was only a onefold increase at moisture content of 3.38%. The difference was significant at 1.27%, 2.02%, 2.34%, and 2.61% soil moisture while the difference did not reach a significant level at 3.38% (Fig. 7). Apparently, the increase in wind-driven sediment flux because of soil thawing is dramatic at lower moisture contents. However, other studies on wellsorted fine quartz sand have shown that frozen soil particles are uncemented at a value of 0.5% gravimetric moisture content (van Dijk and Law, 2003). Although the critical moisture value will change with soil type, it seems that when the moisture contents lie beneath the critical value, the difference in soil resistance to wind erosion between thawed soil and frozen soil will not be significant, owing to the negligible wet (ice) bonding force. Wind erosion is suppressed when soil moisture content reaches a relatively high level, largely because the higher weight of soil particles and bonding forces resist wind erosion. Therefore, the difference in wind-driven sediment flux between thawed and frozen soil becomes insignificant at higher moisture contents. In this
Table 4 Simple correlation coefficients between wind-driven sediment flux and indices of effective soil particle size distribution. Wind-driven Indices of effective soil particle size distribution sediment flux 2–1 mm particle b0.25 mm particle b0.1 mm particle MWD content content content Frozen soil Thawed soil Fig. 6. Relationship between wind-driven sediment flux and soil moisture content. Bars indicate standard deviations. Different letters indicate significant differences (p b 0.05).
⁎ p b 0.05. ⁎⁎ p b 0.01.
−0.762⁎⁎ −0.710⁎⁎
0.770⁎⁎ 0.610⁎
0.671⁎⁎ 0.551⁎
−0.759⁎⁎ −0.670⁎⁎
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Fig. 7. Difference of wind-driven sediment flux between frozen soil and thawed soil at different moisture contents. Bars indicate standard deviations. Different letters indicate significant differences (p b 0.05).
study, moisture content around 3.38% is the transition point that affects the degree of difference between thawed and frozen soil. We can speculate that in the wind erosion regions during late winter–early spring, when soil moisture is relatively low, frozen soil thaw may likely aggravate wind erosion. The factors described above indicate that wind erosion in thawed soil is generally more serious than in frozen soil at the same soil moisture content, and the difference is more pronounced at low moisture content. As mentioned above, the surface moisture content of the frozen soil increased owing to the condensation of the atmospheric moisture and the upward migration of the unfrozen water, which may profoundly reduce the wind erosion during the experiment. However, under natural conditions in the field, thawing occurred from top and bottom in the soil profile but was more rapid from the top, preventing downward movement of water, and thereby the perched water may expose to the evaporation forces near the surface, which may probably contribute to overwinter losses of soil water (Ferguson et al., 1964). Annual soil freezing days declined with increasing mean winter air temperature (Henry, 2008), prolonging the period that thawed soil is exposed to gales, which may aggravate wind erosion. Furthermore, higher temperatures may accelerate the evaporation of soil moisture, leading to the formation of drier topsoil, thus contributing further to the occurrence of wind erosion. The findings of this paper may also apply to the middle latitude regions where ground is commonly frozen in early spring and spring dust outbreaks occur frequently such as the eastern Mongolian Plateau. However, these results are only wind tunnel measurements, and further research is required to confirm them with field observations in the study area. 4. Conclusions In this study, effects of freeze/thaw and soil moisture on wind erosion were identified by wind tunnel experiments, and the conclusions are as follows: • Surface soil moisture content of thawed soil decreased after wind erosion, while that of frozen soil increased because of the condensation of atmospheric moisture and the upward migration of the unfrozen water. Moisture attached to the surface soil seems to create a protective film to prevent soil particles being transported, and a quantity of cold soil particles bond together as long as they creep on the soil surface, which can partly suppress wind erosion. • Fine particles (b 0.1 mm) gathered into large particles (2–1 mm) owing to wet (ice) bonding forces and the phenomenon was increasingly obvious with increasing moisture but less obvious in thawed than in frozen soil. The MWD of frozen and thawed soils increased
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with increasing moisture, largely because of the stronger wet (ice) bonding forces at relatively high moisture, leading to a lower highly erodible fraction (b 0.25 mm particles), which can effectively suppress wind erosion. The MWD of thawed soil was always smaller than that of frozen soil at the same moisture content, on account of the weaker bonding forces of liquid water than that of ice. • Wind-driven sediment flux of thawed and frozen soils both decreased with increasing soil moisture because of heavier particles and stronger interparticle forces which can be partly reflected in the soil particle size distribution. The critical soil moisture content for suppressing wind erosion is around 2.34% for frozen soil and around 2.61% for thawed soil. The wind-driven sediment flux of thawed soil is always more than that of frozen soil, and the difference reaches statistically significant levels at moisture contents b 3.38%, suggesting that frozen soil played a very important role for suppression of wind erosion at low soil moisture content. We can speculate that soil moisture and the number of soil freezing days may decrease as a result of global warming, which may exacerbate wind erosion. Acknowledgments Financial support for this research was provided by the National Natural Science Foundation of China (41271296) and the ‘Hundred-talent Project’ of the Chinese Academy of Sciences. References Arenson, L.U., Johansen, M.M., Springman, S.M., 2004. Effects of volumetric ice content and strain rate on shear strength under triaxial conditions for frozen soil samples. Permafr. Periglac. Process. 15, 261–271. Bajracharya, R.M., Lal, R., Hall, G.F., 1998. Temporal variation in properties of an uncropped, ploughed Miamian soil in relation to seasonal erodibility. Hydrol. Process. 12, 1021–1030. Bullock, M.S., Kemper, W.D., Nelson, S.D., 1988. Soil cohesion as affected by freezing, water content, time and tillage. Soil Sci. Soc. Am. J. 52, 770–776. Bullock, M.S., Larney, F.J., Izaurralde, R.C., Feng, Y., 2001. Overwinter changes in wind erodibility of clay loam soils in southern Alberta. Soil Sci. Soc. Am. J. 65, 423–430. Burt, T.P., Williams, P.J., 1976. Hydraulic conductivity in frozen soils. Earth Surf. Process. 1, 349–360. Cary, J.W., Mayland, H.F., 1972. Salt and water movement in unsaturated frozen soil. Soil Sci. Soc. Am. J. 36, 549–555. Chen, W., Dong, Z., Li, Z., Yang, Z., 1996. Wind tunnel test of the influence of moisture on the erodibility of loessial sandy loam soils by wind. J. Arid Environ. 34, 391–402. Chepil, W.S., 1954. Seasonal fluctuations in soil structure and erodibility of soil by wind. Soil Sci. Soc. Am. 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Academic Press, New York 68–70. Hoeckstra, P., 1966. Moisture movement in soil under temperature gradients with the cold-site temperature below freezing. Water Resour. Res. 2, 241–250. Huang, J.F., Zhang, C., Prospero, J.M., 2010. African dust outbreaks: a satellite perspective of temporal and spatial variability over the tropical Atlantic Ocean. J. Geophys. Res. 115, D05202.
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Kværnø, S.H., Øygarden, L., 2006. The influence of freeze–thaw cycles and soil moisture on aggregate stability of three soils in Norway. Catena 67, 175–182. Layton, J.B., Skidmore, E.L., Thompson, C.A., 1993. Winter-associated changes in dry-soil aggregation as influenced by management. Soil Sci. Soc. Am. J. 57, 1568–1572. Li, F., Zhao, L., Zhang, H., Zhang, T., Shirato, Y., 2004. Wind erosion and airborne dust deposition in farmland during spring in the Horqin Sandy Land of eastern Inner Mongolia, China. Soil Tillage Res. 75, 121–130. Liu, L.Y., Shi, P.J., Zou, X.Y., Gao, S.Y., Erdon, H., Yan, P., Li, X.Y., Dong, Z.B., Wang, J.H., 2003. Short-term dynamics of wind erosion of three newly cultivated grassland soils in northern China. Geoderma 115, 55–64. Logsdail, D.E., Webber, L.R., 1959. Effect of frost action on structure of Haldimand clay. Can. J. Soil Sci. 39, 103–106. Martinez-Mena, M., Alvarez, R.J., Albaladejo, J., Castillo, V.M., 2000. Influence of vegetal cover on sediment particle size distribution in natural rainfall conditions in a semiarid environment. Catena 38, 175–190. Martinez-Mena, M., Castillo, V., Albaladejo, J., 2002. Relations between interrill erosion processes and sediment particle size distribution in a semiarid Mediterranean area of SE of Spain. Geophys. J. Roy. Astron. Soc. 45, 261–275. McKenna-Neuman, C., Nickling, W.G., 1989. A theoretical and wind tunnel investigation of the effect of capillary water on the entrainment of sediment by wind. Can. J. Soil Sci. 69, 79–96. Mitchell, J.K., Mostaghimi, S., Pound, M., 1983. Primary particle and aggregate size distribution of eroded soil from sequenced rainfall events. Trans. ASAE 26, 1773–1777. Müller-Lupp, W., Bölter, M., 2003. Effect of soil freezing on physical and microbiological properties of permafrost-affected soils. Proc. Eighth Int. Conf. Permafrost. AA Balkerna, Lisse, Netherland, pp. 801–805. Nickling, W.G., 1978. Eolian sediment transport during dust storms: Slims River Valle, Yukon Territory. Can. J. Earth Sci. 15, 1069–1084. Nickling, W.G., Bennett, L., 1984. The shear strength characteristics of frozen coarse granular debris. J. Glaciol. 30, 348–357. Okin, G.S., Mahowald, N., Chadwick, O.A., Artaxo, P., 2004. Impact of desert dust on the biogeochemistry of phosphorus in terrestrial ecosystems. Global Biogeochem. Cycles 18, GB2005. Oztas, T., Fayetorbay, F., 2003. Effect of freezing and thawing processes on soil aggregate stability. Catena 52, 1–8. Pawluk, S., 1988. Freeze–thaw effects on granular structure reorganization for soil materials of varying texture and moisture content. Can. J. Soil Sci. 68, 485–494.
Ravi, S., Zobeck, T.M., Over, T.M., Okin, G.S., D'Odorico, P., 2006. On the effect of moisture bonding forces in air-dry soils on threshold friction velocity of wind erosion. Sedimentology 53, 597–609. Ravi, S., D'Odorico, P., Okin, G.S., 2007. Hydrologic and aeolian controls on vegetation patterns in arid landscapes. Geophys. Res. Lett. 34, L24S23. Razbegin, V.N., Vyalov, S.S., Masksimyak, R.V., Sadovvskii, A.V., 1996. Mechanical properties of frozen soils. Soil Mech. Found. Eng. 33, 35–45. Saleh, A., Fryrear, D.W., 1995. Threshold wind velocities of wet soils as affected by wind blown sand. Soil Sci. 160, 304–309. Slattery, M.C., Burt, T.P., 1997. Particle size characteristics of suspended sediment in hillslope runoff and stream flow. Earth Surf. Process. Landf. 22, 705–719. Sletten, R.S., 1988. The formation of pedogenic carbonates on Svalbard: the influence of cold temperatures and freezing. Proc. Fifth Int. Conf. Permafrost. Tapir, Trondheim, Norway, pp. 467–472. SPSS Inc., 2001. SPSS 11.0 for Windows. Chicago, USA. Ting, J.M., Torrence Martin, R., Ladd, C.C., 1983. Mechanisms of strength for frozen sand. J. Geotech. Eng. 109, 1286–1302. Tuller, M., Or, D., Dudley, L.M., 1999. Adsorption and capillary condensation in porous media: liquid retention and interfacial configurations in angular pores. Water Resour. Res. 35, 1949–1964. van Dijk, D., Law, J., 2003. The rate of grain release by pore-ice sublimation in cold-aeolian environments. Geogr. Ann. 85, 99–113. Wang, B.K., Tang, K.L., Zhang, K.L., Zhang, P.C., 1993. Types and intensity of soil erosion and its temporal and spatial distribution in Liudaogou watershed Shenmu County. Bull. Soil Water Conserv. 18, 57–66 (in Chinese, with English Abstr.). Wang, L., Shao, M., Zhang, Q., 2004. Distribution and characters of soil dry layer in north Shaanxi Loess Plateau. Chin. J. Appl. Ecol. 15, 436–442 (in Chinese, with English Abstr.). Wang, E., Cruse, R.M., Chen, X., Daigh, A., 2012. Effects of moisture condition and freeze/ thaw cycles on surface soil aggregate size distribution and stability. Can. J. Soil Sci. 92, 529–536. Wiggs, G.F.S., Baird, A.J., Atherton, R.J., 2004. The dynamic effects of moisture on the entrainment and transport of sand by wind. Geophys. J. Roy. Astron. Soc. 59, 13–30. Yang, X.M., Wander, M.M., 1998. Temporal changes in dry aggregate size and stability: tillage and crop effects on a silty loam Mollisol in Illinois. Soil Tillage Res. 49, 173–183. Yang, H.S., Wu, Q.J., 1980. Physical and Mechanical Properties of Frozen Soil. Science Press, Beijing (135 pp., in Chinese). Yao, X.L., Cheng, Y.S., 1986. Soil Physics. Agriculture Press, Beijing 147–161 (in Chinese).
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Geomorphology journal homepage: www.elsevier.com/locate/geomorph
Freeze/thaw and soil moisture effects on wind erosion L. Wang a,b, Z.H. Shi a,c,⁎, G.L. Wu a, N.F. Fang a a b c
State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, CAS and MWR, Yangling, Shaanxi Province 712100, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, PR China
a r t i c l e
i n f o
Article history: Received 4 July 2013 Received in revised form 29 October 2013 Accepted 30 October 2013 Available online 6 November 2013 Keywords: Freeze/thaw Soil moisture Wind erosion Effective particle size
a b s t r a c t Wind erosion is very pronounced in semiarid regions during late winter–early spring and has major impacts on regional desertification and agriculture. In order to identify the effects of freeze/thaw and soil moisture on wind erosion, wind tunnel experiments were conducted to compare wind erosion effects under various soil moisture gradients in frozen and thawed soil. The variation of surface soil moisture after wind erosion and the effective soil particle size distribution was tested to explain the differences. The results showed that surface soil moisture content decreased in thawed soil and increased in frozen soil after wind erosion. The mean weight diameter, which increased with increasing soil moisture, was smaller in thawed soil than in frozen soil. The wind-driven sediment flux of frozen and thawed soil both decreased with increasing moisture, owing to the heavier soil particle weight and stronger interparticle bonding forces. The critical soil moisture content for suppressing wind erosion was around 2.34% for frozen soil and around 2.61% for thawed soil. The wind-driven sediment flux of thawed soil was always larger than that of frozen soil at the same moisture content, but this difference became negligible at moisture contents above 3.38%. We may speculate that wind erosion will be more severe in the future because of the lower soil moisture content and fewer soil freezing days as a result of global warming. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Wind erosion is defined as the entrainment, transportation, and deposition of soil particles by the air steam. It has been considered as a serious environmental threat that leads to changes in global biochemical cycles, agricultural productivity decline, property damage, and human health hazards and contributes to climate change (Okin et al., 2004; Ravi et al., 2007; Foltz and McPhaden, 2008; Huang et al., 2010). Wind erosion is widespread in arid and semiarid regions around the world (Ravi et al., 2006). The seasonal fluctuation of climate in semiarid regions results in a variation of vegetation cover and soil properties; therefore soil erodibility is a time varying rather than a static characteristic (Chepil, 1954; Bajracharya et al., 1998). Wind erosion is very pronounced during late winter–early spring when frozen soil thaws, largely because of dry, windy weather and bare, loose surface soil (Li et al., 2004). The process of freeze–thaw can effectively alter soil structure (Pawluk, 1988; Layton et al., 1993; Bullock et al., 2001), which plays an important role in determining soil susceptibility to erosion (Chepil, 1954; Layton et al., 1993; Li et al., 2004). The destructive strength of the freeze–thaw impact is governed by the freezing temperature, the number of freeze–thaw cycles, and the moisture content at freezing (Logsdail and Webber, 1959; Bullock et al., 1988; Bajracharya et al., 1998; Oztas and Fayetorbay, 2003; ⁎ Corresponding author at: College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, PR China. Tel.: +86 27 87288249; fax: +86 27 87671035. E-mail address: [email protected] (Z.H. Shi). 0169-555X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geomorph.2013.10.032
Ferrick and Gatto, 2005; Kværnø and Øygarden, 2006). The stability of soil aggregates is considered a good indicator of soil structure and declines with decreasing freezing temperature (Oztas and Fayetorbay, 2003) and increasing number of freeze–thaw cycles (Bullock et al., 1988; Bajracharya et al., 1998; Kværnø and Øygarden, 2006). However, the effect of the number of freeze–thaw cycles on wet aggregate stability is not consistent: aggregate stability increased as the number of cycles increased from three to six but decreased afterward (Oztas and Fayetorbay, 2003). Additionally, the stability of soil aggregates is negatively correlated with soil moisture content at the time of freezing (Oztas and Fayetorbay, 2003). Soil with high moisture will form more ice with stronger expending forces, which probably breaks interparticle bonds, while at low moisture content ice crystals just complete their growth in the soil pores (Nickling and Bennett, 1984; Bullock et al., 1988; Ferrick and Gatto, 2005). Seemingly, freeze–thaw may cause destruction of the bonds holding aggregates together (Bullock et al., 1988) leading to variation of soil aggregate size distribution. Generally, soil aggregate size has an overall decreasing trend after freeze–thaw impact (Yang and Wander, 1998; Wang et al., 2012), which may influence wind erosion profoundly (Colazo and Buschiazzo, 2010). Many studies focus on the variation of soil erodibility before or after freeze–thaw impact, neglecting the erodibility of soil that stays frozen (Chepil, 1954; Pawluk, 1988; Layton et al., 1993; Bajracharya et al., 1998; Oztas and Fayetorbay, 2003). The sensitivity of soil to erosion after freeze–thaw impact is well-known, while the difference of soil erodibility between frozen soil and thawed soil is still unclear. Some research has shown that the ability of soil to resist the shear forces, which
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greatly influences soil erodibility, declines when frozen soil thaws (Ting et al., 1983). Moreover, stronger wind speeds are needed for dust uplift on frozen grounds (15.7 m s−1) than on thawed grounds (12.6 m s−1) (Han et al., 2011). Seemingly, wind erosion appears more frequently on thawed soil than on the frozen soil, but the exact reason is ambiguous. Investigating the changes in wind erosion owing to frozen soil thaws in advance from global warming is meaningful for better predicting wind erosion in the future. Soil moisture may also effectively influence the freeze–thaw impact, so soil moisture should also be taken into account. Within the last few decades, the soil surface conditions have become an important aspect of studies of geomorphological processes related to wind erosion (McKenna-Neuman and Nickling, 1989; Fécan et al., 1999; Wiggs et al., 2004; Han et al., 2009, 2011). The purpose of this paper was to identify the effects of freeze/thaw and soil moisture on wind erosion, providing a basis for research on wind erosion in cold seasons of semiarid regions under global warming. The differences of wind erosion under various soil moisture gradients in frozen and thawed soil were compared. The Liudaogou catchment of the Loess Plateau was selected as the sampling area because it is within a semiarid region suffering from serious wind erosion and because the ground is commonly frozen from late autumn to early spring. The variation of surface soil moisture, effective soil particle size distribution, and wind-driven sediment flux was measured to investigate wind erosion variation when frozen soil thaws at different moisture levels. 2. Materials and methods
Fig. 1. Time series of (A) monthly average precipitation, evaporation, and temperature; (B) monthly average wind speed and days of gale (NBeaufort force 8).
2.1. Soil samples The sampling area is in the Liudaogou catchment (38°49′ N., 110°23′ E.) of Shenmu County in the northern part of the Loess Plateau, China. It is characterized as a continental semiarid and seasonal wind climate with an average temperature of 8.4 °C. Monthly mean temperatures range from − 9.7 °C in January to 23.7 °C in July, so that part of the ground suffers from freezing during the winter and early spring. The mean annual precipitation is 437 mm with 77.4% occurring from June to September with evaporation also in this period. The mean annual gale days (N Beaufort force 8) are 16.2, and gales occur mainly in spring. Detailed meteorological data are shown in Fig. 1. Loess soil with poor ability to resist erosion is widely distributed in the study area, thus the local soil erosion rate approximates 10,890 t km− 2 y− 1 (Wang et al., 1993). The particle size distribution (with dispersion treatment) and other soil properties are shown in Table 1. 2.2. Sample tray preparation Soil samples were crushed and passed through a 2-mm sieve, wetted with a known amount of distilled water by a sprayer in the form of a fine mist, then covered with a Perspex sheet for 24 h to make sure the water was evenly distributed. The specific soil moisture contents were 1.27 ± 0.03%, 2.02 ± 0.09%, 2.34 ± 0.05%, 2.61 ± 0.12%, and 3.38 ± 0.05% (gravimetric moisture content) according to the varying surface soil moisture contents of different positions of the slopes in the study area (Wang et al., 2004). The prepared soils in the sample trays (79.5 × 50.0 × 9.7 cm3) were packed with a known quantity based on field bulk density obtained from the sampling area. To minimize differences in bulk density, the unit was divided horizontally into half and each packed separately. Afterward, each sample tray was covered with a Perspex sheet before wind tunnel tests to keep the moisture constant. In order to prevent disturbance of the soil surface, the Perspex sheet was supported with foam and wooden sticks to maintain a specific distance from the soil surface. Six sample trays were prepared for each moisture content; three frozen and three thawed. Three of the sample trays were put into a freezer set at −11 °C for 24 h, and put into the
wind tunnel immediately. The other three sample trays were put into the freezer set at −11 °C for 24 h, but thawed at room temperature for 24 h before wind tunnel experiments. The room temperature during all experiments was 10.5–12.0 °C.
2.3. Effective soil particle size distribution tests Pre-wetted soil was put into aluminum boxes (85 cm3) following the same procedure as for the sample trays. Soil samples in the aluminum boxes that stayed frozen or thawed were all passed through a 2-mm sieve. Subsamples (b2 mm) were separated by moving a 1-mm sieve (by hand) right and left by 3 cm with 50 repetitions for 2 min. The N1-mm particles were collected, and the sieving was repeated for the b1-mm factions with the next smaller sized sieve. This procedure was repeated for every sieve size (0.5, 0.25, 0.15, and 0.1 mm). All fractions were oven dried at 65 °C and weighed for calculating effective soil particle size distribution. Eroded sediments in the field consist of both primary particles and soil aggregates (Mitchell et al., 1983), which constitute what can be termed the ‘effective particle size distribution’ (Martinez-Mena et al., 2000). By contrast, ‘ultimate particle size distribution’ data are commonly evaluated after sediment has been fully dispersed into its primary particles (Slattery and Burt, 1997; MartinezMena et al., 2002). Therefore, considering the effective particle size of
Table 1 Selected physical and chemical properties of the study soila. Bulk density (g cm−3)
Mean values Standard deviations
1.35 0.07
pH
8.96 0.21
SOC (g kg−1)
1.23 0.08
Particle-size distribution (%) Sand
Silt
Clay
65.57 2.11
31.24 1.87
3.19 0.24
a Sand (2–0.05 mm), silt (0.05–0.002 mm), clay (b0.002 mm). SOC = soil organic carbon.
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sediment is crucial because this may govern the actual behavior of the transported sediment. 2.4. Wind tunnel tests The wind tunnel experiment was carried out at the Institute of Soil and Water Conservation, Chinese Academy of Science. The total length of the wind tunnel was 24 m, including a power section (3.55 m), a speed regulation section (1.5 m), a rectification section (10 m), a test section (1.28 m), a sediment collection section (3.02 m) and a diversion section, following the design criteria for a low speed wind tunnel. The 1.2-m-wide and 1-m-high cross section can produce free turbulent airflow and a stable airflow field. Its airflow velocity uniformity is b1%, and the static pressure gradient in axial direction is b0.005. Wind velocity was measured by a pitot tube installed 30 cm above the tunnel floor and can be changed continuously from 0 to 15 m s−1 via the control panel. A prepared sampling tray was placed into the wind tunnel and the soil surface was exactly paralleled to the test section's floor. A wind speed of 14 m s−1 was selected as it is close to the maximum monthly average wind speed at 30 cm above the soil surface. Blowing duration was 7 min according to pre-experiment tests. The wind-driven sediment of each sample was trapped in the deposition chamber at the end of the wind tunnel with a capture efficiency of ~90%. It was then collected and weighed after oven drying at 105 °C with an electronic balance to calculate the wind-driven sediment flux (g m−2 s−1). The gravimetric soil moisture content was measured in three replicates before and after each test on small samples taken down to 2 mm by scraping with a sharp knife. 2.5. Data analyses All data were analyzed using SPSS 11.0 (SPSS Inc., 2001). The effects of freeze/thaw and soil moisture on wind erosion were tested by twoway ANOVA. Mean values were compared using S-N-K (Student Newman Keuls). Data were transformed through ‘compute procedure’ prior to statistical analysis to make its distribution more normal when necessary. 3. Results and discussion 3.1. Variation of surface soil moisture contents after wind erosion The ratio of surface soil (0–2 mm) moisture contents after wind erosion (w1) to before wind erosion (w2) were all b 1 in thawed soil and N 1
Fig. 2. Ratios of surface soil (0–2 mm) moisture contents after wind erosion (w1) to before wind erosion (w2) at different soil moisture contents. Bars indicate standard deviations.
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in frozen soil (Fig. 2) at each moisture content. This meant that, after wind erosion, surface soil moisture decreased in the thawed soil but increased in the frozen soil although the blowing duration was only 7 min. The surface soil moisture of the frozen soil increased slightly with wind blowing probably owing to the condensation of the atmospheric moisture on the topsoil. Additionally, during the freezing process, water in the liquid and the vapor phases within the soil may significantly redistribute (Hoeckstra, 1966; Hansson et al., 2004) owing to the pressure-induced gradients and temperature gradients (Burt and Williams, 1976). The water was moved from the subsoil to the freezing zone nearer the surface (Ferguson et al., 1964; Cary and Mayland, 1972), leading to an increase of surface soil moisture content as well. The amount of movement probably depends on available soil water, temperatures of the frozen zone, length of frozen period, and physical properties of the soil (Ferguson et al., 1964). Cold soil particles absorb the atmospheric moisture as long as they creep on the soil surface so that the wet particles bond together (Fig. 3). Some ice may be sublimated by the strong wind (van Dijk and Law, 2003), and then the wind-dry soil particles are blown away and the wet particles are retained on the topsoil. The surface soil moisture of thawed soil decreased immediately with wind blowing until it was reduced to a threshold (which depended on the soil physicochemical properties and particle distribution) when entrainment would occur (Wiggs et al., 2004). As the dry surface soil was blown away, a ‘new’ surface layer with higher moisture appears and this too will be removed once it is dry enough. This phenomenon circulates during the whole wind erosion process.
3.2. Effective particle size distribution of frozen/thawed soil Fig. 4 shows the effective particle size distribution of frozen and thawed soil at different moisture contents. Effective particles in the size of 2–1 and b 0.1 mm showed an obvious change with soil moisture while the variation of other effective particle sizes was negligible in frozen soil and in thawed soil (Fig. 4). The 2–1 mm particle content increased with increasing moisture while the b0.1 mm particle content decreased, indicating that fine particles (b0.1 mm) probably gathered into large particles (2–1 mm) through soil moisture. Much research has found that the bonding forces among soil particles are, besides the electrostatic and van der Waals forces, a result of wet bonding forces owing to the presence of water, which increase with increasing soil moisture in a certain range (Yao and Cheng, 1986; McKenna-Neuman and Nickling, 1989; Saleh and Fryrear, 1995; Fécan et al., 1999; Cornelis et al., 2004). When soil water is frozen, ice in the soil has a tensile strength and is therefore responsible for the cementing of solid particles. With increasing volumetric ice content, this effect becomes more important and the cohesion increase too (Arenson et al., 2004). A decrease in temperature below the freezing point and/or an increase in soil initial moisture content may lead to an increase in volumetric ice content (Yang and Wu, 1980; Müller-Lupp and Bölter, 2003). Thus the mean weight diameter (MWD) of thawed soil and frozen soil increased with each successive addition of moisture (Table 2). Soil particles (without any dispersion treatment) of b 0.25 mm can be considered as fractions highly erodible by wind (Liu et al., 2003). Fig. 5 shows that b0.25 mm soil particles decreased with increasing moisture in frozen and in thawed soil, leading to the reduction of erodible fractions. Additionally, the phenomenon of fine particles (b0.1 mm) gathering into large particles (2–1 mm) was more obvious in frozen soil than that of thawed soil (Fig. 4). The bonding forces of liquid water are lower than that of ice, because the activation energy of the hydrogen atom reduces when the temperature decreases, which consequently strengthens the bonding forces of molecules (Yang and Wu, 1980). As a result, the MWD of thawed soil was smaller than that of frozen soil at the same moisture (Table 2). Thawed soil contained more highly erodible
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A
B
C
D
Fig. 3. Soil surfaces at the moisture content of 2.61%. (A) Frozen soil before wind erosion; (B) frozen soil after wind erosion; (C) thawed soil before wind erosion; (D) thawed soil after wind erosion. Note the gathering soil particles after wind erosion on the frozen soil surface.
fractions (b0.25 mm) than frozen soil, and the difference increased with increasing moisture (Fig. 5). 3.3. Influence of freeze/thaw and soil moisture on wind-driven sediment flux 3.3.1. Wind-driven sediment flux of frozen soil varied with soil moisture The two-way ANOVA revealed freeze/thaw and soil moisture can interactively influence wind-driven sediment flux (Table 3). Fig. 6 shows that the wind-driven sediment flux of frozen soil decreased with increasing moisture. For instance, the wind-driven sediment flux
at 1.27% moisture content is about six times higher than at 3.38%. Ice around the individual soil particles could increase the weight of the particles, making them heavier and more difficult to drag and lift from the soil surface by wind. As previously stated, more large soil particles are present in frozen soil at high moisture content because of stronger ice-bonding forces (Figs. 4, 5 and Table 2), which resulted in less erodible fractions and suppression of wind erosion. Simple correlation coefficients between wind-driven sediment flux and indices of effective soil particle size distribution were calculated (Table 4). The result showed that wind-driven sediment flux was found to be negatively correlated with MWD and 2–1 mm particle content and positively correlated
Fig. 4. Effective particle size distribution of frozen and thawed soil at different soil moisture contents. Bars indicate standard deviations.
L. Wang et al. / Geomorphology 207 (2014) 141–148 Table 2 Mean weight diameter (μm) of frozen/thawed soil at different soil moisture contentsa. Freeze/thaw impact
Frozen Standard deviations Thawed Standard deviations
Soil moisture content 1.27%
2.02%
2.34%
2.61%
3.38%
155.54Aa 1.95 145.18Ba 1.08
166.99Aab 0.96 151.27Bb 2.87
178.76Ab 2.31 158.75Bc 1.39
268.88Ac 12.4 175.40Bd 0.44
309.13Ad 12.5 185.81Be 5.66
a Values of different moisture contents followed by the same lowercase letter are not significantly different, and values for frozen/thawed soil followed by the same uppercase letter are not significantly different at p b 0.05.
with b0.25 and b 0.1 mm particle content, suggesting that large particles are effective in controlling wind erosion, which is consistent with the finding of Colazo and Buschiazzo (2010). Additionally, the shear resistance of frozen soil, which affects wind erosion profoundly, increases with increasing ice content until it reaches a threshold (Nickling and Bennett, 1984). This threshold is seldom reached in wind-eroded regions owing to very low soil moisture contents. All the above factors will influence the reduction in wind-driven sediment flux with increasing soil moisture. The difference in wind-driven sediment flux was negligible between 1.27% and 2.02% moisture content and between 2.61% and 3.38% moisture content (Fig. 6). We can speculate that when soil moisture is low, small increases cannot change the antierodibility of frozen soil significantly. At high moisture contents, the antierodibility of frozen soil is sufficiently strong to suppress wind erosion, so the increase in moisture has little effect on wind-driven sediment flux reduction. Soil moisture content around 2.34% is critical for frozen soil to resist wind erosion. This critical soil moisture level was probably influenced by soil particle distribution. Soil containing more fine particles has a larger specific surface area, which strongly adsorbs water around the particles leading to more unfrozen water in the frozen soil compared to soil with less fine particles; thus the mechanical strength of the former soil is weaker than the latter soil at the same soil moisture content and negative temperature (Yao and Cheng, 1986).
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3.3.2. Wind-driven sediment flux of thawed soil varied with soil moisture The wind-driven sediment flux of thawed soil decreased with increasing moisture (Fig. 6). For instance, wind-driven sediment flux at 1.27% moisture is ~200 times more than that at 3.38% moisture. Similarly to frozen soil, water films around the soil particles in the thawed soil may increase the weight of the particles, and more large soil particles are present in thawed soil at high moisture content because of stronger wet bonding forces (Figs. 4, 5 and Table 2), leading to suppression of wind erosion. Simple correlation coefficients between wind-driven sediment flux of thawed soil and indices of effective soil particle size distribution are shown in Table 4. In Fig. 6, significant differences in wind-driven sediment flux of thawed soil at moisture contents of 1.27%, 2.02%, and 2.34% are shown, while at 2.61% and 3.38% moisture levels, the difference was not significant, suggesting that the impact of moisture on wind erosion is greater at low moisture contents. Apparently, moisture content around 2.61% is critical to prevent wind erosion of thawed soil. The critical moisture content varies in soil types. Nickling (1978) found 3–4% (gravimetric) was the critical moisture content to hamper wind erosion of surface soil consisting of fine sand and silt. Chen et al. (1996) found that the soil moisture content of 4–6% (gravimetric) could prevent wind erosion of loessial sandy loam, and Han et al. (2009) found that moisture contents above 0.802% (gravimetric) could resist wind erosion of sand. The differences in these results are likely owing to differences in physicochemical properties of the soils, especially the primary soil particle distribution. Together with the results of the present study, we can say that finer soil particles need more moisture to suppress wind erosion. Wet bonding forces consist of capillary forces and adhesive forces of adsorbed water films surrounding the particles (Yao and Cheng, 1986; McKenna-Neuman and Nickling, 1989; Saleh and Fryrear, 1995; Fécan et al., 1999; Cornelis et al., 2004). The former is the result of the tension exerted by the air–water interface and by the pressure difference inside and out of the water wedge at the contact point of particles (Fisher, 1926), while the latter is the result of wet layer bonding because of the electronic, van der Waals and hydration forces (Tuller et al., 1999). Capillary and adhesion forces cannot be easily separated, since the capillary ‘wedges’ are at a state of internal equilibrium with the adsorption
Fig. 5. Cumulative percentage content of effective particles of frozen and thawed soil at different moisture contents.
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Table 3 Effects of soil moisture and freeze/thaw on wind-driven sediment flux (g m−2 s−1)a. Freeze/thaw impact
Frozen Thawed
Soil moisture content 1.27%
2.02%
2.34%
2.61%
3.38%
0.80 ± 0.17 42.32 ± 1.73
0.71 ± 0.13 7.55 ± 1.15
0.30 ± 0.02 0.67 ± 0.08
0.20 ± 0.05 0.31 ± 0.02
0.15 ± 0.03 0.19 ± 0.02
Factors
F-ratio
P-value
Soil moisture content (A) Freeze/thaw impact (B) Interaction (A × B)
646.63 446.71 160.63
0.000 0.000 0.000
a
Two-way ANOVA results: F-ratio and P-value for main effects and interaction.
‘films,’ and the ones cannot be changed without affecting the others (Hillel, 1982). However, Fécan et al. (1999) pointed out the existence of a minimum moisture content, corresponding to the maximum amount of water that the adsorption molecular can trap (Yao and Cheng, 1986), below which the equilibrium shifted into adsorbed water and the capillary water does not induce strong wet bonding forces. The critical moisture value mentioned above can be considered as the minimum moisture according to Han et al. (2009). He proposed that a slight increase in moisture can dramatically enhance the wet bonding forces resulting from adsorbed water films before the critical moisture value. While exceeding the critical moisture value, wet bonding forces mainly resulted from capillary forces that did not vary intrinsically with slight increases in moisture content, leading to the different degrees of wind erosion variations among different soil moisture contents. Fig. 6 reveals that a slight moisture increase of b 2.61% can dramatically reduce wind erosion modulus thus supporting the findings of Han et al. (2009). 3.3.3. Difference of wind-driven sediment flux between frozen soil and thawed soil Wind-driven sediment flux of thawed soil is more than that of frozen soil at the same moisture content (Table 3, Fig. 7), probably owing to the weaker shear resistance of thawed soil than frozen soil (Yang and Wu, 1980; Ting et al., 1983). Frozen soil is made of mineral solid particles,
ice, unfrozen water, and gases of various chemical compositions (Razbegin et al., 1996). During freezing, solutes are excluded from the ice phase and concentrated within films of unfrozen water, leading to the formation of inorganic precipitates as a consequence of exceeding their solubility limit, which may enhance soil structural stability (Sletten, 1988). Furthermore, ‘soil strengthening’ may occur after freezing because of the increased dilatancy, structural hindrance, and tension in the unfrozen water film, and consequently the strength of frozen soil generally exceeds the sum of ice strength plus soil strength (Ting et al., 1983). After wind erosion, surface soil moisture decreased in the thawed soil while it increased in the frozen soil (Fig. 2). Moisture attached to the surface of frozen soil during the wind erosion process seems to create a protective film to prevent soil particles being transported, although the film is shallow. Additionally, more large soil particles are present in frozen soil than thawed soil at the same moisture content (Figs. 4, 5 and Table 2), leading to a relatively higher erodibility of thawed soil. The critical soil moisture value for suppressing wind erosion is around 2.34% for frozen soil and around 2.61% for thawed soil, indicating that thawed soil needed more moisture to resist wind erosion. Fig. 7 shows that the difference in wind driven sediment flux between frozen soil and thawed soil decreased with increasing soil moisture. For instance, a fiftyfold increase in wind-driven sediment flux was found when frozen soil thawed at moisture content of 1.27%, but was only a onefold increase at moisture content of 3.38%. The difference was significant at 1.27%, 2.02%, 2.34%, and 2.61% soil moisture while the difference did not reach a significant level at 3.38% (Fig. 7). Apparently, the increase in wind-driven sediment flux because of soil thawing is dramatic at lower moisture contents. However, other studies on wellsorted fine quartz sand have shown that frozen soil particles are uncemented at a value of 0.5% gravimetric moisture content (van Dijk and Law, 2003). Although the critical moisture value will change with soil type, it seems that when the moisture contents lie beneath the critical value, the difference in soil resistance to wind erosion between thawed soil and frozen soil will not be significant, owing to the negligible wet (ice) bonding force. Wind erosion is suppressed when soil moisture content reaches a relatively high level, largely because the higher weight of soil particles and bonding forces resist wind erosion. Therefore, the difference in wind-driven sediment flux between thawed and frozen soil becomes insignificant at higher moisture contents. In this
Table 4 Simple correlation coefficients between wind-driven sediment flux and indices of effective soil particle size distribution. Wind-driven Indices of effective soil particle size distribution sediment flux 2–1 mm particle b0.25 mm particle b0.1 mm particle MWD content content content Frozen soil Thawed soil Fig. 6. Relationship between wind-driven sediment flux and soil moisture content. Bars indicate standard deviations. Different letters indicate significant differences (p b 0.05).
⁎ p b 0.05. ⁎⁎ p b 0.01.
−0.762⁎⁎ −0.710⁎⁎
0.770⁎⁎ 0.610⁎
0.671⁎⁎ 0.551⁎
−0.759⁎⁎ −0.670⁎⁎
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Fig. 7. Difference of wind-driven sediment flux between frozen soil and thawed soil at different moisture contents. Bars indicate standard deviations. Different letters indicate significant differences (p b 0.05).
study, moisture content around 3.38% is the transition point that affects the degree of difference between thawed and frozen soil. We can speculate that in the wind erosion regions during late winter–early spring, when soil moisture is relatively low, frozen soil thaw may likely aggravate wind erosion. The factors described above indicate that wind erosion in thawed soil is generally more serious than in frozen soil at the same soil moisture content, and the difference is more pronounced at low moisture content. As mentioned above, the surface moisture content of the frozen soil increased owing to the condensation of the atmospheric moisture and the upward migration of the unfrozen water, which may profoundly reduce the wind erosion during the experiment. However, under natural conditions in the field, thawing occurred from top and bottom in the soil profile but was more rapid from the top, preventing downward movement of water, and thereby the perched water may expose to the evaporation forces near the surface, which may probably contribute to overwinter losses of soil water (Ferguson et al., 1964). Annual soil freezing days declined with increasing mean winter air temperature (Henry, 2008), prolonging the period that thawed soil is exposed to gales, which may aggravate wind erosion. Furthermore, higher temperatures may accelerate the evaporation of soil moisture, leading to the formation of drier topsoil, thus contributing further to the occurrence of wind erosion. The findings of this paper may also apply to the middle latitude regions where ground is commonly frozen in early spring and spring dust outbreaks occur frequently such as the eastern Mongolian Plateau. However, these results are only wind tunnel measurements, and further research is required to confirm them with field observations in the study area. 4. Conclusions In this study, effects of freeze/thaw and soil moisture on wind erosion were identified by wind tunnel experiments, and the conclusions are as follows: • Surface soil moisture content of thawed soil decreased after wind erosion, while that of frozen soil increased because of the condensation of atmospheric moisture and the upward migration of the unfrozen water. Moisture attached to the surface soil seems to create a protective film to prevent soil particles being transported, and a quantity of cold soil particles bond together as long as they creep on the soil surface, which can partly suppress wind erosion. • Fine particles (b 0.1 mm) gathered into large particles (2–1 mm) owing to wet (ice) bonding forces and the phenomenon was increasingly obvious with increasing moisture but less obvious in thawed than in frozen soil. The MWD of frozen and thawed soils increased
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with increasing moisture, largely because of the stronger wet (ice) bonding forces at relatively high moisture, leading to a lower highly erodible fraction (b 0.25 mm particles), which can effectively suppress wind erosion. The MWD of thawed soil was always smaller than that of frozen soil at the same moisture content, on account of the weaker bonding forces of liquid water than that of ice. • Wind-driven sediment flux of thawed and frozen soils both decreased with increasing soil moisture because of heavier particles and stronger interparticle forces which can be partly reflected in the soil particle size distribution. The critical soil moisture content for suppressing wind erosion is around 2.34% for frozen soil and around 2.61% for thawed soil. 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