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Implications of surface properties for dust emission from gravel deserts (gobis) in the Hexi Corridor
Geoderma 268 (2016) 69–77
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Implications of surface properties for dust emission from gravel deserts (gobis) in the Hexi Corridor Zhengcai Zhang a,⁎, Zhibao Dong a, Jiyan Li a,b, Guangqiang Qian a, Chanwen Jiang a,c a b c
Key Laboratory of Desert and Desertification, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China Taiyuan Normal University, Jinzhong 030619, China Key Laboratory for Ecology and Environment of River Wetlands in Shaanxi Province, School of Chemistry and Environment, Weinan Normal University, Weinan 714099, China
a r t i c l e
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Article history: Received 1 September 2015 Received in revised form 7 January 2016 Accepted 12 January 2016 Available online 5 February 2016 Keywords: Dust emission Gravel desert Gobi Hexi Corridor Surface sediments
a b s t r a c t Gravel deserts (gobis) may be primary dust sources in China, where they occupy almost the same area as sandy deserts. The obvious difference between gravel and sandy deserts is the dominant grain size in the surface sediments: fine in sandy deserts versus coarse in gravel deserts. Potential sand transport, gravel cover, and the mean grain size of the surface sediments of gravel deserts are the main factors that affect dust release. However, little data is available on these properties. In the present study, we estimated gravel cover in surface photographs using ImageJ software, determined the proportion of the total weight accounted for by gravel (diameter N 2 mm), and described the grain size distribution for study areas in the Hexi Corridor of northern China. We found statistically significant differences between the proportions of total weight as gravel among five sub-regions. This proportion ranged from 22 to 91% of the total (66 ± 17%; mean ± SD), and the proportions in most of the samples (73%) ranged from 40 to 80%. The gravel cover ranged from 15 to 87% (52 ± 17%), which was within the range in previous research that produced maximum aerodynamic roughness. The sandy material in the surface sediments was mainly medium sand, which accounted for 52.5% of the total sample. Potential sand transport was N 200 vector units in most gravel deserts, and 75% of the study sites had a physical soil crust. The high gravel cover and frequency of surface crusts is likely to decrease dust emission from the gravel deserts of the Hexi Corridor. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The wind-driven emission, transport, and deposition of aeolian sediments are important processes in desert areas (Kok et al., 2012). The magnitudes of these phenomena are controlled by the regional wind regime and by surface characteristics (e.g., sediment grain size distribution, soil moisture content, vegetation cover). Long-term aeolian sediment transport can carry dust in the air for hundreds or thousands of kilometers from source regions (Gillette and Walker, 1977; Zender et al., 2003, Miller et al., 2006). This dust can cause severe environmental and social problems, and has therefore attracted considerable attention from governments and researchers. Previous research indicated that the world's dominant sources of natural mineral dust are located in the northern hemisphere, where there are many deserts and other areas of dry land (Kok et al., 2012). In Asia, dust sources include sandy and gravel deserts in southern Mongolia, the Taklimakan Desert, the Badain Jaran Desert, the Tengger Desert, and the Ulan Buh Desert (Zhang et al., 1996). In China, the ⁎ Corresponding author at: Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, No. 260, West Donggang Road, Lanzhou, Gansu Province 730000, China. E-mail address: [email protected] (Z. Zhang).
http://dx.doi.org/10.1016/j.geoderma.2016.01.011 0016-7061/© 2016 Elsevier B.V. All rights reserved.
major source regions are the Taklimakan Desert in western China, the arid to semi-arid region of northwestern China, and eastern Inner Mongolia (Zhang et al., 2008a; Kok et al., 2012). The dust emission from these source regions accounts for about 70% of the total Asian dust emission (Zhang and Gong, 2005). However, the specific dust sources are still being debated, mainly because of inaccurate modeling (due to inaccurate input parameters), unsound study methodology, a lack of detailed information on surface characteristics, or a combination of these factors. Some researchers think that sandy deserts are the main dust sources (Laurent et al., 2006); others think that gravel deserts are the main dust sources (Wang et al., 2011). Laurent et al. (2006) proposed that China's sandy deserts generated more dust than its gravel deserts. Dust storms have been studied by means of field observations (Jugder et al., 2011), wind tunnel studies (Wang et al., 2011, 2012), numerical modeling (Laurent et al., 2006), satellite remote sensing (Gu et al., 2003; Zhang et al., 2008a), and chemical analysis (Zhang et al., 1996, 1997). However, due to a lack of field data on the properties of desert surfaces, most previous studies mainly analyzed synoptic data on dust storms (Laurent et al., 2006; Park et al., 2010). This approach can introduce much uncertainty in efforts to identify the dust source and clarify entrainment and transport of dust materials in models. For example, the lack of field observations has created large uncertainties in key model parameters such as the surface grain size distribution,
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threshold wind velocity, and aerodynamic roughness length. Uno et al. (2006) found that the estimated dust concentration could differ by a factor of 2 to 4 in their analysis of the effects of changes in such parameters in eight dust transport models. The area of sandy desert in northwestern China is about 56 × 104 km2, versus about 68 × 104 km2 for gravel desert; these areas occupy about one-third of the total land area in China and are therefore a major part of the central Asian arid region and one of the region's major dust sources. Gravel deserts are likely to be a significant dust source because of their large area, and have therefore attracted the attention of aeolian and dust researchers (Qu et al., 2001; Zhang et al., 2008a; Jugder et al., 2011; Wang et al., 2012; Qian et al., 2014). Dust emissions are usually caused by the action of strong winds acting on dry, fine, and loose soil surface materials (Pye, 1987; Cook et al., 1993) and are related to the proportion of the cover as erodible grains and the fine grain content of the surface material (Wang et al., 2012). A strong wind regime and favorable meteorological conditions (e.g., low surface soil water content, relative humidity, and precipitation) can dramatically increase the likelihood of dust storm occurrence, particularly when combined with a low gravel content in the surface materials and a low vegetation cover. In contrast, a high gravel content, high soil moisture content, a weak wind regime, and high vegetation cover can decrease the entrainment of dust (Laurent et al., 2006; Rostagno and Degorgue, 2011). Vegetation cover can decrease sediment loss by wind because it reduces the near-surface wind speed and soil erodibility, and increases the capacity for capturing windblown eroded material (Van De Ven et al., 1989; Dong et al., 1996; Leenders et al., 2011; Munson et al., 2011). In sandy deserts, there is no doubt that the sandy surface is the main sand source (Zhang et al., 1996, 2008a). The grain size distribution in this surface (Bagnold, 1941), its soil moisture content (Chepil, 1956; McKenna–Neuman and Nickling, 1989; Bisal and Hsieh, 1996; Shao et al., 1996; Dong et al., 2007), and vegetation all control sand and dust entrainment in these deserts (Hupy, 2004). However, in gravel deserts, it is unclear whether the gravel surface is a significant dust source.
In northwestern China, the gravel cover in these deserts ranges between 32 and 85% (Qian et al., 2014). Wind tunnel experiments have indicated that the gravel surface is relatively stable at gravel cover ranging from 40 to 70% (Dong et al., 2002a,b; Zhang et al., 2004), but Wang et al. (2013) recently found that high gravel cover did not necessarily decrease dust entrainment; in fact, With a total gravel coverage below 40%, wind erosion increased as the gravel cover increased. However, due to aerodynamic roughness and drag coefficients on the gravel surface, a gravel cover above 40% resulted in the surface becoming aerodynamically stable and a reduction in wind erosion (Wolfe and Nickling, 1996; Dong et al., 2002a,b). In contrast, Lyles (1988) found that when the cover of non-erodible material reached 80%, erosion of the erodible material ceased. This summary of previous research suggests the importance of gravel cover in aeolian sediment transport. However, apart from the study of Qian et al. (2014), who studied only a small region of the Zhongyang Gravel Desert and Gashun Gravel Desert, there is little data on the distribution of gravel cover in China's desert areas. In the present study, we obtained additional data for a key desert area in northwestern China, the Hexi corridor. Based on our field study, we describe the properties of the region's gravel desert surfaces (grain size distribution, gravel cover proportion, gravel as a proportion of total weight, and the presence of a surface physical soil crust). Our first goal was to provide basic data that could be used to predict wind erosion and dust emission in these areas. Our second goal was to use the properties of the gravel surfaces to support future efforts to assess the potential dust sources in the study area in numerical models. 2. Material and methods 2.1. Study region The Hexi Corridor lies in northwestern China (Fig. 1). The desert area is surrounded by the Qilian Mountains to the south and southwest, by the Kumtagh Desert to the west, by the Mazun Mountains, Heli
Fig. 1. Location of the Hexi Corridor in northwestern China, and locations of the weather stations, the field sample sites and of the five sub-regions of the Hexi Corridor: (A) the Kumtagh Desert, surrounding a gravel desert; (B) the northern gravel desert; (C) the southern gravel desert; (D) the Jiuquan gravel desert; and (E) the Hexi Desert, surrounding a gravel desert.
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Mountains, and Longshou Mountains to the north, by the Badain Jaran Desert to the northeast, and by the Tengger Desert to the southeast. The sandy deserts in this region are the Kumtagh Desert, Hexi Desert, and Tengger Desert. The gravel deserts are mostly found in the plateau areas of the study region. Gravel deserts can be divided into two main categories: erosion-deposition (formed by the peneplain process, mainly Mazun Mountains and adjacent region) and deposition (formed by the aggradation process, mainly Hexi Corridor gravel desert, where the rock fragments were transported by water flow and deposited at the lower region) types (Wang, 2003). The climate of the Hexi Corridor is a typical continental arid climate. Average annual rainfall decreases from east to west, ranging from 158 mm in the southeast (at Wuwei) to 37 mm in the northwest (at Dunhuang). Most (60%) of the rain falls from June through August. The annual potential evaporation is more than 2500 mm, and increases from south to north. The annual average temperature ranges from 5 to 9.5 °C, and increases from 7.7 °C in the southeast to 8.2 °C in the northwest. 2.2. Methods We sampled the surface sediments at 33 sites in the Hexi Corridor (Fig. 1) in December 2013. Before collecting the samples, we photographed the gravel surface and a ruler (to allow calculation of the scale) using a Nikon 7100 digital camera to permit calculation of the gravel cover. Fig. 2 shows typical gravel surfaces from the five subregions of the Hexi Corridor (described in the next paragraph). Our sediment samples included the surface layer (0 to 0.5 cm) and the subsurface layer (0.5 to 4 cm). At each site, we collected the surface materials
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from a 20 × 20 cm quadrat; the total weight of each sample was 0.5 to 1 kg. To describe variations in the characteristics of the grain size distribution throughout the Hexi Corridor, we divided the region into five subregions: A, the Kumtagh Desert, surrounding a gravel desert; B, the northern gravel desert; C, the southern gravel desert; D, the Jiuquan gravel desert; and E, the Hexi Desert, surrounding a gravel desert (Fig. 1). Although no research has been done to identify the sources of the sediments in the Hexi Corridor, the topography shown in Fig. 1 suggests that the source materials for sub-region A were mainly alluvial deposits from the western Qilian Mountains and the Beishan Mountains; those for sub-region B mainly originated from alluvial deposits from the Beishan Mountains and were separated from sub-region C by the Shule River; those for sub-region C mainly originated from alluvial deposits from the Qilian Mountains and were separated from sub-region B by the Shule River; and those for sub-regions D and E mainly originated from alluvial deposits from the Qilian Mountains. We determined the following properties of the surface sediments: the weight of gravel, the gravel cover, the distribution of surface crusts, and the grain size distribution (the content of sand, silt, and clay particles with a grain diameter ≤ 2 mm). The weight of gravel (grain diameter N 2 mm) was determined for about 400 g of the total sediment after removal of the gravel using a 2-mm sieve, using an electronic balance with a precision of 1 mg. The gravel cover was calculated from the field photos using version 1.36b of the ImageJ software (http://imagej. nih.gov/ij/). Fig. 2f shows how gravel particles were identified and measured for a typical gravel surface. Firstly, the dimensions in pixels of particles identified on the photographs were converted to physical dimensions. The photo image was then converted into an 8-bit, gray,
Fig. 2. Typical gravel surfaces in the five sub-regions shown in Fig. 1: (a) sampling site A2, (b) sampling site B2, (c) sampling site C10, (d) sampling site D2, (e) sampling site E3, and (f) identification of gravel particles and calculation of the gravel cover for a typical gravel surface using ImageJ software. Site locations are shown in Fig. 1.
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binary image. Particles N 2 mm in diameter were only measured from which the particle coverage (%) was calculated by using the “Analyze particles” menu in the software. At present, there is no broadly accepted classification system for gravel cover, but wind tunnel experiments using sediments from the study area suggest that gravel cover is related to aerodynamic roughness length; when the gravel cover is b 40%, aerodynamic roughness length increased with increasing gravel cover, reached its maximum
value at a gravel cover ranging from 40 to 70%, and then decreased with increasing gravel cover (Dong et al., 2002a,b; Liu and Dong, 2003). In this paper, we classified gravel cover into three categories based on these relationships: low gravel cover (b40%), moderate gravel cover (40 to 70%), and high gravel cover (N70%). We analyzed the grain size distribution of the samples using a Malvern MasterSizer 2000 (Malvern Instruments Ltd., Malvern, England), which operates in the range of 0.02 to 2000 μm. The grain size
Fig. 3. Wind regime in the study region for (a) mean annual wind speed (m s−1) and (b) potential sand transport (drift potential, DP, in VU) and sand transport direction (resultant drift direction, RDD; arrows).
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and sorting parameters were calculated using the equations devised by Folk and Ward (1957). Potential for dust emission was directly related to the amount of surface erodible material and gravel weight (% of total sampler). We defined the dust material ratio as the ratio of b63 μm particles divided by the % gravel weight to assess the potential for dust emissions. Wind data were measured using automatic weather stations at 27 locations in the Hexi Corridor (Fig. 1). The data were acquired at 10min intervals every hour for 24 h per day from 1 January to 31 December 2009 in an open area at a height of 10 m above the surface. The characteristics of the wind regime (i.e., the mean annual wind speed and annual potential sand transport parameters) were calculated using data from the automatic weather stations. We calculated two key sand drift parameters using the methods of Fryberger and Dean (1979): sand drift potential (DP, in vector units [VU]) and the resultant drift direction (RDD), which represents the net direction in which sand will be transported. 3. Results and discussion 3.1. Wind speed and potential sand transport Wind speed controls the entrainment of sediment materials. The larger the wind speed, the greater the emission of dust and the transport of coarser particles; the magnitude of this transport is represented by DP. The present study represents the first detailed record of wind speed and potential sand transport characteristics in the Hexi Corridor. We used version 9.1 of ArcGIS (www.esri.com) to map the data from our study, then used ordinary kriging interpolation to calculate the distribution of wind speed and potential sand transport between sample points. The annual mean wind speed showed a clear spatial distribution (Fig. 3a), with values ranging from 1.0 to 4.8 m s− 1, and the largest values in the northeastern Hexi Desert and northwestern Tengger Desert.
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Potential sand transport was intermediate to high according to Fryberger and Dean's (1979) classification. There were three regions with particularly high DP (N500 VU): northwest of Dunhuang, northeast of Dunhuang, and northwest of Jiuquan (Fig. 3b). Previous research indicated that winds in the Hexi Corridor were controlled by the Mongolia–Siberia high-pressure system in late spring and early summer. Because of the obstruction created by the western Qilian Mountains, the local wind regime is divided into two branches: one crosses the middle and southwestern parts of the Hexi Corridor, moving southwest, and the other crosses the northern and eastern parts of the Hexi Corridor, moving east to southeast (Wang et al., 2005). Our results suggest that the potential sand transport direction (RDD) follows this pattern and can also be divided into two branches (Fig. 3b). Although the potential sand transport is high (DP N 400 VU) in the northwestern part of the Hexi Corridor (Fig. 3a), this region may not supply much dust for long distance transport to the southeastern Hexi Corridor or even over longer distances to southeastern China, since RDD is primarily to the southwest in this region, and the transported material would be deposited on the northern slopes of the western Qilian Mountains (where there is moderate to low DP because of the higher elevation; i.e., the presence of the Qinghai–Tibetan Plateau). In the central and southeastern Hexi Corridor, DP is lower than in the northwest, but the dust transport direction is to the east and southeast, so the transported dust material would not be blocked by mountains and could potentially be transported over long distances. 3.2. Gravel weight and cover The surface sediments of the gravel deserts had a high gravel content, and this is likely to protect the sediments against erosion and to decrease dust emission. Fig. 4 shows the gravel weight and cover values for the five sub-regions. We used ANOVA to compare these gravel parameters among the sub-regions, and found significant differences in gravel weight as a proportion of the total among the five sub-regions. The
Fig. 4. Gravel weights as a proportion of total sediment weight in the study region for (a) the average weight in each of the five sub-regions shown in the map and (b) for all field samples combined. Gravel cover as a proportion of the total surface area for (c) the average cover in each of the five sub-regions and (d) for all field samples combined. Values are means ± SD. Bars labeled with different letters differ significantly (ANOVA, p b 0.05).
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Fig. 5. Physical soil crust distribution, and the grain size parameters: (a) mean grain size, (b) sorting, (c) skewness, and (d) kurtosis. Bars labeled with different letters differ significantly (ANOVA, p b 0.05).
gravel weight in sub-region A was significantly lower than in the other four sub-regions. The mean gravel weights were similar in sub-regions B and D (77 ± 9% and 77 ± 7%, respectively), and smallest in subregion A (26 ± 4%) (Fig. 4a). Overall, the gravel weight ranged from 22 to 90% of the total (mean ± SD, 65 ± 17%), and most of the samples (73%) had contents of 40 to 80% (Fig. 4b). The study region is a diluvial–alluvial gravel desert (Qian et al., 2014), and the regional geomorphology could have affected the weight of the gravel, since the Qilian Mountains piedmont appear to be the source of most of the materials in the study area. Gravel size decreases with increasing distance from the apex of the alluvial fan to the end of the alluvial fan as the energy of the flowing water decreases (Our results show that from C7 to C9, the gravel weight and cover both increased). This may explain why the gravel weight was largest in sub-region D, which is close to the mountains, and smallest in sub-region A, which is farthest from the mountains. Fig. 4c and d shows the mean gravel cover in the study region for the five sub-regions. We used ANOVA to compare gravel cover among the five sub-regions, and found no statistically significant differences in gravel cover.
ensure the entrainment of this material because this material can form a surface crust that will decrease dust entrainment even under strong winds (Belnap and Gillette, 1998; Gomes et al., 2003). Surface crusts have been observed in many deserts (Mabbutt, 1965). Pye (1987) reported that sand and dust production in the gravel deserts of northwestern China may be higher than in some other gravel deserts due to the particularly arid and windy conditions. However, Pye did not consider the effects of surface crusts in this region. In the Hexi Corridor, crusts have formed in most (75%) of the gravel surfaces (Fig. 5). The distribution of the gravel crusts exposed between gravel clasts is likely to have been affected by a combination of geomorphology and potential sand transport. The gravel crust exposed between gravel clasts was mainly distributed in the alluvial fan, where rainfall and fine materials in the surface sediments produced a thin coating. However, in areas with higher DP, the stronger winds would have deflated the surface by transporting fine material, thereby preventing crust formation, even though the amount of rainfall was similar.
3.3. Physical soil crust distribution In arid regions, surface physical soil crusts often form as a result of interactions between rainfall and fine materials such as clays in the surface sediments, particularly in the presence of large concentrations of soluble salts (Valentin and Bresson, 1992). These crusts form a strong, dense, relatively impermeable layer that can protect the underlying fine materials from wind erosion and stabilize the surface (Cook et al., 1993). Wind tunnel experiments indicated that the threshold wind speed for entrainment of particles above the surface of typical crusts is 4 to 5 times that above surfaces that lack a crust (Zhang et al., 2008b), so crusts could greatly decrease sediment entrainment in the study area, thereby decreasing wind erosion and dust storm occurrence. Alluvial fans and gravel deserts are major sources of the fine materials that produce dust (Pye, 1987). However, a high silt and clay content does not
Fig. 6. Relationship between gravel weight and gravel cover for all samples combined.
Z. Zhang et al. / Geoderma 268 (2016) 69–77 Table 1 Grain size distribution in the five sub-regions (A to E). Locations of the sub-regions are shown in Fig. 2. Size (μm)
b31 31–63 63–125 125–250 250–500 500–1000 N1000 Silt + clay Sand Gravel
Proportion of total (%) A
B
C
D
E
12.6 ± 4.2 7.6 ± 3.6 23.7 ± 5.8 27.0 ± 7.6 16.8 ± 5.0 10.6 ± 4.9 1.8 ± 1.4 20.2 ± 6.6 78.1 ± 6.2 1.8 ± 1.4
20.2 ± 10.6 3.8 ± 1.4 9.6 ± 1.0 21.2 ± 3.7 26.5 ± 4.1 16.7 ± 4.5 1.9 ± 2.0 23.95 ± 11.7 74.1 ± 10.6 1.9 ± 2.0
15.9 ± 8.2 7.3 ± 2.6 16.3 ± 4.8 18.3 ± 5.7 18.3 ± 3.7 18.4 ± 7.0 5.4 ± 3.6 23.3 ± 10.6 71.4 ± 9.0 5.4 ± 3.6
30.2 ± 10.6 10.5 ± 3.0 18.3 ± 6.8 18.7 ± 7.2 12.8 ± 6.2 7.8 ± 8.1 1.8 ± 3.7 40.6 ± 13.3 57.5 ± 12.0 1.8 ± 3.7
10.6 ± 2.8 3.2 ± 1.0 22.0 ± 7.9 35.7 ± 13.2 19.4 ± 5.1 9.8 ± 11.1 3.3 ± 2.9 9.8 ± 3.7 86.9 ± 5.7 3.3 ± 2.9
3.4. Gravel mass and cover Gravel could decrease wind speed and increase the surface aerodynamic roughness on the gravel surface, which could decrease dust emissions. Rostagno and Degorgue (2011) defined the gravel mass as the particles larger than 2.0 mm that either lay on the soil surface or were embedded in the soil by b50% of their volume. They found that the gravel cover could be expressed by a linear equation. Unlike in their study,
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we found a power relationship for the sediment in the Hexi Corridor gravel deserts (Fig. 6). This may have resulted from different geomorphological characteristics in the two study areas. Our data show that more than 61% of the gravel cover values were N40% in the Hexi Corridor (Fig. 4d), which means that most of the gravel surfaces would be stable, and that it would be difficult for the wind to entrain dust materials. Wind tunnel experiments have shown that wind erosion increased with increasing gravel cover when the gravel cover was b40% (Wang et al., 2013) or that erosion decreased when gravel cover was N30% (Wang et al., 2012). In the study region, 24% of the sites had gravel cover values between 30 and 40%. Lyles (1988) found that a gravel surface with gravel cover N80% experienced no erosion. Zhang et al. (2004) indicated that gravel cover of 40 to 60% could stabilize gravel surfaces and prevent erosion. At the border between China and Mongolia, gravel cover was b40% (Wang et al., 2013), and in eastern Xinjiang, which lies west of Mongolia, the gravel cover in the Gashun Gobi and Zhongyang Gobi ranged from 32 to 85%, and most of the gravel cover ranged from 40 to 70% (Qian et al., 2014). 3.5. Grain size distribution Dust storms usually result from the action of strong winds on dry, fine, and loose soil surfaces. The silt and clay content is thought to determine the resistance of sediments to wind erosion (Marticorena and
Fig. 7. Grain size distributions for particles ≤1 mm in diameter for (a to e) the five sub-regions shown in Fig. 1, and (f) ratio of b63 μm dust percentage and gravel weight percentage of total sample.
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Bergametti, 1995). Table 1 shows the grain size distribution for samples from the five sub-regions. ANOVA revealed significant differences in the silt plus clay among the five sub-regions. The mean silt plus clay content ranged from 9.8 to 40.1%, with the largest values in sub-region D (20 ± 7%, mean ± SD) and the smallest in sub-region E (10 ± 4%). The mean sand content ranged from 57.5 to 86.9%, with the largest value in subregion E (86.9 ± 5.7%) and the smallest value in sub-region D (57.5 ± 12.0%). In sub-region A, the silt plus clay content and the sand content ranged from 9.0 to 27.1% and from 72.5 to 88.8%, respectively. In sub-region B, the silt plus clay content and the sand content ranged from 6.7 to 41.1% and from 58.3 to 89.5%, respectively. In sub-region C, the silt plus clay content and the sand content ranged from 7.9 to 36.3% and from 60.5 to 86.9%, respectively. In sub-region D, the silt plus clay content and the sand content ranged from 26.3 to 59.9% and from 39.0 to 69.0%, respectively. In sub-region E, the silt plus clay content and the sand content ranged from 6.0 to 14.8% and from 78.4 to 89.6%, respectively. This variation in the silt plus clay content affected the extent of the surface crust, since this content at sites with a crust (20.7 ± 10%) was about 2 times the content at sites with no crust (9.0 ± 7.8%). The crust contains higher amounts of fine materials, and protects the fine material under the crust from wind erosion, thus the silt plus clay content is larger in this underlying material. On surfaces with no crust, strong winds and the resulting high DP led to deflation of the surface material by transport of silt and clay particles, thereby greatly decreasing the silt plus clay content. The entrainment of dust involves a complex group of processes that are controlled by the gravel cover and the abundance of dust materials at the surface. Dust materials in the gravel surface provide potential dust entrainment sources. For a given set of wind and surface conditions (such as surfaces with no crust), a higher silt and clay content at the gravel surface increases entrainment of dust materials. Fig. 7 shows the grain size distribution of the samples from each of the five-subregions and for the study region as a whole. Our data reveals both unimodal and bimodal distributions, but most of the curves (83.8%) were bimodal. Dust material ratio was largest in sub-region E (4.90 ± 1.49) and smallest in sub-region A (0.91 ± 0.17) (Fig. 7f). In general, stronger winds and higher DP would transport more surface silt and clay into the air. However, we lack sufficient data on the mean wind speed and DP at each sample site to confirm whether these factors affected the surface silt and clay contents in the study region. In sub-regions A, B, and C, the mean silt and clay contents were similar (they ranged from 20 to 24%), but the wind speed and DP were significantly higher in subregion B than in sub-regions A and C. 4. Conclusions Gravel deserts are an important part of the landscape in the arid and semi-arid regions of China. Previous research suggested that these deserts are a significant dust source, but until the present study, there had been little research on the properties of the gravel surface that could be used to support the development of numerical models of dust emission and transport from these deserts. The present study provides much data that can be used to support such modeling. Based on our results, we have the following main conclusions: 1) In the Hexi Corridor, the gravel cover is mainly moderate (40 to 70% in 53% of the total samples). 2) Most (75%) of the gravel surfaces have developed a crust and formed between the gravel clasts, with the main exceptions being in areas with high DP and at the margins of the desert. 3) The content of the erodible materials (sand, silt, and clay) showed a clear spatial distribution. Most of the gravel surface sediments were medium and fine sands, which accounted for 52.5 and 25.0% of the total, respectively. The silt plus clay content ranged from 9.8 to
40.1%, and most of the content (about 73% of the samples) ranged from 10 to 30%. Dust material ratio, crust distribution, and potential sand transport indicated that the northern region of the Hexi Corridor was the primary dust source. 4) The spatial heterogeneity of the wind regime and the gravel surface properties means that there will be similarly large variation in the dust entrainment processes and amount of dust material that is entrained. However, this theoretical conclusion must be verified by additional field sampling (e.g., collection of blowing sediments in wind tunnel tests). Additional data about differences in dust emission from gravel surfaces would let researchers build more robust dust emission models capable of providing more accurate regional and global dust emission estimates.
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Implications of surface properties for dust emission from gravel deserts (gobis) in the Hexi Corridor Zhengcai Zhang a,⁎, Zhibao Dong a, Jiyan Li a,b, Guangqiang Qian a, Chanwen Jiang a,c a b c
Key Laboratory of Desert and Desertification, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China Taiyuan Normal University, Jinzhong 030619, China Key Laboratory for Ecology and Environment of River Wetlands in Shaanxi Province, School of Chemistry and Environment, Weinan Normal University, Weinan 714099, China
a r t i c l e
i n f o
Article history: Received 1 September 2015 Received in revised form 7 January 2016 Accepted 12 January 2016 Available online 5 February 2016 Keywords: Dust emission Gravel desert Gobi Hexi Corridor Surface sediments
a b s t r a c t Gravel deserts (gobis) may be primary dust sources in China, where they occupy almost the same area as sandy deserts. The obvious difference between gravel and sandy deserts is the dominant grain size in the surface sediments: fine in sandy deserts versus coarse in gravel deserts. Potential sand transport, gravel cover, and the mean grain size of the surface sediments of gravel deserts are the main factors that affect dust release. However, little data is available on these properties. In the present study, we estimated gravel cover in surface photographs using ImageJ software, determined the proportion of the total weight accounted for by gravel (diameter N 2 mm), and described the grain size distribution for study areas in the Hexi Corridor of northern China. We found statistically significant differences between the proportions of total weight as gravel among five sub-regions. This proportion ranged from 22 to 91% of the total (66 ± 17%; mean ± SD), and the proportions in most of the samples (73%) ranged from 40 to 80%. The gravel cover ranged from 15 to 87% (52 ± 17%), which was within the range in previous research that produced maximum aerodynamic roughness. The sandy material in the surface sediments was mainly medium sand, which accounted for 52.5% of the total sample. Potential sand transport was N 200 vector units in most gravel deserts, and 75% of the study sites had a physical soil crust. The high gravel cover and frequency of surface crusts is likely to decrease dust emission from the gravel deserts of the Hexi Corridor. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The wind-driven emission, transport, and deposition of aeolian sediments are important processes in desert areas (Kok et al., 2012). The magnitudes of these phenomena are controlled by the regional wind regime and by surface characteristics (e.g., sediment grain size distribution, soil moisture content, vegetation cover). Long-term aeolian sediment transport can carry dust in the air for hundreds or thousands of kilometers from source regions (Gillette and Walker, 1977; Zender et al., 2003, Miller et al., 2006). This dust can cause severe environmental and social problems, and has therefore attracted considerable attention from governments and researchers. Previous research indicated that the world's dominant sources of natural mineral dust are located in the northern hemisphere, where there are many deserts and other areas of dry land (Kok et al., 2012). In Asia, dust sources include sandy and gravel deserts in southern Mongolia, the Taklimakan Desert, the Badain Jaran Desert, the Tengger Desert, and the Ulan Buh Desert (Zhang et al., 1996). In China, the ⁎ Corresponding author at: Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, No. 260, West Donggang Road, Lanzhou, Gansu Province 730000, China. E-mail address: [email protected] (Z. Zhang).
http://dx.doi.org/10.1016/j.geoderma.2016.01.011 0016-7061/© 2016 Elsevier B.V. All rights reserved.
major source regions are the Taklimakan Desert in western China, the arid to semi-arid region of northwestern China, and eastern Inner Mongolia (Zhang et al., 2008a; Kok et al., 2012). The dust emission from these source regions accounts for about 70% of the total Asian dust emission (Zhang and Gong, 2005). However, the specific dust sources are still being debated, mainly because of inaccurate modeling (due to inaccurate input parameters), unsound study methodology, a lack of detailed information on surface characteristics, or a combination of these factors. Some researchers think that sandy deserts are the main dust sources (Laurent et al., 2006); others think that gravel deserts are the main dust sources (Wang et al., 2011). Laurent et al. (2006) proposed that China's sandy deserts generated more dust than its gravel deserts. Dust storms have been studied by means of field observations (Jugder et al., 2011), wind tunnel studies (Wang et al., 2011, 2012), numerical modeling (Laurent et al., 2006), satellite remote sensing (Gu et al., 2003; Zhang et al., 2008a), and chemical analysis (Zhang et al., 1996, 1997). However, due to a lack of field data on the properties of desert surfaces, most previous studies mainly analyzed synoptic data on dust storms (Laurent et al., 2006; Park et al., 2010). This approach can introduce much uncertainty in efforts to identify the dust source and clarify entrainment and transport of dust materials in models. For example, the lack of field observations has created large uncertainties in key model parameters such as the surface grain size distribution,
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threshold wind velocity, and aerodynamic roughness length. Uno et al. (2006) found that the estimated dust concentration could differ by a factor of 2 to 4 in their analysis of the effects of changes in such parameters in eight dust transport models. The area of sandy desert in northwestern China is about 56 × 104 km2, versus about 68 × 104 km2 for gravel desert; these areas occupy about one-third of the total land area in China and are therefore a major part of the central Asian arid region and one of the region's major dust sources. Gravel deserts are likely to be a significant dust source because of their large area, and have therefore attracted the attention of aeolian and dust researchers (Qu et al., 2001; Zhang et al., 2008a; Jugder et al., 2011; Wang et al., 2012; Qian et al., 2014). Dust emissions are usually caused by the action of strong winds acting on dry, fine, and loose soil surface materials (Pye, 1987; Cook et al., 1993) and are related to the proportion of the cover as erodible grains and the fine grain content of the surface material (Wang et al., 2012). A strong wind regime and favorable meteorological conditions (e.g., low surface soil water content, relative humidity, and precipitation) can dramatically increase the likelihood of dust storm occurrence, particularly when combined with a low gravel content in the surface materials and a low vegetation cover. In contrast, a high gravel content, high soil moisture content, a weak wind regime, and high vegetation cover can decrease the entrainment of dust (Laurent et al., 2006; Rostagno and Degorgue, 2011). Vegetation cover can decrease sediment loss by wind because it reduces the near-surface wind speed and soil erodibility, and increases the capacity for capturing windblown eroded material (Van De Ven et al., 1989; Dong et al., 1996; Leenders et al., 2011; Munson et al., 2011). In sandy deserts, there is no doubt that the sandy surface is the main sand source (Zhang et al., 1996, 2008a). The grain size distribution in this surface (Bagnold, 1941), its soil moisture content (Chepil, 1956; McKenna–Neuman and Nickling, 1989; Bisal and Hsieh, 1996; Shao et al., 1996; Dong et al., 2007), and vegetation all control sand and dust entrainment in these deserts (Hupy, 2004). However, in gravel deserts, it is unclear whether the gravel surface is a significant dust source.
In northwestern China, the gravel cover in these deserts ranges between 32 and 85% (Qian et al., 2014). Wind tunnel experiments have indicated that the gravel surface is relatively stable at gravel cover ranging from 40 to 70% (Dong et al., 2002a,b; Zhang et al., 2004), but Wang et al. (2013) recently found that high gravel cover did not necessarily decrease dust entrainment; in fact, With a total gravel coverage below 40%, wind erosion increased as the gravel cover increased. However, due to aerodynamic roughness and drag coefficients on the gravel surface, a gravel cover above 40% resulted in the surface becoming aerodynamically stable and a reduction in wind erosion (Wolfe and Nickling, 1996; Dong et al., 2002a,b). In contrast, Lyles (1988) found that when the cover of non-erodible material reached 80%, erosion of the erodible material ceased. This summary of previous research suggests the importance of gravel cover in aeolian sediment transport. However, apart from the study of Qian et al. (2014), who studied only a small region of the Zhongyang Gravel Desert and Gashun Gravel Desert, there is little data on the distribution of gravel cover in China's desert areas. In the present study, we obtained additional data for a key desert area in northwestern China, the Hexi corridor. Based on our field study, we describe the properties of the region's gravel desert surfaces (grain size distribution, gravel cover proportion, gravel as a proportion of total weight, and the presence of a surface physical soil crust). Our first goal was to provide basic data that could be used to predict wind erosion and dust emission in these areas. Our second goal was to use the properties of the gravel surfaces to support future efforts to assess the potential dust sources in the study area in numerical models. 2. Material and methods 2.1. Study region The Hexi Corridor lies in northwestern China (Fig. 1). The desert area is surrounded by the Qilian Mountains to the south and southwest, by the Kumtagh Desert to the west, by the Mazun Mountains, Heli
Fig. 1. Location of the Hexi Corridor in northwestern China, and locations of the weather stations, the field sample sites and of the five sub-regions of the Hexi Corridor: (A) the Kumtagh Desert, surrounding a gravel desert; (B) the northern gravel desert; (C) the southern gravel desert; (D) the Jiuquan gravel desert; and (E) the Hexi Desert, surrounding a gravel desert.
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Mountains, and Longshou Mountains to the north, by the Badain Jaran Desert to the northeast, and by the Tengger Desert to the southeast. The sandy deserts in this region are the Kumtagh Desert, Hexi Desert, and Tengger Desert. The gravel deserts are mostly found in the plateau areas of the study region. Gravel deserts can be divided into two main categories: erosion-deposition (formed by the peneplain process, mainly Mazun Mountains and adjacent region) and deposition (formed by the aggradation process, mainly Hexi Corridor gravel desert, where the rock fragments were transported by water flow and deposited at the lower region) types (Wang, 2003). The climate of the Hexi Corridor is a typical continental arid climate. Average annual rainfall decreases from east to west, ranging from 158 mm in the southeast (at Wuwei) to 37 mm in the northwest (at Dunhuang). Most (60%) of the rain falls from June through August. The annual potential evaporation is more than 2500 mm, and increases from south to north. The annual average temperature ranges from 5 to 9.5 °C, and increases from 7.7 °C in the southeast to 8.2 °C in the northwest. 2.2. Methods We sampled the surface sediments at 33 sites in the Hexi Corridor (Fig. 1) in December 2013. Before collecting the samples, we photographed the gravel surface and a ruler (to allow calculation of the scale) using a Nikon 7100 digital camera to permit calculation of the gravel cover. Fig. 2 shows typical gravel surfaces from the five subregions of the Hexi Corridor (described in the next paragraph). Our sediment samples included the surface layer (0 to 0.5 cm) and the subsurface layer (0.5 to 4 cm). At each site, we collected the surface materials
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from a 20 × 20 cm quadrat; the total weight of each sample was 0.5 to 1 kg. To describe variations in the characteristics of the grain size distribution throughout the Hexi Corridor, we divided the region into five subregions: A, the Kumtagh Desert, surrounding a gravel desert; B, the northern gravel desert; C, the southern gravel desert; D, the Jiuquan gravel desert; and E, the Hexi Desert, surrounding a gravel desert (Fig. 1). Although no research has been done to identify the sources of the sediments in the Hexi Corridor, the topography shown in Fig. 1 suggests that the source materials for sub-region A were mainly alluvial deposits from the western Qilian Mountains and the Beishan Mountains; those for sub-region B mainly originated from alluvial deposits from the Beishan Mountains and were separated from sub-region C by the Shule River; those for sub-region C mainly originated from alluvial deposits from the Qilian Mountains and were separated from sub-region B by the Shule River; and those for sub-regions D and E mainly originated from alluvial deposits from the Qilian Mountains. We determined the following properties of the surface sediments: the weight of gravel, the gravel cover, the distribution of surface crusts, and the grain size distribution (the content of sand, silt, and clay particles with a grain diameter ≤ 2 mm). The weight of gravel (grain diameter N 2 mm) was determined for about 400 g of the total sediment after removal of the gravel using a 2-mm sieve, using an electronic balance with a precision of 1 mg. The gravel cover was calculated from the field photos using version 1.36b of the ImageJ software (http://imagej. nih.gov/ij/). Fig. 2f shows how gravel particles were identified and measured for a typical gravel surface. Firstly, the dimensions in pixels of particles identified on the photographs were converted to physical dimensions. The photo image was then converted into an 8-bit, gray,
Fig. 2. Typical gravel surfaces in the five sub-regions shown in Fig. 1: (a) sampling site A2, (b) sampling site B2, (c) sampling site C10, (d) sampling site D2, (e) sampling site E3, and (f) identification of gravel particles and calculation of the gravel cover for a typical gravel surface using ImageJ software. Site locations are shown in Fig. 1.
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binary image. Particles N 2 mm in diameter were only measured from which the particle coverage (%) was calculated by using the “Analyze particles” menu in the software. At present, there is no broadly accepted classification system for gravel cover, but wind tunnel experiments using sediments from the study area suggest that gravel cover is related to aerodynamic roughness length; when the gravel cover is b 40%, aerodynamic roughness length increased with increasing gravel cover, reached its maximum
value at a gravel cover ranging from 40 to 70%, and then decreased with increasing gravel cover (Dong et al., 2002a,b; Liu and Dong, 2003). In this paper, we classified gravel cover into three categories based on these relationships: low gravel cover (b40%), moderate gravel cover (40 to 70%), and high gravel cover (N70%). We analyzed the grain size distribution of the samples using a Malvern MasterSizer 2000 (Malvern Instruments Ltd., Malvern, England), which operates in the range of 0.02 to 2000 μm. The grain size
Fig. 3. Wind regime in the study region for (a) mean annual wind speed (m s−1) and (b) potential sand transport (drift potential, DP, in VU) and sand transport direction (resultant drift direction, RDD; arrows).
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and sorting parameters were calculated using the equations devised by Folk and Ward (1957). Potential for dust emission was directly related to the amount of surface erodible material and gravel weight (% of total sampler). We defined the dust material ratio as the ratio of b63 μm particles divided by the % gravel weight to assess the potential for dust emissions. Wind data were measured using automatic weather stations at 27 locations in the Hexi Corridor (Fig. 1). The data were acquired at 10min intervals every hour for 24 h per day from 1 January to 31 December 2009 in an open area at a height of 10 m above the surface. The characteristics of the wind regime (i.e., the mean annual wind speed and annual potential sand transport parameters) were calculated using data from the automatic weather stations. We calculated two key sand drift parameters using the methods of Fryberger and Dean (1979): sand drift potential (DP, in vector units [VU]) and the resultant drift direction (RDD), which represents the net direction in which sand will be transported. 3. Results and discussion 3.1. Wind speed and potential sand transport Wind speed controls the entrainment of sediment materials. The larger the wind speed, the greater the emission of dust and the transport of coarser particles; the magnitude of this transport is represented by DP. The present study represents the first detailed record of wind speed and potential sand transport characteristics in the Hexi Corridor. We used version 9.1 of ArcGIS (www.esri.com) to map the data from our study, then used ordinary kriging interpolation to calculate the distribution of wind speed and potential sand transport between sample points. The annual mean wind speed showed a clear spatial distribution (Fig. 3a), with values ranging from 1.0 to 4.8 m s− 1, and the largest values in the northeastern Hexi Desert and northwestern Tengger Desert.
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Potential sand transport was intermediate to high according to Fryberger and Dean's (1979) classification. There were three regions with particularly high DP (N500 VU): northwest of Dunhuang, northeast of Dunhuang, and northwest of Jiuquan (Fig. 3b). Previous research indicated that winds in the Hexi Corridor were controlled by the Mongolia–Siberia high-pressure system in late spring and early summer. Because of the obstruction created by the western Qilian Mountains, the local wind regime is divided into two branches: one crosses the middle and southwestern parts of the Hexi Corridor, moving southwest, and the other crosses the northern and eastern parts of the Hexi Corridor, moving east to southeast (Wang et al., 2005). Our results suggest that the potential sand transport direction (RDD) follows this pattern and can also be divided into two branches (Fig. 3b). Although the potential sand transport is high (DP N 400 VU) in the northwestern part of the Hexi Corridor (Fig. 3a), this region may not supply much dust for long distance transport to the southeastern Hexi Corridor or even over longer distances to southeastern China, since RDD is primarily to the southwest in this region, and the transported material would be deposited on the northern slopes of the western Qilian Mountains (where there is moderate to low DP because of the higher elevation; i.e., the presence of the Qinghai–Tibetan Plateau). In the central and southeastern Hexi Corridor, DP is lower than in the northwest, but the dust transport direction is to the east and southeast, so the transported dust material would not be blocked by mountains and could potentially be transported over long distances. 3.2. Gravel weight and cover The surface sediments of the gravel deserts had a high gravel content, and this is likely to protect the sediments against erosion and to decrease dust emission. Fig. 4 shows the gravel weight and cover values for the five sub-regions. We used ANOVA to compare these gravel parameters among the sub-regions, and found significant differences in gravel weight as a proportion of the total among the five sub-regions. The
Fig. 4. Gravel weights as a proportion of total sediment weight in the study region for (a) the average weight in each of the five sub-regions shown in the map and (b) for all field samples combined. Gravel cover as a proportion of the total surface area for (c) the average cover in each of the five sub-regions and (d) for all field samples combined. Values are means ± SD. Bars labeled with different letters differ significantly (ANOVA, p b 0.05).
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Fig. 5. Physical soil crust distribution, and the grain size parameters: (a) mean grain size, (b) sorting, (c) skewness, and (d) kurtosis. Bars labeled with different letters differ significantly (ANOVA, p b 0.05).
gravel weight in sub-region A was significantly lower than in the other four sub-regions. The mean gravel weights were similar in sub-regions B and D (77 ± 9% and 77 ± 7%, respectively), and smallest in subregion A (26 ± 4%) (Fig. 4a). Overall, the gravel weight ranged from 22 to 90% of the total (mean ± SD, 65 ± 17%), and most of the samples (73%) had contents of 40 to 80% (Fig. 4b). The study region is a diluvial–alluvial gravel desert (Qian et al., 2014), and the regional geomorphology could have affected the weight of the gravel, since the Qilian Mountains piedmont appear to be the source of most of the materials in the study area. Gravel size decreases with increasing distance from the apex of the alluvial fan to the end of the alluvial fan as the energy of the flowing water decreases (Our results show that from C7 to C9, the gravel weight and cover both increased). This may explain why the gravel weight was largest in sub-region D, which is close to the mountains, and smallest in sub-region A, which is farthest from the mountains. Fig. 4c and d shows the mean gravel cover in the study region for the five sub-regions. We used ANOVA to compare gravel cover among the five sub-regions, and found no statistically significant differences in gravel cover.
ensure the entrainment of this material because this material can form a surface crust that will decrease dust entrainment even under strong winds (Belnap and Gillette, 1998; Gomes et al., 2003). Surface crusts have been observed in many deserts (Mabbutt, 1965). Pye (1987) reported that sand and dust production in the gravel deserts of northwestern China may be higher than in some other gravel deserts due to the particularly arid and windy conditions. However, Pye did not consider the effects of surface crusts in this region. In the Hexi Corridor, crusts have formed in most (75%) of the gravel surfaces (Fig. 5). The distribution of the gravel crusts exposed between gravel clasts is likely to have been affected by a combination of geomorphology and potential sand transport. The gravel crust exposed between gravel clasts was mainly distributed in the alluvial fan, where rainfall and fine materials in the surface sediments produced a thin coating. However, in areas with higher DP, the stronger winds would have deflated the surface by transporting fine material, thereby preventing crust formation, even though the amount of rainfall was similar.
3.3. Physical soil crust distribution In arid regions, surface physical soil crusts often form as a result of interactions between rainfall and fine materials such as clays in the surface sediments, particularly in the presence of large concentrations of soluble salts (Valentin and Bresson, 1992). These crusts form a strong, dense, relatively impermeable layer that can protect the underlying fine materials from wind erosion and stabilize the surface (Cook et al., 1993). Wind tunnel experiments indicated that the threshold wind speed for entrainment of particles above the surface of typical crusts is 4 to 5 times that above surfaces that lack a crust (Zhang et al., 2008b), so crusts could greatly decrease sediment entrainment in the study area, thereby decreasing wind erosion and dust storm occurrence. Alluvial fans and gravel deserts are major sources of the fine materials that produce dust (Pye, 1987). However, a high silt and clay content does not
Fig. 6. Relationship between gravel weight and gravel cover for all samples combined.
Z. Zhang et al. / Geoderma 268 (2016) 69–77 Table 1 Grain size distribution in the five sub-regions (A to E). Locations of the sub-regions are shown in Fig. 2. Size (μm)
b31 31–63 63–125 125–250 250–500 500–1000 N1000 Silt + clay Sand Gravel
Proportion of total (%) A
B
C
D
E
12.6 ± 4.2 7.6 ± 3.6 23.7 ± 5.8 27.0 ± 7.6 16.8 ± 5.0 10.6 ± 4.9 1.8 ± 1.4 20.2 ± 6.6 78.1 ± 6.2 1.8 ± 1.4
20.2 ± 10.6 3.8 ± 1.4 9.6 ± 1.0 21.2 ± 3.7 26.5 ± 4.1 16.7 ± 4.5 1.9 ± 2.0 23.95 ± 11.7 74.1 ± 10.6 1.9 ± 2.0
15.9 ± 8.2 7.3 ± 2.6 16.3 ± 4.8 18.3 ± 5.7 18.3 ± 3.7 18.4 ± 7.0 5.4 ± 3.6 23.3 ± 10.6 71.4 ± 9.0 5.4 ± 3.6
30.2 ± 10.6 10.5 ± 3.0 18.3 ± 6.8 18.7 ± 7.2 12.8 ± 6.2 7.8 ± 8.1 1.8 ± 3.7 40.6 ± 13.3 57.5 ± 12.0 1.8 ± 3.7
10.6 ± 2.8 3.2 ± 1.0 22.0 ± 7.9 35.7 ± 13.2 19.4 ± 5.1 9.8 ± 11.1 3.3 ± 2.9 9.8 ± 3.7 86.9 ± 5.7 3.3 ± 2.9
3.4. Gravel mass and cover Gravel could decrease wind speed and increase the surface aerodynamic roughness on the gravel surface, which could decrease dust emissions. Rostagno and Degorgue (2011) defined the gravel mass as the particles larger than 2.0 mm that either lay on the soil surface or were embedded in the soil by b50% of their volume. They found that the gravel cover could be expressed by a linear equation. Unlike in their study,
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we found a power relationship for the sediment in the Hexi Corridor gravel deserts (Fig. 6). This may have resulted from different geomorphological characteristics in the two study areas. Our data show that more than 61% of the gravel cover values were N40% in the Hexi Corridor (Fig. 4d), which means that most of the gravel surfaces would be stable, and that it would be difficult for the wind to entrain dust materials. Wind tunnel experiments have shown that wind erosion increased with increasing gravel cover when the gravel cover was b40% (Wang et al., 2013) or that erosion decreased when gravel cover was N30% (Wang et al., 2012). In the study region, 24% of the sites had gravel cover values between 30 and 40%. Lyles (1988) found that a gravel surface with gravel cover N80% experienced no erosion. Zhang et al. (2004) indicated that gravel cover of 40 to 60% could stabilize gravel surfaces and prevent erosion. At the border between China and Mongolia, gravel cover was b40% (Wang et al., 2013), and in eastern Xinjiang, which lies west of Mongolia, the gravel cover in the Gashun Gobi and Zhongyang Gobi ranged from 32 to 85%, and most of the gravel cover ranged from 40 to 70% (Qian et al., 2014). 3.5. Grain size distribution Dust storms usually result from the action of strong winds on dry, fine, and loose soil surfaces. The silt and clay content is thought to determine the resistance of sediments to wind erosion (Marticorena and
Fig. 7. Grain size distributions for particles ≤1 mm in diameter for (a to e) the five sub-regions shown in Fig. 1, and (f) ratio of b63 μm dust percentage and gravel weight percentage of total sample.
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Bergametti, 1995). Table 1 shows the grain size distribution for samples from the five sub-regions. ANOVA revealed significant differences in the silt plus clay among the five sub-regions. The mean silt plus clay content ranged from 9.8 to 40.1%, with the largest values in sub-region D (20 ± 7%, mean ± SD) and the smallest in sub-region E (10 ± 4%). The mean sand content ranged from 57.5 to 86.9%, with the largest value in subregion E (86.9 ± 5.7%) and the smallest value in sub-region D (57.5 ± 12.0%). In sub-region A, the silt plus clay content and the sand content ranged from 9.0 to 27.1% and from 72.5 to 88.8%, respectively. In sub-region B, the silt plus clay content and the sand content ranged from 6.7 to 41.1% and from 58.3 to 89.5%, respectively. In sub-region C, the silt plus clay content and the sand content ranged from 7.9 to 36.3% and from 60.5 to 86.9%, respectively. In sub-region D, the silt plus clay content and the sand content ranged from 26.3 to 59.9% and from 39.0 to 69.0%, respectively. In sub-region E, the silt plus clay content and the sand content ranged from 6.0 to 14.8% and from 78.4 to 89.6%, respectively. This variation in the silt plus clay content affected the extent of the surface crust, since this content at sites with a crust (20.7 ± 10%) was about 2 times the content at sites with no crust (9.0 ± 7.8%). The crust contains higher amounts of fine materials, and protects the fine material under the crust from wind erosion, thus the silt plus clay content is larger in this underlying material. On surfaces with no crust, strong winds and the resulting high DP led to deflation of the surface material by transport of silt and clay particles, thereby greatly decreasing the silt plus clay content. The entrainment of dust involves a complex group of processes that are controlled by the gravel cover and the abundance of dust materials at the surface. Dust materials in the gravel surface provide potential dust entrainment sources. For a given set of wind and surface conditions (such as surfaces with no crust), a higher silt and clay content at the gravel surface increases entrainment of dust materials. Fig. 7 shows the grain size distribution of the samples from each of the five-subregions and for the study region as a whole. Our data reveals both unimodal and bimodal distributions, but most of the curves (83.8%) were bimodal. Dust material ratio was largest in sub-region E (4.90 ± 1.49) and smallest in sub-region A (0.91 ± 0.17) (Fig. 7f). In general, stronger winds and higher DP would transport more surface silt and clay into the air. However, we lack sufficient data on the mean wind speed and DP at each sample site to confirm whether these factors affected the surface silt and clay contents in the study region. In sub-regions A, B, and C, the mean silt and clay contents were similar (they ranged from 20 to 24%), but the wind speed and DP were significantly higher in subregion B than in sub-regions A and C. 4. Conclusions Gravel deserts are an important part of the landscape in the arid and semi-arid regions of China. Previous research suggested that these deserts are a significant dust source, but until the present study, there had been little research on the properties of the gravel surface that could be used to support the development of numerical models of dust emission and transport from these deserts. The present study provides much data that can be used to support such modeling. Based on our results, we have the following main conclusions: 1) In the Hexi Corridor, the gravel cover is mainly moderate (40 to 70% in 53% of the total samples). 2) Most (75%) of the gravel surfaces have developed a crust and formed between the gravel clasts, with the main exceptions being in areas with high DP and at the margins of the desert. 3) The content of the erodible materials (sand, silt, and clay) showed a clear spatial distribution. Most of the gravel surface sediments were medium and fine sands, which accounted for 52.5 and 25.0% of the total, respectively. The silt plus clay content ranged from 9.8 to
40.1%, and most of the content (about 73% of the samples) ranged from 10 to 30%. Dust material ratio, crust distribution, and potential sand transport indicated that the northern region of the Hexi Corridor was the primary dust source. 4) The spatial heterogeneity of the wind regime and the gravel surface properties means that there will be similarly large variation in the dust entrainment processes and amount of dust material that is entrained. However, this theoretical conclusion must be verified by additional field sampling (e.g., collection of blowing sediments in wind tunnel tests). Additional data about differences in dust emission from gravel surfaces would let researchers build more robust dust emission models capable of providing more accurate regional and global dust emission estimates.
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