wind erosion characteristics of sahelian surface typesdust.ess.uci.edu/ppr/ppr_mhs10.pdf · 2010....

EARTH SURFACE PROCESSES AND LANDFORMSEarth Surf. Process. Landforms (2010)Copyright © 2010 John Wiley & Sons, Ltd.Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/esp.1975

Wind erosion characteristics of Sahelian surface typesThomas Maurer1, Ludger Herrmann2 and Karl Stahr2

1 Research Centre for Landscape Development, Brandenburg University, Germany2 Institute for Soil Science and Land Evaluation, Hohenheim University, Germany

Received 20 January 2009; Revised 12 November 2009; Accepted 16 November 2009

*Correspondence to: T. Maurer, Brandenburg University, Germany. E-mail: [email protected]

ABSTRACT: The assessment of wind erosion magnitudes for a given area requires knowledge of wind erosion susceptibilities of the dominant local surface types. Relative wind erosion potentials of surfaces can hardly be compared under fi eld conditions, as each erosion event is unique in terms of duration, intensity and extent.

The objective of this study was to determine and compare relative wind erosion potentials of the most representative surface types over a transect comprising most parts of southwestern Niger. For this purpose, mobile wind tunnel experiments were run on 26 dominant surface types. The effects of surface disturbance were additionally determined for 13 of these surfaces. The results, namely measurements of wind fi elds and mass fl uxes, can be classifi ed according to specifi c surface characteristics.

Three basic surface groups with similar emission behaviour and aerodynamic characteristics were identifi ed: (1) sand surfaces, (2) rough stone surfaces and (3) fl at crusted surfaces. Sand surfaces feature a turbulent zone close to the surface due to the devel-opment of a saltation layer. Their surface roughness is medium to high, as a consequence of the loss of kinetic energy of the wind fi eld to saltating particles. Sand surfaces show the highest mass fl uxes due to the abundance of loose particles, but also fairly high PM10 fl uxes, as potential dust particles are not contained in stable crusts or aggregates. Rough stone surfaces, due to their frag-mented and irregular surface, feature the highest surface roughness and the most intense turbulence. They are among the weakest emitters but, due to their relatively high share of potential dust particles, PM10 emissions are still average. Flat crusted surfaces, in contrast, show low turbulence and the lowest surface roughness. This group of surfaces shows rather heterogeneous mass fl uxes, which range from moderate to almost zero, although the share of PM10 particles is always relatively high. Topsoil disturbance always results in higher total and PM10 emissions on sand surfaces and also on fl at crusted surfaces. Stone surfaces regularly exhibit a decrease in emission after disturbance, which can possibly be attributed to a reorganization which protects fi ner particles from entrainment.

The results are comparable with fi eld studies of natural erosion events and similar wind tunnel fi eld campaigns. The broad range of tested surfaces and the standardized methodology are a precondition for the future regionalization of the experimental point data. Copyright © 2010 John Wiley & Sons, Ltd.

KEYWORDS: mobile wind tunnel; emission potential; turbulence; Niger; PM10

Introduction

Wind erosion has considerable ecologic and socioeconomic impacts such as desertifi cation and nutrient depletion. Countries in semi-arid climates with agriculture-based econo-mies especially suffer from its consequences. Among these areas, the Sahel comprises the most prominent and largest region (Hillel, 1991). Nutrient depletion of already poor soils and ongoing desertifi cation are among the most pressing envi-ronmental problems in this part of the world (Buerkert and Hierneaux, 1998). The wind erosion susceptibility of land-scapes is to a great extent determined by specifi c surface properties. A thorough understanding of the individual response of local surfaces to wind erosion processes is essen-tial for fi nding proper attenuation strategies.

A number of fi eld studies observing natural wind erosion events were carried out in the last two decades in the Sahel.

The intention of most of these studies was the evaluation of protective wind erosion measures (Enninga, 1994; Buerkert et al., 1996; Sterk et al., 1997; Michels et al., 1998; Bielders et al., 2000) or the estimation of nutrient losses (Sterk et al., 1996, 1997; Bielders et al., 2002; Michels and Bielders, 2006; Visser and Sterk, 2007). One of the most comprehensive fi eld measurement campaigns in the region, the African Monsoon Multidisciplinary Analysis (AMMA) experiment, is currently running in Niger (Rajot et al., 2008). However, comparability is a major problem: each wind erosion event or storm is unique and varies in duration, intensity and extent. Infl ux of adjacent surfaces cannot be neglected, and vegetation can vary over the same surface type, resulting in different mass fl uxes over the same surface type.

For better comparability, standardized wind conditions are required. They can be achieved either by fi eld wind tunnel measurements during individual natural wind erosion events

T. MAURER ET AL.

Copyright © 2010 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms (2010)

or by the reproduction of the natural surface under a stationary wind-tunnel. The best compromise between the two methods is a portable wind tunnel, which is able to generate standard-ized conditions at the cost of very little disturbance of the natural surfaces. The focus of most fi eld wind tunnel studies was on solving problems regarding specifi c physical wind erosion processes on natural surfaces (Gillette, 1978; Borrmann and Jaenicke, 1987; Leys et al., 1996; Shinfu and Chuenjinn, 2001), or on comparing only a small number of surfaces (Leys and Raupach, 1991; Leys et al., 2002; Argaman et al., 2006). Wind tunnel experiments with the objective of systematically comparing wind erosion characteristics of natural surfaces are scarce, e.g. the work of Nickling and Gillies (1993) in Mali.

This study aims to determine the relative wind erosion char-acteristics and source strengths of surfaces in a Sahelian tran-sect using a portable wind tunnel. The experiments were conducted along a transect in the southwestern part of the Republic of Niger. Here, a variety of soils and geomorphologic features can be found that are typical for a wide area of the West African Sahel (Greigert and Pougnet, 1967).

Remote-sensing data (Prospero et al., 2002; Engelstaedter et al., 2006) and fi eld observations (Washington et al., 2006) confi rm that dust emission potentials vary greatly over the Saharan and sub-Saharan regions. We assume that large-scale heterogeneities of emission potentials can also be found on a smaller scale for the various surface types in our transect. The governing factors are expected to be particle availability, texture, aggregation and crusting, climate, topography and land use. Vegetation and soil humidity are highly variable and will therefore be neglected when considering true emission potentials. The infl uence of variable surface modifi cators on wind erosion susceptibility was investigated in a separate study (Maurer et al., 2009).

Comparability with natural wind erosion events is restricted due to the inherent particle supply limitation in the wind tunnel, whereas the huge areas affected by natural events provide an almost unlimited sediment supply. This results in the early depletion of entrainable material and thus a rapid change of wind erosion characteristics of surfaces. Another restriction of wind tunnel experiments is the inability to repro-duce the abrading (sand-blasting) effect of saltating particles that move long distances over surfaces. This can result in the misinterpretation of emission potentials of highly crusted surfaces.

Soil disturbance goes along with reduced aggregation and/or crusting, which probably has an increasing effect on emis-sion potentials. However, the presence or absence of sieving crusts seems to have no infl uence on dust emission potentials of sandy soils (Rajot et al., 2003). We try to confi rm these fi ndings with a systematic examination of disturbed/undis-turbed surface counterparts. This will also help to assess the effect of human activity on specifi c surface emission potentials in the region.

Wind Tunnel Experiments

Experimental set-up

A detailed description of the wind tunnel can be found in Maurer et al. (2006). The test section is composed of seven aluminum segments, each 1·5 m long, 0·6 m broad and 0·7 m high. The total length of the test section from the end of the diffusor to the instruments is 9·4 m. The lower tunnel edges have an extension of 0·05 m length that can be put into the ground to avoid lateral advection. Air movement is driven by a medium-pressure axial fan. A honeycomb diffusor laminates

the air stream after the entrance. The diffusor is partially masked to modify the air stream at the beginning of the test section. The instruments are housed in the last tunnel segment downstream. The instrumentation relevant for this study com-prises fi ve modifi ed Wilson and Cook (MWAC) sediment catchers and four Pitot tube anemometers, both arranged in a logarithmic height profi le.

During each experiment, wind speed was increased step-wise by increasing the power output to the fan by 6% (= 3 Hz). Each increment step lasted 1 min. Initial particle movement and the evolution of the wind profi le was observed and recorded. After reaching maximum power, the experiment continued for 20 min to gain enough material in the sediment catchers. Wind tunnel sites were chosen according to the fol-lowing criteria:

• The surface type had to represent a signifi cant proportion of the regional area, i.e. it had to be a major geomorpho-logic unit.

• The surface had to be vegetation-free to exclude its effect on aerodynamic roughness. If necessary, small plants and plant residues had to be carefully removed manually.

• The surface had to be suffi ciently fl at and even to make wind tunnel placement possible. Hardened surfaces had to be carefully prepared to allow proper insertion into the ground.

• The surface had to adequately represent the local conditions.

• The surface needed to have a minimum distance from anthropogenic structures (roads, villages, fences, etc.) to avoid infl uence of allochthonous material.

In the southwestern part of the transect, major surface types were identifi ed on the basis of the SOTER (Soil and Terrain digital database) map by Graef (1999). In other areas, a mixture of own ground truth data and various map information was used. In SOTER and other classifi cations, surface types are elements of superordinate geomorphologic units (section Surface types). Three zones, positioned in the southwestern, central and northeastern part of the transect were focal points of measurement activity (Figure 1).

Data and sample analysis

Wind profi les and roughness lengthsWind speed measurements at 0·04, 0·12, 0·2 and 0·5 m heights (z) were conducted every 5 s. Figure 2 shows an example of wind speed development during the stepwise incrementation of wind speed for the surface F-P6. Depending on surface conditions, the logarithmic wind profi les recorded show different shapes. The main factor controlling the shape is the surface roughness z0:

uu

zz

z z*

= ( ) ≥1

00κ

ln , , (1)

with u being the wind velocity in height z, u* being the thresh-old friction velocity and with κ being the von Kármán constant (0·4).

Determination of friction velocities, threshold friction veloci-ties and mass fl uxThe friction velocity u* is defi ned as a measure of the shear stress that prevails immediately above the surface, whereas u*t, the threshold friction velocity, is the friction velocity that is capable of initiating particle movement. In the present case,

WIND EROSION CHARACTERISTICS OF SAHELIAN SURFACES


Figure 1. Location of the transect, geological conditions and the position of the wind tunnel experiment sites. Geological units: Quaternary: 1 fossil valley fi lls, 2 aeolian deposits without dune structures, 3 aeolian deposits with dune structures. Tertiary/Continental Terminal: 4 CT1, 5 CT2, 6 CT3, 7 undifferentiated, 8 marine Paleocene. Cretaceous (Senonian): 9 upper marine sandstones, 10 lower marine sandstones, 11 Continental Hamadien. Birrimian Basement: 12 synorogenic granites, 13 lower metamorphic greywackes and schists, 14 Gourma complex, metamorphites. Geological map simplifi ed after Greigert and Pougnet (1965).

Figure 2. Diagram showing the development of wind speeds at four sample heights (0·04, 0·12, 0·2 and 0·5 m) during one experiment on a stone surface.

T. MAURER ET AL.


these u*t are averaged values, which were calculated from Eq. (1) with the roughness lengths z0 already known from linear regression. The threshold friction velocity u*t was initially to be measured by a saltiphone, but since this part of the instru-mentation did not work properly under fi eld conditions, u*t was estimated from visual observations of the beginning of the erosion processes. Initial particle movement was hard to detect properly under these circumstances, but in some cases could be assuredly observed at around 27–30 Hz. Lacking other indicators, it was assumed that, at this point of time, initial particle movement occurs regularly on all surfaces, and u*t was calculated accordingly. The development of a saltation layer, however, was observed on almost every surface. This was regularly the case at about 39 to 42 Hz or 78–84% fan power output, possibly because of the relatively high content of medium sand on almost every surface. The threshold velocity for initial saltation layer development thus is based on much more resilient evidence and is given addi-tionally as u*ts.

The height-resolved sediment samples were gravimetrically analyzed. The results can be fi tted using a model developed by Fryrear and Saleh (1993) and refi ned by Sterk and Raats (1996) in order to calculate the specifi c mass fl ux q(z) for each surface. The fi rst component of the model is an expression for the suspension fl ux. The other component is an expression for the saltation fl ux (dominating the fl ux at ≤0·3 m). The regres-sion coeffi cients a and b were determined by fi tting this sus-pension equation to samples collected above a height of 0·3 m. Combining the two components (while using the already determined regression coeffi cients a and b) results in:

q z a z czb( ) = +( ) + −⎛

⎝⎜⎞⎠⎟

−1 exp .β

(2)

This procedure ameliorates the results of the fi nal regression, where sample data from all points were fi tted to regression coeffi cients a, b and c (Sterk and Raats, 1996). Prior to regres-sion, all sample data were corrected for the respective catcher effi ciences found by Goossens and Offer (2000). In most cases, erosion ceased before the end of wind tunnel experi-ments, which makes an exact determination of mass fl uxes per time unit diffi cult. Hence, mass fl uxes will be given in the form ‘g m−1 per experiment’.

Laboratory analysisThe fi rst two topsoil horizons were sampled to characterize soil profi les. We used sieving of the sand fractions and subse-quent determination of silt and clay via sedimentation (pipette method). Characterization of particle distribution in catcher samples was essential for the determination of emission poten-tials. Due to low quantities, texture analysis of MWAC samples was carried out with a laser particle counter (Coulter LS130) instead of the pipette method. The particle counter is capable to measure particle sizes from 0·4 to 900 μm in 128 logarith-mic size classes in a suspension stream. No dispersing agents were applied to the samples to stay as close as possible to the dry particle distribution.

Surface types

Transect dimensions are about 500 km × 200 km, extending from about 13 to 16°N and 1 to 6°E (Figure 1). The climatic conditions range from the northern Sudanian zone (growing season of 100–125 days, 600–800 mm annual rainfall) to the southern Sahelian zone (growing season of 60–100 days, 400–600 mm annual rainfall) (Sivakumar, 1989). This climatic

south–north gradient results in different vegetation covers, land use conditions and surface conditions. Also, geological conditions vary considerably over the transect (Figure 1). On the transect, spatially dominating surface types were chosen as experimental sites (Figures 3 and 4). A total of 40 wind tunnel experiments were carried out on 26 different surface types (Figure 1 and Table I). Surface types can be grouped according to their affi liation to the respective geomorphologic unit (e.g. plateaus and sand sheets, see below). They fi t in existing classifi cation systems for regional surface types estab-lished by Graef (1999) and d’Herbes and Valentin (1997). Tested surface types were assigned to the following major geomorphologic units:

Dallols are important features in the southwestern parts of Niger. Surfaces in the upper and lower Dallol Bosso (Filingué, Birni N’Gaouré) and the Dallol Maouri (Dogon Doutchi) were thus incorporated in the measurement program. Surfaces are normally covered by fl uvial-aeolian sand deposits and capped dunes, and are thus very similar to the sand-covered surfaces described above, the exception being the fl at, intensely crusted, loamy-sandy ‘Mare’ surfaces in the Dallol depres-sions. Crusts are normally of the depositional type. Due to the better availability of (ground) water, Dallols are more densily populated than other areas, and land use and vegetation density are both more intense.

Plateaus are a dominating geomorphologic unit in south-western Niger. Surfaces are either stone-covered, or covered by loamy and intensily crusted material, especially in depres-sions. Crusts are normally of the depositional type. Microbial crusts dominate in depressions, where water stagnates even during the early dry season. Patches of loose sand cover are common in the leeward position of sand-fi lled valleys or depressions. Inselbergs are also counted in this category as they are characterized by stone surfaces as well, although their topography is different. Due to the intense crusting and/or high skeleton contents, the land use is restricted to extensive pasture or felling. On sporadic sand patches, the cultivation of millet is common. Remains of the original ‘brousse tigré’ vegetation are the dominating vegetation.

Sand surfaces cover a great deal of the transect area. Among the surface types belonging to this geomorphologic unit are active dunes, inactivated dunes and interdunes, but mostly capped dune fi elds or sand sheets. Sand-covered pediments are also added to this category. With the exception of dunes, surfaces are generally fl at, and show wind ripples. A typical morphologic feature is the existence of a structural crust underneath the uppermost loose sand layer. Structural crusts observed in the transect are always sieving crusts, and can either show a well-developed vesicular layer or only several layers of densely packed, mostly fi ne sand material without vesicules in the upper fi ne microlayers. Most of the surfaces are under agricultural use or fallow fi elds.

Pediments are transition zones between plateaus (plateau slopes) and sand-fi lled depressions or intraplateau valleys. They are often covered by a thick sand layer and thus are similar to sand surfaces. Sand-free pediments have completely different surface characteristics. Defl ated erosion crusts or structural crusts prevail on the intermediate section of typical pediments, while in the upslope direction, closer to the plateau or inselberg edge, stone cover increases in the typical case, until the actual slope is reached. Due to intense crusting, these surfaces are normally not under cultivation.

Wherever possible, an additional experiment was run on a disturbed counterpart of the respective surface type. Disturbance was created either manually by the use of hoes or by simply placing the wind tunnel on cattle trails or pists. Normally, experiments were carried out in the immediate



neighbourhood of the undisturbed surface test sites, with the exception of sites CB1g and Fk-CC2g, which were located several kilometres away from their undisturbed counterparts, but were chosen as good examples of deeply disturbed trails. Disturbances always have in common that they cause destruc-tion of stable and somehow protective structures like aggre-gates, clods and crusts and liberate more particles that can be

entrained in a wind erosion event. During the fi eld campaign, adequate and sometimes differing methods of disturbance had to be chosen. Considerable care was taken that disturbance remained comparative despite the use of different methods. It was assumed that cattle and cars running over a surface have quite similar effects due to the repeated use of the surface as a traffi c way. Where disturbance was to be applied manually

Figure 3. Characteristic Sahelian landscapes of southwest Niger: (1) is a northern Sahelian landscape about 30 km north of Abalak, which is extensively used as pasture. Note the erosion crust in the foreground. (2) A typical sand surface under agricultural use about 30 km west of Tahoua. (3) A stabilized inactive dune at Namaro, about 45 km northwest of Niamey. (4) CT2 plateaus, situated about 20 km southeast of Filingué. The slope in the foreground is the corresponding crusted pediment. (5) Rough stone surface near Inselbergs consisting of metamorphic precambrian rocks, some 50 km northwest of Niamey. (6) An intensely crusted plateau surface, halfway between Niamey and Dosso.

T. MAURER ET AL.


Figure 4. Examples for typical surface types. F-P6 and T-PN2 are examples for stone-covered plateaus, whereas HS1 is a typical surface on quartzitic inselbergs. A-CRT is a sand sheet on Continental Intercalaire; the vegetation was removed after taking the picture. CC2 is a typical sand sheet on Continental Terminal and T-DAD is a surface on an active dune. T-DA2 is a crusted surface found in depression (Mares) in the Dallols. F-LC1 is a typical crusted surface developed on deeply alterated Precambrian granite. LB1g is a disturbed fl at crusted surface. The length of the bar at the lower left of each frame is 0·2 m.



with hoes, or by driving the Toyota pickup or the 5-ton truck over a surface several times, it was ensured that the destruc-tion of crusts and aggregates was approximately equally deep.

Surface Effects on Wind Field and Particle Emission

Wind profi les

Wind fi eld development over timeRaw wind speed data already show distinct surface- dependent characteristics. In all experiments, lower pitot tubes sampled higher amplitudes, indicating a higher degree of turbulence. Regarding wind profi le development and shape types, three basic groups of surface types – called ‘surface classes’ in the following – can be distinguished:

• Flat crusted surfaces. This class is mostly consists of loamy pediment and stone-free plateau surfaces, but also defl ation

crusts on sand sheet surfaces. These surfaces are character-ized by the narrow arrangement of all four wind speed curves, which indicates a rather small degree of energy loss due to surface roughness. Another sign for a smooth surface is the relatively small increase in turbulence in the lower areas of the wind profi le (section Turbulence below). Wind profi les from these surfaces are most similar to the wind profi le gained during wind tunnel calibration.

• Sand surfaces. These have a rather characteristic progres-sion of wind speeds. Distances between the wind speed curves become bigger, indicating a higher surface rough-ness than the fl at loamy surfaces described above. This can be mostly ascribed to the initially larger roughness due to the presence of sand ripples and other unevenness, and in the later stages of the experiments to the transfer of energy from the fl owing medium to the moving particles. Turbulence is much more signifi cant near the surface, which is refl ected by the high amplitude of wind speeds recorded at z1 = 0·04 m (section Turbulence). One possible explanation for this behaviour could be an increased

Table I. Overview of the wind tunnel experiments. Site nomenclature is partially oriented on the SOTER system established by Graef (1999)

Experim. name†

PositionDisturbance description

Geology§

(bedrock)Geomorph.

unitSurface class Land use Crust type*

Geometric roughness

Loose material (kg m−2)Lat Long

CB1 13·82548 1·57984 None Birrimian Sand sheet Sand Grazing Struct 1 Trampling 13·90CB1g 13·85866 1·34201 Cattle trail Birrimian Sand sheet Sand Trail Struct 1 Trampling 41·70CC2 13·23567 2·28193 None CT1 Sand sheet Sand Fallow Struct 1/2 Trampling 41·70DA1 13·09105 2·88312 None Quatern. Dallol Sand Fall./Graz. Struct 1 Trampling 69·50D-P2 13·65684 4·08532 None CT3 Plateau Crust Barren Sedim/Bio Single stones 1·40D-DA1 13·63861 4·05775 None Quatern. Dallol Sand Grazing Struct 1 Trampling 55·60F-CC2 14·33437 3·29900 None CT2 Sand sheet Sand Agric. Struct 1 Wind ripples 55·60F-DA1-1 14·15881 3·43611 None Quatern. Dallol Sand Grazing Struct 1 Wind ripples 25·27F-DA1-2 14·15881 3·43607 None Quatern. Dallol Crust Grazing Erosion Undulated 0·21F-DA2 14·25380 3·44391 None Quatern. Dallol/Mare Crust Barren Sedim. Smooth 0·03F-DA2g 14·25365 3·44390 Hoeing Quatern. Dallol/Mare Crust Barren Sedim. Small clods 0·98F-LC1 14·25143 3·46316 None CT2 Pediment Crust Barr./Graz. RO dep. Single stones 0·70F-LC1g 14·25140 3·46316 PU wheels CT2 Pediment Crust Barr/Graz. RO dep. Small clods 12·64F-P6 14·36329 3·29119 None CT2 Plateau Crust Barren Sedim. Stone patches 1·05F-P6g 14·36328 3·29117 Car piste CT2 Plateau Crust Traffi c Sedim. Stone patches 4·21Fk-CC2 13·42494 2·75780 None CT3 Sand sheet Sand Grazing Struct 1 Trampling 69·50Fk-CC2g 13·41961 2·73452 Cattle trail CT3 Sand sheet Sand Trail Struct 1 Trampling 125·10HS1 13·67627 1·82656 None Birrimian Inselberg Stone Barr/Graz. None Dense stones 1·40HS1g 13·67620 1·82650 Hoeing Birrimian Inselberg Stone Barr/Graz. None Dense stones 2·81LB1 13·91414 1·59822 None Birrimian Pediment Crust Barr/Graz. Erosion Sand patches 0·84LB1g 13·91441 1·59818 Car piste Birrimian Pediment Crust Traffi c Erosion Small clods 1·05LC3 13·54074 2·30152 None CT3 Sand sheet Sand Fall/Graz. Struct 1 Trampling 41·70LC3g 13·54069 2·30155 Cattle trail CT3 Sand sheet Sand Fall/Graz. Struct 1 Trampling 69·50P2 13·61623 1·94735 None CT1 Plateau Stone Barr/Graz. Sedim. Dense stones 2·11P2g 13·61625 1·94745 Hoeing CT1 Plateau Stone Barr/Graz. Sedim. Clods+stones 2·81P6 13·21249 2·69576 None CT3 Plateau Crust Barren Sedim/Bio Stone patches 0·25P6g 13·21240 2·69555 Car piste CT3 Plateau Crust Traffi c Sedim/Bio Clods+stones 2·11T-DFD 15·13099 5·69219 None Senon Fixed dune Sand Grazing Struct 1 Trampling 27·80T-DFI 15·29536 5·77015 None Senon Fixed dune Sand Grazing Struct 1/2 Trampling 23·87T-PC 14·76798 5·35168 None CT1 Sand sheet Sand Agric. Struct 1 Wind ripples 98·28T-PCg 14·76636 5·34971 Cattle trail CT1 Sand sheet Sand Trail Struct 1 Trampling 140·40T-PN1 14·66211 5·38928 None CT1 Plateau Crust Barr/Graz. None Single stones 0·35T-PN2 14·66262 5·38930 None CT1 Plateau Stone Barr/Graz. Sedim/Bio Dense stones 1·05T-PNg 14·66220 5·38960 Car piste CT1 Plateau Crust Traffi c Sedim/Bio Small clods 5·97T-PS 14·86887 5·13353 None CT2 Sand sheet Sand Fall/Graz. Struct 2 Trampling 41·70T-PSg 14·86888 5·13423 Cattle trail CT2 Sand sheet Sand Trail Struct 2 Trampling 55·60T-VC 14·75010 5·81421 None Senon Floodplain Sand Agric. Sedim. Small ridges 55·60VV1 13·23567 2·28193 None CT1 Sand sheet Crust Fallow Erosion Smooth 0·28

* after Casenove and Valentin (1992).† A preceding F, T, or D means sample site is located in the region of Filingué, Tahoua or Dogondoutchi. Sites with no prefi x are located around Niamey. A ‘g’ at the end marks a disturbed surface counterpart.§ The notation CT (bedrock) stands for the cenocoic sediment series of the ‘Continental Terminal’ and their variations. Barr/Graz. = Barren/Grazing; Fall/Graz. = Fallow/Grazing; RO dep. = Runoff depositional; PU wheels = Pickup wheels.

T. MAURER ET AL.


exchange of momentum due to increased particle move-ment, especially in the saltation layer, and therefore a higher rate of vertical momentum.

• Rough stone surfaces. This class of surface is mostly repre-sented by stony plateau surfaces and inselberg outcrops, as well as stone-covered pediments. A typical characteristic of the corresponding wind speed curves seems to be a signifi -cant gap between z4 = 0·5 m and z3 = 0·2 m. The steep loss of kinetic energy in this zone can only be explained by the high surface roughness, that seems especially to affect the zone between the surface and 0·2–0·3 m. In the lower parts of the profi le, the degree of turbulence is even less signifi -cant than on sand surfaces.

Wind profi les on different surface typesWind profi les at medium wind speeds (at 39 Hz or 78% fan power) are best suited to compare the impact of surface char-acteristics on the fl ow fi eld, as wind speeds are suffi ciently high to provide accurate measurements of the pressure probes while still staying below the erosion threshold. This is essen-tial, as energy transfer to particles will alter wind profi les and surface roughness elements (ripples or clods) could be altered or removed by erosion processes.

The survey of the resulting diagrams confi rms the existence of three wind-profi le-related surface classes. Criteria for class separation are the inclination of the profi le curves and the intersection points with the x-axis (= roughness lengths). Linear regression through the data points yields the individual rough-ness length, z0, which allows an extrapolation of the curve between the last measurement height, z1 = 0·04 m, and the roughness length z0 = 0 m. The intersection points of each surface class centre around specifi c regions on the x-axis. This suggests that roughness lengths are surface-specifi c. The average inclinations for the fl at crusted surfaces are the most shallow, while differences between the two other classes are less distinctive. Flat crusted surfaces show clear differences in surface roughness compared with the two other classes with values centred around z0 = 2·7 × 10−4 m, while differences between the sand surfaces and the rough stone surfaces are only marginal, with z0 = 1·2 × 10−3 m to 1·5 × 10−3 m.

Wind profi les on disturbed surfacesCrusted surfaces exhibit a tendency towards a clear increase of average roughness lengths, as could be expected because of the treatment and keying of the surface. The increase in surface roughness magnitude is about 360%.

Wind profi les on disturbed sand surfaces show a trend towards less energy loss and reduced surface roughness lengths of about 75%. This circumstance can be partly explained by the smoothing of the sand surface through ripple destruction, but the increased mass fl ux also drags away fl ow energy during the erosion stage.

Wind profi les of the tested stone surfaces seem to be insen-sitive to disturbance, with even a slight decrease of the average surface roughness length by about 14%. Due to the small number of comparable surface couples – especially in the case of the stone rough surfaces – a reliable statistical confi rmation of these observations is not possible with the available data.

Effect of surface roughness

Average roughness lengths for the total experiment duration were calculated, as well as for the phase prior to erosion and for the erosion phase. Roughness length comparisons for the complete experiment runtime basically confi rm the former observations: the class of crusted fl at surfaces show the small-

est z0, followed at some distance by sand and stone surfaces, which exhibit average z0 values 3·5 and 3·8 times larger, respectively. Disturbed fl at crusted surfaces show z0 at 1·8 times higher, while the average roughness length of disturbed sand surfaces is only one third of their undisturbed counter-parts. Stone surfaces seem rather unaffected by disturbance, showing only a negligible decrease in z0 (Figure 5).

Friction velocities and threshold friction velocities

Values for u* and u*t were averaged for the three basic surface classes (Table II, Figure 5). The comparison of the friction velocities before and during the erosion stage clearly exhibits a regular increase of u* by a factor of about 1·8 for each surface type. Friction velocities, and estimated threshold fric-tion velocities, show an increase for crusted surfaces, while sand surfaces show a signifi cant decrease and rough stone surfaces show a small decrease. The behaviour of u* and u*t is simply explained by the mathematical and physical connec-tion to the surface roughness lengths of the respective surfaces.

Turbulence

The degree of turbulence was derived indirectly from the amplitude of wind speed changes at the different measure-ment heights, using the statistic variance of the wind speed values. On almost every surface, turbulence increases with decreasing height z. This is a strong positive indicator for the accuracy of the described method. The average turbulence near the ground seems to be higher for fl at crusted and sand surfaces than for stone surfaces, while in the higher regions of the boundary layer, the opposite seems to be the case. Apparently, rough stone surfaces induce larger eddies that reach higher up the wind profi le, whereas turbulent structures on the other surface classes seems to be restricted to a zone closer to the surface.

Disturbance seems to have the biggest effect on sand sur-faces, causing a strong increase in turbulence of about one third in the lowest regions of the wind profi le. This is most probably a consequence of an increased number of loose particles due to the destruction of the underlying structural crusts, which leads to increased momentum exchange during erosion. Rough stone surfaces seem to be least infl uenced by disturbance, which correlates with other observations described above. The fl at crusted surfaces gain surface rough-ness by disturbance, and thus the turbulence pattern becomes more similar to the rough stone surfaces.

Mass fl uxes

In the following, the term total mass fl ux, Qt, will be used as expression for the combined mass fl ux of all particle sizes, while PM10 mass fl ux, QPM10, depicts the mass fl ux of the particle class <10 μm.

Total mass fl uxThe gravimetric analysis of the sediment caught in the MWAC catchers up to height z4 = 0·5 m shows signifi cant differences in the amount of emitted material. The three basic surface classes can be revisited. Regarding emission quantity, the sand-covered surfaces clearly dominate over the two other surface classes with mass fl uxes being 2 to 100 times higher (Table III).



Figure 5. Box plots showing the bandwidth and the average values of roughness lengths, z0 (above), estimated classical values for threshold friction velocities, u*t (centre) and turbulence indicators (below) before (left) and after (right) disturbance.

T. MAURER ET AL.


Table II. Overview of wind tunnel experiments indicating measured roughness lengths z0, estimated threshold friction velocity u*t for initial particle movement, and total mass fl ux Qt per experiment

Experim. name

Surface descriptionAverage* z0

(10−4 m)Estimated† u*t

(m s−1)Turbulence indicator

Total mass fl ux Qt (g m−1 per exp.)

Supply limit reached Geomorph. Class Disturbance

CB1 Sand sheet Sand None 23·10 0·61 1·3 2·2 yesCB1g Sand sheet Sand Cattle trail 1·00 0·39 2·1 4·4 yesCC2 Sand sheet Sand None 1·04 0·38 0·4 1·3 yesDA1 Dallol Sand None 1·10 0·40 0·4 2·2 yesD-P2 Plateau Crust None 0·69 0·28 0·3 0·2 yesD-DA1 Dallol Sand None 1·06 0·30 0·4 2·4 noF-CC2 Sand sheet Sand None 13·50 0·55 1·5 4·3 yesF-DA1-1 Dallol Sand None 5·78 0·51 0·6 4·3 yesF-DA1-2 Dallol Crust None 13·50 0·55 1·9 1·0 yesF-DA2 Dallol/Mare Crust None 1·54 0·34 0·5 0·004 yesF-DA2g Dallol/Mare Crust Hoeing 4·48 0·42 0·4 0·1 yesF-LC1 Pediment Crust None 3·36 0·48 5·3 1·6 yesF-LC1g Pediment Crust Pickup wheels 9·86 0·50 2·3 2·0 yesF-P6 Plateau Crust None 5·30 0·51 0·5 0·02 yesF-P6g Plateau Crust Car piste 3·36 0·45 0·4 0·01 yesFk-CC2 Sand sheet Sand None 13·30 0·56 1·2 7·8 noFk-CC2g Sand sheet Sand Cattle trail 12·50 0·42 2·9 6·2 noHS1 Inselberg Stone None 17·30 0·64 0·8 0·3 yesHS1g Inselberg Stone Hoeing 16·60 0·63 0·4 0·2 yesLB1 Pediment Crust None 1·26 0·43 0·5 0·01 yesLC3 Sand sheet Sand None 8·01 0·48 0·8 1·6 yesLC3g Sand sheet Sand Cattle trail 3·92 0·47 – 10·2 noP2 Plateau Stone None 13·20 0·59 0·7 0·2 yesP2g Plateau Stone Hoeing 17·40 0·57 0·6 0·01 yesP6 Plateau Crust None 0·19 0·31 0·4 0·04 yesP6g Plateau Crust Car piste 1·77 0·42 0·3 0·08 yesT-DFD Fixed dune Sand None 5·84 0·50 0·3 0·02 yesT-DFI Interdune Sand None 4·07 0·44 0·5 0·01 yesT-PC Sand sheet Sand None 5·36 0·49 1·0 3·6 yesT-PCg Sand sheet Sand Cattle trail 1·81 0·41 0·9 3·6 yesT-PN1 Plateau Crust None 1·79 0·43 0·3 0·08 yesT-PN2 Plateau Stone None 15·70 0·61 – 0·2 yesT-PNg Plateau Crust Car piste 3·54 0·45 0·4 0·1 yesT-PS Sand sheet Sand None 29·10 0·65 2·4 0·6 yesT-PSg Sand sheet Sand Cattle trail 2·46 0·44 1·2 1·2 yesT-VC Floodplain Sand None 19·20 0·56 0·5 0·4 yesVV1 Sand Sheet Crust None 0·96 0·36 0·4 0·08 yes

* Averaged z0 values taken in the 21–33 Hz interval (5 min) prior to erosion.† Only rough estimate due to saltiphone malfunction.

Sand surfaces. Mass fl uxes measured on sand surfaces exhibit a wide range of intensities from 0·4 to almost 8 g m−1 per experiment. This class of surface can be divided into two sub-classes of strong and moderate sources. Strong sources are not only characterized by signifi cantly larger emissions, but also by a larger proportion of mass fl ux near the surface, thus suggesting relatively higher transport rates in the lower zones of the saltation layer. According to our observations, the differences between sand surfaces can have several reasons:

• Higher concentrations of fi ne and middle sand fractions. Generally, but not always, strongly emitting sand surfaces have higher contents of more easily entrainable sand frac-tions. This can be the result of different genesis of the respective surfaces, e.g. Dallol (alluvial, e.g. DA1) versus corresponding leeward sand surfaces (aeolian, e.g. LC3).

• Struct. consolidation. Younger aeolian sediments are less consolidated, with large amounts of loose material above a weakly developed structural crust (e.g. T-PC).

• Higher contents of clay and medium and fi ne silt fractions. Some sand surfaces show transitional attributes towards

crusted surfaces, thus having potentially higher contents of aggregates and more intense superfi cial crusting (e.g. T-VC and VV1).

Flat crusted surfaces and rough stone surfaces. These two classes of bare, uncovered surfaces were well distinguishable on the basis of their wind profi le characteristics. However, differences between the two surface classes are less evident. Total mass fl uxes are in the same range of 0·004 to 0·3 g m−1 per experiment, with the signifi cant exception of surface F-LC1, a pediment surface with less consolidated erosion crust. Differences between the two surface classes become clearer when examining the distribution of mass fl ux with height. Here, rough stone surfaces exhibit a noticably larger proportion of sediment fl ux in higher regions of the wind profi le than do fl at crusted surfaces. This can be directly con-nected to the increased roughness of stone surfaces, and espe-cially their height-resolved distribution of turbulence intensities. It seems that the higher degree of turbulence in the upper regions of the wind profi les of the rough stone surfaces enhances momentum exchange, and hence provides better particle transfer in the vertical direction.



Table III. Overview of the total mass fl uxes (Qt) and the sampled contents of particles ≤10 μm (QPM10) per experiment. The values for disturbed counterparts are also given where possible, together with the percental change of emissions after disturbance

Surface name

Qt (g m−1) per exp.

ΔQt (%)

QPM10 (mg m−1) per exp.

ΔPM10 (%)undisturbed disturbed undisturbed disturbed

Sand surfaces (strong sources)CB1 2·4 4·4 +101 63 98 +55DA1 2·2 – – – – –Fak-CC2 7·8 – – 121 – –F-CC2 4·3 – – 83 – –F-DA1-1 4·3 – – 83 – –T-PC 3·6 3·6 −1 109 70 −35

Sand surfaces (moderate sources)CC2 1·3 6·2 +390 37 123 +230D-DA1 1·2 – – 86 – –LC3 1·6 10·2 +548 62 123 +98T-PS 0·6 1·2 +92 50 75 +49T-VC 0·4 – – 77 – –

Flat crusted surfacesF-DA2 0·004 0·1 +2924 0·04 1 +2957F-LC1 1·6 2·0 +23 57 46 −20LB1 0·08 0·2 +85 14 23 +63P6 0·04 0·08 +115 4 – –T-PN1 0·08 0·1 +22 29 16 −45D-P2 0·2 – – – – –

Rough stone surfacesF-P6 0·02 0·01 −18 5 2 −60HS1 0·3 0·2 −27 76 36 −53P2 0·2 0·06 −74 38 1 −97T-PN2 0·2 – – – – –

Impact of disturbance on total emissions. Topsoil distur-bance has considerable consequences on most surface classes, as more loose particles are made available to wind erosion processes. Disturbance also results in restructuring of surfaces, with the removal or creation of roughness elements or the change of the shape of roughness elements, as well as possibly changing the texture of the topsoil material by mixing.

Mass fl uxes on sand surfaces increase by a factor of 2 to 5·5 (with one exception), and for fl at crusted surfaces by a factor in the range of 1·2 to almost 30. Total mass fl uxes on rough stone surfaces are reduced by a factor of 1·2 to about 1·8. For sand surfaces, this can be explained by the destruction of the underlying structural crusts, the increasing thickness of the loose sand layer and the break-up of fragile fi ne sand or silt aggregates. Additionally, the surface roughness of the sand surfaces is reduced on disturbance because of the smoothen-ing of the surface and the removal of roughness elements such as wind ripples.

In the case of the fl at crusted surfaces, the generation of a large number of additional erodible particles clearly outbal-ances the adverse effect of increased roughness element pro-duction in the form of aggregates or clods. Another reason for increased mass fl uxes may in some cases also be an increase of the proportion of fi ner material (fi ne sand) close to the surface.

On rough stone surfaces, neither roughness nor threshold velocities nor turbulence are altered signifi cantly by topsoil disturbance. Nevertheless, emissions were generally reduced as a consequence of the deprivation of entrainable particles from the surface. Obviously, the restructuring of the surface resulted in increased separation of skeleton and fi ne earth fraction. Crusts and aggregates between larger stones and rocks are destroyed and fi ne earth material is liberated and

translocated beneath the skeletal components due to keying. Below the skeletal components, the fi ne particles are even more protected from wind erosion processes. Thus it seems that disturbance of skeleton-rich surfaces constrains the effects of wind erosion, while it has an increasing effect on surfaces with no or little skeleton. Future research should aim at verify-ing this hypothesis.

PM10 emissions of different surfacesRestrictions of PM10 measurements. Goossens and Offer (2000) describe declining MWAC catcher effi ciencies for small particles up to a size of 63 μm. It can be expected that catcher effi ciencies are much smaller for particles <10 μm (PM10). Also, texture analysis was accomplished using wet sieving, which means that larger aggregates were broken up. Nevertheless, we suppose that the content of PM10 in the sediment samplers give useful information about relative PM10 mass fl uxes, for two reasons: (a) under natural condi-tions aggregates will break up due to abrasion after longer-distance saltation transport. This effect, in its consequence similar to wet sieving, did not occur suffi ciently in the wind tunnel due to the short transport distance; (b) we suppose that catcher effi ciencies are equal for all surfaces. Thus, it should be possible to compare the relative PM10 fl uxes for the differ-ent surfaces and thus compare their potential to emit PM10. ‘Mass fl uxes’ of PM10 given here are thus, unlike saltation mass fl uxes, not absolute values, but rather describe the rela-tive ability of surfaces to emit PM10.

Relative PM10 contents. PM10 emissions from sand sur-faces are among the highest measured quantities, with the strongly emitting class of sand surfaces also being the strongest PM10 emitters, due to the high total mass fl ux. Nevertheless, the strong accentuation of emissions is less obvious in the case

T. MAURER ET AL.


Table IV. Wind tunnel threshold friction velocities on comparable surfaces, measured in the present study (left) and by Nickling and Gillies (1991)

Surface class Estimated Estimated Surface class MeanPresent study u*ts (saltation) u*t Nickling and Gillies (1991) u*t

Crusted fl at surfaces 0·66 0·41 Bare crusted wash 0·30Interdune surface 0·67 0·44 Coppice Interdune 0·32Sand surfaces 0·82 0·51 Sandy lateritic lag 0·37

of the dust fraction due to lower contents of fi ne particles. The average percentage of PM10 in the total emitted material from sand surfaces ranges only around 2–3%, with one major exception (T-VC).

The fl at crusted surfaces display a greater variety of emission quantities, while the percentage of potential topsoil PM10 seems to have no infl uence on PM10 proportions in the samples. The amount and texture of the loose material layer is likely to be the most decisive factor. The highest emissions from the crusted surfaces overlap with those from the lowest emitting sand surfaces. These crusted surfaces show high sand content and less stable crusts. Rough stone surfaces emit mate-rial within the range of the fl at crusted surfaces. However, they regularly show higher percentages of PM10 in the total emitted material. This seems to be a consequence of a bigger share of free particles between skeleton components, which are bound in aggregates on crusted surfaces.

Impact of disturbance on PM10 emissions. Topsoil distur-bance changes the PM10 emissions for the three basic surface classes basically in the same way as the total emissions. However, PM10 emissions increase not as much as total emis-sions on surfaces with increasing total emissions upon distur-bance, whereas the decrease of PM10 emissions is higher on surfaces with decreasing total emissions. In some cases, PM10 emissions decrease while total emissions increase. Disturbed sand surfaces generally show an increase of PM10 emission, but always less intense than the increase in total emission. Also, the PM10 fraction in topsoils did not signifi cantly change after disturbance. Results for fl at crusted surfaces are hetero-geneous. PM10 emissions of disturbed surfaces drop or rise, while total emissions always rise in comparison to the undis-turbed counterparts. These observations can be made on crusted surfaces with a fi ner grained layer on top and a coarser grained layer below. In some cases, PM10 emissions rise disproportionally, possibly due to the destruction of crusts and aggregates and the dispoportional increase of erodible sand particles. The surface F-DA2 is in many ways an exception, as it is by far the most clayey and most intensely crusted surface in this class. On this surface, disturbance has a signifi -cant effect, with a drastic increase of both total and PM10 emissions, albeit on a very low level of absolute emissions and with a very high proportion of sand-sized aggregates.

On disturbed rough stone surfaces, PM10 emissions drop disproportionally more than the total emissions. Again, a pos-sible explanation could be the reorganization of the surface upon disturbance, with an increased protection of the fi nest particle fractions by ‘dropping’ underneath skeleton and coarser fractions.

The Results in the Scientifi c Context

Aerodynamic properties of natural surfaces in other fi eld studies

The characteristics of wind profi les can be explained by roughness-induced differences in Reynolds shear stress and

are largely plausible for most surface classes. Another impor-tant factor for particle entrainment is the degree and kind of turbulence which provides energy for the vertical uplift of particles. Studies in fl uid dynamics confi rm that the surface roughness is directly related to the development and the characteristics of turbulent structures (Keirsbulck et al., 2002). The existence of a lower turbulent layer in the fi rst centimetre of the surface boundary layer was confi rmed by Leenders et al. (2005). This zone is governed by small-scale eddies and has to be distinguished from the zone of larger eddies in the fi rst 10–20 m. Field measurements during wind erosion events showed that turbulence near the surface is more or less decoupled from turbulence in the atmospheric boundary layer. This suggests that turbulence was repro-duced adequately as it produced similar small micro-relief eddies.

Similar fi eld wind tunnel experiments in the Sahelian region

Nickling and Gillies (1991, 1993) described their fi eld cam-paign in Mali, where they measured wind erosion parameters, erosion and dust emission potentials. Nickling and Gillies (1991) determined wind erosion parameters, like threshold friction velocities and potential vertical aerosol fl ux (potential dust emissions), with a similar portable fi eld wind tunnel. Natural dust background deposition was measured on the same test sites by the means of a 10 m aluminum sampling tower during a three-year measurement campaign. The wind tunnel experiments were in general slightly differently designed as in the present study, especially in respect to sediment catch-ing. The values of threshold friction velocities u*t found by Nickling and Gillies (1991) were of the same magnitude, although with a tendency to be generally lower. This can pos-sibly be ascribed to differing surface conditions in Mali or a lack of accuracy in the subjective optical determination of initial particle movement in this study. Some authors suggest the use of the threshold wind velocity at which a continuous saltation layer has developed (Kardous et al., 2005). The advantage of this defi nition of u*t is that it has a much safer observability in practice. Table IV shows both estimated threshold velocities in comparison with the data of Nickling and Gillies (1991).

Nickling and Gillies (1991, 1993) did not provide detailed information about the total mass fl uxes. They focussed on the determination of vertical mass fl uxes F as dependency on shear or friction velocity on only the most important surface types, rather than on the comparison of total mass fl uxes Qt for as many surface types as possible. A direct comparison of surface types or surface classes with their emission behaviour is therefore not possible. Most of the measured surface types in the Niger inland delta and around Bamako are suspected to be different from those in south-west Niger, consequently different results are to be expected (Table IV).



Mass fl ux: comparability of fi eld measurements and wind tunnel experiments

A problem in comparing different fi eld wind tunnel studies and fi eld experiments is the difference in fl ow conditions of artifi cial and natural storm events. Each natural storm event is unique and features large-scale atmospheric gusts, while wind tunnels produce less turbulent fl ow conditions which are not an adequate reproduction of natural events but rather an approximation. Varying wind direction and strength, and the much bigger particle reservoir constitute further fundamental differences. While the standardized fl ow conditions were nec-essary for the comparison of emission potentials, the question remains to what extent the wind tunnel data can be translated to natural conditions. The ideal case for this evaluation would be a dataset from similar storm events recorded on the respec-tive test surfaces (or at least similar surfaces), that can be correlated with wind tunnel data.

A comparable wind erosion fi eld experiment was performed by Sterk and Raats (1996) on one single surface type on the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) Sahelian Center (ISC). They recorded mass fl uxes of four convective storm events at the beginning of the 1993 rainy season together with meteorological data. The mass of the sampled material is not comparable to the amounts caught during the wind tunnel runs at the same location because the available reservoir of (re)mobilizable material was almost infi -nite compared with the small surface covered by a wind

tunnel. This enabled a strong and constant mass fl ux during the whole event, while in contrast emission activity on wind tunnel surfaces had only a short peak and decreased in the course of the experiment due to the lack of sediment replen-ishment. On the other hand, height distributions of emitted material are well comparable, which indicates an acceptable reproduction of fl ow conditions and suffi cient accuracy in the characterization of surface-specifi c emission behaviour (Figure 6). Nevertheless, small discrepancies in height distributions can be observed: the natural events seem to induce a higher vertical mass fl ux of suspended particles, which can be attrib-uted to increased vertical mixing due to large-scale turbu-lence. The duration of the erosion stage in wind tunnel experiments varies, so a direct comparison of mass fl uxes per time unit is somehow problematic. More plausible durations can be found by thoroughly monitoring the end of the erosion stage in the wind tunnel or by using data from experiments without supply limitation.

Topsoil disturbance: implications for land use

Our results indicate that mass fl uxes increase strongly on surfaces that are used by agriculture, especially Arenosols. In contrast to the present fi ndings, Rajot et al. (2003) found in their observations of natural storm events in southwest Niger that mass fl uxes are not limited by the presence or absence of a structural crust on sandy soils, mainly because no source

Figure 6. Measured mass fl uxes (A) by Sterk during a fi eld experiment at the ISC, (B) from a wind tunnel experiment at the same location; (C) and (D) show characteristic wind tunnel mass fl uxes from other surface types.

T. MAURER ET AL.


limitations occurred under the open fi eld conditions. This implies that, under natural conditions, disturbance (i.e. the destruction of structural crusts and aggregates) has no measur-able effect on wind erosion susceptibilities. However, the textural differences in topsoils of directly adjoining disturbed and undisturbed surface counterparts (Figure 7, T-PS, LC3, T-PC) suggest that continous disturbance leads to increased depletion of silt and clay-sized particles.

Unlike most Arenosols, surfaces under extensive use are less susceptible to an increase in emission potentials after distur-

bance. Land use is most likely to increase on sand surfaces, which would further increase the overall degree of distur-bance and hence wind erosion susceptibilities. An increase in the degree of disturbance mainly means not a deepening of the disturbed horizon, but an increase of the area covered by disturbed and unprotected surfaces (Thiombiano, 2000). This will lead to an increased, area-wide depletion of soils, which in turn forces farmers to resume tillage of fallows. An escape from this vicious circle is only possible by the consequent implementation of protective measures (crop residues, stub-

Figure 7. Wet particle distributions of the topsoils of all experimental sites. Surfaces are assigned to the respective surface classes for better comparability. Surface T-VC was assigned to sand surfaces because of its distinct layer of loose erodible particles.



bles, wind barriers) in combination with new agroforestry systems (parkland system) (Leenders et al., 2007). Countermeasures are most effi cient when the most susceptible areas are known. Future quantifi cation of area-wide nutrient losses due to increased land use can be done by coupling observed values for disturbed topsoils with scenarios describ-ing the increase and the distribution of land use activities on a regional scale.

Acknowledgements—The authors wish to thank the German Climate Research Program DEKLIM for the opportunity for this study and the German Ministry for Education and Research (BMBF) for funding it. The warmest thanks also go to the ICRISAT Sahelian Center and INRAN Tahoua for their friendly cooperation during the fi eld work, to Prof. Issa Ousseini for the detailed introduction to the landscape of southwestern Niger and to Jean-Louis Rajot for the careful and pro-found revision of the manuscript. Special thanks also go to Dr Horst H. Gerke, Centre for Agricultural Landscape Research (ZALF), Muencheberg, Germany, and the Transregional Collaborative Research Centre (SFB/TRR 38), Brandenburg University of Technol-ogy, Cottbus, Germany, for the friendly support upon completion of this article.

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