GRF Davos Planet@Risk, Volume 2, Number 2, April 2014
117
A Wind Erosion Case Study in an Alpine Environment (Davos,
Switzerland) Compared to Wind Tunnel Experiments with Live
Plants¹
GRAF, Frank𝑎
𝑎
WSL Institute for Snow and Avalanche Research SLF, CH-7260 Davos Dorf, Switzerland. [email protected]
Abstract – It is generally accepted that the (re-)establishment of a protective vegetation cover is
the most promising and efficient measure in restoring degraded land in the long term. Sustainable protection against wind erosion requires adequate information about suitable plant species
regarding ecological aspects as well as with respect to their proper contribution to wind erosion
control. e laer, however, is widely lacking. e goals of the presented field study are to record
reliable data on windblown erosion rates under natural alpine conditions and to cross-link the
findings with the results of wind tunnel experiments. A wind erosion test field was established at
2409 m a.s.l. in an alpine meadow including two test tracks. One track is le as is, representing
the naturally alpine vegetated soil (10-20% plant cover). e other track is equipped with a plastic
covering sheet, mimicking desertified soil (0% plant cover). Blue and red quartz sand was spread
on the vegetated and sheet-covered track, respectively, to visualise and measure the effect of vegetation on wind erosion control. Compared to the bare soil it was found that only small amounts
of sand from the vegetated plot were transported, even during heavy wind events. Related to the
seasonal course, the overall ratio varied from 1:19 to 1:717. alitatively similar findings, however
quantitatively less pronounced, resulted from the wind tunnel experiments (ratio = 1:15). Under
consideration of all available information, the comparison with data from the field experiment
considering only the configuration that best coincide with the wind tunnel set-up yields at least a
70-fold higher impact of plants on wind erosion control under natural conditions. e difference
implies that the sheltering effect of vegetation in nature is much higher than found for wind tunnel
runs, even when using live plants.
Keywords – vegetation, wind erosion control.
1.
Introduction
Worldwide and, in particular, in the Himalayas and on the
Tibetan Plateau, snow and glaciers are important both on
a regional scale, providing water supply, and on a global
scale, indicating climatic changes. Observations in the
field and experiments have consistently been confirming
that thin debris layers accelerate the ablation rate of underlying snow or ice (Han et al. 2006). e rapid increase
in aeolian sediment transport due to large-scale devastation of protecting vegetation has been verified (Wang &
Shen 2009). Wind erosive processes considerably intensify desertification and contribute to glacier decline and
speed-up of snow melting both inducing climate change,
erosion and sliding (Fujita et al. 2007). e increase in de-
sertification, snow and ice melting as well as soil instabilities are cross-linked phenomena. us, successfully inhibiting the main trigger contributes to the amelioration
at large.
Re-vegetation is accepted as the most efficient strategy to combat wind erosion and desertification in the
long term. Intact vegetation considerably contributes to
the stability of the near-surface soil structure (Burri et
al. 2011b, Suer-Burri et al. 2013) with important consequences regarding the soil-atmosphere interface including water and radiation balance and topography. Moreover, vegetation shelters soil from the impact of wind by
reducing its erosive force, trapping windborne particles,
and providing loci for deposition (Okin et al. 2009). With
a certain lag, vegetation recovery and the reduction of ae-
¹is article is based on a presentation given during the 4th International Disaster and Risk Conference IDRC Davos 2012, held 26-30 August 2012 in
Davos, Switzerland (hp://idrc.info/home/).
118
GRF Davos Planet@Risk, Volume 2, Number 2, April 2014
olian sediment transport should have a decelerating effect
on the ”sediment-driven” climate, with a positive feedback
mechanism on plant development and on the regain of arable land (Okin et al. 2009). Related to the presented field
study, it was recently demonstrated in the wind tunnel
using for the first time live plants that the total sediment
mass flux decreases exponentially with increasing canopy
density (Burri et al. 2011a, b). A naturally grown canopy
was analysed, mimicking the behaviour of natural vegetation canopies much more accurately than previous studies. However, this set-up, too, was considerably simplified
compared to real nature but allowed for concentrating on
specific parameters and, in particular, guaranteed the repeatability.
Field studies offer the possibility to address the full
complexity of a particular wind erosion situation, however, at the expense of unambiguous assignment of response and explanatory variables as well as comparability. As a consequence, the results of such investigations
are strongly coupled with their specific site characteristics, and the general conditions of the individual experiments. us, it is rather difficult to find a sound correlation between data of wind tunnel and field investigations.
erefore, the extrapolation from wind tunnel based models to natural scales is still not straightforward. Here we
combine the natural conditions of an alpine test field with
parts of the set-up applied in the wind tunnel experiments,
namely using the same sandy soil and comparing bare and
vegetated soil of the same plant cover (15-20%). e objective of the field study was to reliably measure windblown
erosion rates under natural alpine conditions in order to
validate the findings of the associated wind tunnel experiments. e synthesis of the two studies aims to assess
the extent and intensity of desertification and to evaluate the effectiveness of counter-measures, i.e. to quantify
the impact of natural plant cover on wind erosion control
(Zobeck et al. 2003).
2.
Material and methods
A wind erosion test field with two tracks was established
at 2409 m a.s.l. in an alpine meadow dominated by Phleum
alpinum L. and Poa alpina L. (Fig. 1). One track was le as
is, representing the naturally alpine vegetated soil (15-20%
plant cover). e other track was equipped with a plastic covering sheet (10x2 𝑚2 ), mimicking desertified soil
(0% plant cover). A direct link to the wind tunnel experiments (Burri et al. 2011a, b) was provided as experiments
with bare soil and comparable vegetation cover were performed. Blue and red quartz sand (♯: 0.2-0.6 mm) was
spread on both tracks (2x2 𝑚2 ) to visualise and measure
the effect of vegetation on wind erosion control. Leeward
of the two test tracks, 4 sets of panels and ground-plates
were installed in 3 lanes each, equipped with sticky foils to
trap and quantify particle transport (Fig. 1, 2). e le and
right lanes diverge with an angle of 15° related to the centre lane. Each lane consists of four vertical panels (10x50
𝑐𝑚2 ) mounted at a height of 40 cm with the corresponding
ground-plate (30x3 𝑐𝑚2 ) installed at their foot (Fig. 2).
Figure 1: Test field with the 2 tracks with coloured sand, the meteorological stations and sand trapping devices.
Figure 2: e 3 lanes of sand trapping devices; note the red sand
in the first row of the centre and le lane.
e distances between the panel-plate sets in the three
lanes were 3, 5, 10, and 15 m measured from the end of
the test tracks and 7, 9, 14, and 19 m measured from the
front of the two sand sources. In addition, wind direction,
wind speed at 50, 100, and 200 cm height, air temperature,
humidity, incoming and reflected short- and long wave
radiation, as well as precipitation were recorded (Fig. 1,
le). Periods of sediment transport were supposed to meet
the subsequent requirements: wind speed (ℎ = 50cm)
>6ms−1 , wind direction 230-290°, no precipitation, humidity >6 %, and temperature >0 ℃.
For the present article only the foils of the groundplates of three selected experiments (ex. 15, ex. 21, ex. 33)
were considered. A step by step practice was applied to
the jpg-files resulting from the scanning procedure of the
sticky foils (Xerox WorkCentre 7345: 300 dpi). e soware ImageJ (ver. 1.46 m; http://imagej.nih.gov/
ij/) was used and the different steps were: 1) edge cropping (100 pixels on each side), 2) subtraction of particles
> 0.6 mm, 3) colourthresholding (red and blue particles)
based on the HSB colour space limiting Hue, Saturation,
Brightness, and 4) filtering particles between 0.2 and 0.6
mm. e field experiments presented were performed in
2011 from 21-25 June (ex. 15), 21 July – 4 August (ex. 21),
and from 9-14 September (ex. 33). e wind tunnel experiments with bare sand and sand planted with live Peren-
GRF Davos Planet@Risk, Volume 2, Number 2, April 2014
the first row (ct1) conform most closely. e corresponding grain numbers of ct1 for the experiments 15, 21, and 33
are 8, 2, and 182, for blue and 10’809, 20’970, and 12’804
for red sand. Likewise the ratios yield 1:1’351, 1:10’485,
and 1:70 (Tab. 2).
In Figure 3 the detailed sand grain distribution from
bare and vegetated soil is shown for ex. 15. e total sediment flux [kg 𝑚−2 𝑠−1 ] in the wind tunnel experiments
was 0.261 for bare and 0.017 for vegetated soil, yielding
a ratio of 1:15, i.e. a 15-fold reduction in sediment transport due to vegetation. Related to the overall trapped sand
grains of each of the three field experiments, it may be
concluded that under natural conditions this positive effect of plants on wind erosion control is 1.3 to 48 times
higher than found in the wind tunnel. Restricted to corresponding ct1 (first sediment trap of centre lane) the factors range from 4.7 to 700. Taking further into account
the seasonal changes of plant cover and plant mechanical
properties in the field, the wind tunnel set-up and field
configuration most soundly coincide with ex. 15 (Fig. 2).
Consequently, the positive effect of vegetation on wind
erosion control is roughly 90 times higher in nature (ct1
of ex. 15: 1:1’350) compared to the findings in the wind
tunnel (1:15).
nial Ryegrass (Lolium perenne L.) were performed at the
WSL Institute for Snow and Avalanche Research SLF. For
providing a vegetated surface, eight 1 x 1 𝑚2 trays were
planted with L. perenne and aligned in the 8 m test section
of the wind tunnel. Sediment sampling was conducted
with a modified WITSEG sampler (Dong et al. 2004) positioned 7 m down-wind of the test section. In this article
the results of the runs with bare sand and the configuration with a canopy density of 25 tussocks per square meter yielding a plant cover of 15-20% are considered. e
mean free stream velocity measured at h=60 cm was 15
m 𝑠−1 (Burri et al. 2011b).
3.
119
Results
During the season the vegetation cover of the planted plot
changed from 15% (ex. 15) to 20% (ex. 21) to 10% (ex. 33).
Concomitant, mechanical properties of the plants (flexibility/stiffness) have been changing, altering their behaviour towards wind and sediment trapping. e flexibility of the shoots increased slightly from ex. 15 to ex.
21 and was strongly decreased in ex. 33. e proposed
requirements for potential wind erosion risk and, thus,
sediment transport were met during eight (ex. 15), nine
(ex. 21) and ten (ex. 33) periods lasting from 10 to 200
minutes. e corresponding total time of potential sediment transport summed up to 200, 350, and 260 minutes.
e maximum wind velocities varied between 6.2 and 10
m 𝑠−1 (Tab. 1).
e respective overall number of trapped grains for
the three experiments (ex. 15, ex. 21, ex. 33) was 105, 166,
and 1’712, for blue and 28’180, 119’009, and 32’840 for red
sand. e corresponding ratio between blue and red sand
– i.e. the reduction of sediment transport due to vegetation – was 1:268, 1:717, and 1:19 (Tab. 2). If rows and lanes
are considered individually, the factor for the reduction in
sediment transport by plants ranged from 5 to 9’748 for
the rows and from 12 to 6’897 for the lanes. In all three
experiments significantly less sand was transported from
the vegetated compared to the bare soil (p-value < 0.001).
Referred to the wind tunnel set-up with sediment sampling at 7 m downwind in the middle of the test section
(Burri et al. 2011b) the first row and the centre lane of the
field configuration and, in particular, the centre trap of
4.
Added value to integrative risk management
It has impressively shown that plants are indispensable in
stabilisation and restoration concepts for regions prone to
wind erosion. Additionally, seasonal variations as well as
the important interactions between plants and their living
environment both below and above ground and referring
to succession processes need to be integrated in the planning from the start. If, and only if these key factors are set,
wind erosion control can be tackled reliably with respect
to long-term success.
5.
Discussion and conclusion
Wind tunnel and field experiments point into the same
direction, emphasising the importance of plants in wind
erosion control. However, the positive effect of appropriately applied vegetation is still deeply underestimated.
Discrepancies may have several sources. Apart from vari-
Table 1: Time periods of potential wind erosion risk with corresponding mean and maximum wind velocities during the three experiments.
experiment
period
1
2
3
4
5
6
7
8
9
10
ex. 15: 21-25/06
duration [min]
v𝑚𝑒𝑎𝑛 [𝑚𝑠−1 ]
v𝑚𝑎𝑥 [𝑚𝑠− 1]
10
7.0
7.0
30
7.2
7.5
30
7.2
7.8
70
8.1
9.8
30
6.6
7.2
10
6.8
6.8
10
6.2
6.2
10
6.8
6.8
–
–
–
–
–
–
∑= 200
weight. = 7.3
max = 9.8
ex. 21: 27/07-04/08
duration [min]
v𝑚𝑒𝑎𝑛 [𝑚𝑠−1 ]
v𝑚𝑎𝑥 [𝑚𝑠−1 ]
10
6.2
6.2
10
6.8
6.8
30
6.3
6.5
30
6.8
6.8
200
7.6
10.0
10
6.2
6.2
10
6.2
6.2
30
6.8
7.2
20
6.8
7.2
–
–
–
∑350
∅weight. = 7.1
max = 10.0
ex. 33: 09-14/09
duration [min]
v𝑚𝑒𝑎𝑛 [𝑚𝑠−1 ]
v𝑚𝑎𝑥 [𝑚𝑠−1 ]
100
7.7
9.0
10
6.2
6.2
10
7.0
7.0
20
7.6
7.8
10
6.5
6.5
50
7.6
8.2
10
6.5
6.5
20
7.0
7.2
20
6.8
7.2
10
6.5
6.5
∑260
∅weight. = 7.3
max = 9.0
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GRF Davos Planet@Risk, Volume 2, Number 2, April 2014
Table 2: Number and ratio of blue and red sand particles trapped on the plates in the different rows (1-4) and lanes (le, centre, right)
during the three experiments (ex. 15, ex. 21, ex. 33). Framed: ct1 (first trap of centre lane) that best corresponds to the wind
tunnel set-up of Burri et al. (2011b).
ex. 15
row number (dist. from sand source [m]
lane
colour
1 (7)
2 (9)
3 (14)
4 (19)
sum
row ratio
le
blue
red
blue
red
blue
red
3
5’943
8
10’809
22
2’355
1
2’751
5
4’205
13
1’071
0
24
1
399
20
230
0
8 19
4
108
28
42
4
8’961
18
15’521
83
3’698
1:2’240
lane ratio
1:579
1:422
1:42
1:5
centre
right
ex. 21
∑ blue
∑ red
∅1:268
105
28’180
∑ blue
∑ red
∅1:717
166
119’009
∑ blue
∑ red
1’712
32’840
1:862
1:45
row number (dist. from sand source [m]
lane
colour
1 (7)
2 (9)
3 (14)
4 (19)
sum
row ratio
le
blue
red
blue
red
blue
red
4
34’480
2
20’970
0
3’036
4
6’903
13
34’557
0
544
0
0
0
12’000
101
351
0
0
2
6’030
42
138
6
41’383
17
73’557
143
4069
1:6’897
lane ratio
1: 9’748
1: 2’800
1:122
1:140
centre
right
ex. 33
1:4’327
1:28
row number (dist. from sand source [m]
lane
colour
1 (7)
2 (9)
3 (14)
4 (19)
sum
row ratio
le
blue
red
blue
red
blue
red
20
11’099
182
12’804
832
1’383
15
1’751
88
5’090
493
311
0
8
7
356
75
38
0
0
0
0
0
0
35
12’858
277
18’250
1’400
1’732
1:367
lane ratio
1: 24
1: 12
1:5
NA
centre
right
ations in flow characteristics (vertical profile, turbulence
structure, eddies, …), differences in soil structure are most
decisive. Although live plants were used in the wind tunnel, the short growing phase (4-5 months) and restriction
to one species did not allow for a functional soil ecosystem. As against nature, interactions between plants and
micro-organisms as well as hydrological processes (hydraulic li, evapo-transpiration, …) were drastically reduced or lacking. Compared to the field investigations
that account primarily for an alpine environment having undergone natural pedogenesis and maintaining functional interactions, the wind tunnel experiments rather
represent a first development stage in a re-colonisation
process of a completely unstructured and abiotic substrate, e.g. the abiotic part of the spoil of road construction. In contrast, under natural conditions at least parts of
the soil structure, organisms (living individuals, propagules), and organic maer, particularly in deeper layers, remain even aer heavy erosive processes. e extreme difference in sediment transport between the blue coloured
sand of the vegetated and the red one of the bare soil is reflected by an overall ratio of 1:268 (ex. 15), 1:717 (ex. 21),
1:66
1:12
∅1:19
and 1:19 (ex. 33), however, with a huge variation within
the different experiments depending on the location of
the traps and among the three experiments indicating a
seasonal trend which is related to a change in vegetation
cover as well as to plant mechanical properties, particularly the elasticity of the above ground biomass. e vast
variation within the short horizontal distance of 17 m and
the small covering area of 275 m2 reflects to a certain extent the complexity of wind erosion field experiments.
e considerable difference in sediment transport between the blue sand of the vegetated and the red one of
the bare soil fits well with the total mass flux results of
the wind tunnel experiment. In turn, the exponential decrease in total sediment mass flux with increasing canopy
density found in the wind tunnel study with the grass
Lolium perenne is in accordance with field observations
by Allgaier (2008), Hesse and Simpson (2006), Lancaster
and Baas (1998), and Li et al. (2007). e relationship between plant cover and erosion reduction is similar to the
one found for Salt Grass (Distichlis spicata) by Lancaster
and Baas (1998) and does not significantly differ from their
proposed model.
GRF Davos Planet@Risk, Volume 2, Number 2, April 2014
121
Figure 3: Distribution of windblown sand of ex. 15 (le: vegetated soil, right: bare soil). Rhombuses indicate the test tracks and filled
symbols the correspondingly coloured sand. e blue and red doed lines mark the first row of sediment traps at a distance 7
m off the front of the sand sources. Width: distance between sediment traps (rows); length: distance off the front of the sand
sources.
ough, the data of the present field investigation
gives evidence to suggest that the sheltering effect of vegetation is even more pronounced than worked out by the
wind tunnel experiment. Based on the overall ratio between bare and vegetated soil of the field investigation,
the findings of the wind tunnel experiment with the comparable medium-density configuration (ratio = 1:15) are
surpassed by a factor ranging from 1.3 to 48.
In the wind tunnel experiment, sediment sampling
was conducted with a WITSEG sampler according to
Dong et al. (2004) positioned near the end of the test section, at approximately 7 m down-wind (Burri et al. 2011b).
From this perspective and to beer sustain comparison
with the data of the field investigation, only the sticky foils
of the centre lane (clt), starting in a distance of 7 m off the
front of the two sand sources (blue and red sand) and, in
particular the first trap of the centre lane (ct1) should be
taken into consideration. To take these circumstances as a
basis, the difference in sheltering effect of the vegetation
between wind tunnel and field experiments diverges even
more. e mean ratio of bare to vegetated soil of clt and
cl1 of the three experiments ranges from 1:66 to 1:4’327
and from 1:70 to 1:10’485, respectively compared to 1:15
in the wind tunnel experiment and, therefore, amounts to
an increase by up to three orders of magnitude (Tab. 3).
However, the wind tunnel experiments were conducted with reference wind speeds of 15.3 m s−1 during
2 minutes for the bare soil and of 15.7 m s−1 during 10
minutes for the vegetated soil (medium-density configuration). In the field study, only one 10 minutes measurement interval reached a maximum wind speed of 10.0 m
s−1 with an average wind speed of 7.1 m s−1 . e corresponding total wind erosion period was 350 min. Due
to the lower maximum wind speed compared to the wind
tunnel experiments, it can be speculated that the protection function is overestimated in the field. On the other
hand, during the field experiments the wind exposed period with potential wind erosion risk was many times
longer with 3 h 20 min. for ex. 15, 4 h 20 min. for ex.
21, and 5 h 50 min. for ex. 33 (Tab. 1).
Despite these inconveniences, it seems most likely
that in nature the sheltering effect against wind erosion processes is more pronounced than eventuated by
the wind tunnel experiments, notwithstanding live plants
were applied. e uniform planting paern and the homogenous wind flow of the wind tunnel set-up differ considerably from natural conditions and are comprehensible
reasons behind the differences between wind tunnel and
field experiment data. Further explanation is found in the
field of the mutually dependent hydrologic balance and
soil structure, which are additional key aspects in wind
erosion control by plants.
In the wind tunnel investigations the 1 cm thick quartz
sand layer was on top of 6 cm thick 3:1 mixture of crushed
sand and earth (Burri et al. 2011b) filled in wooden trays
with drainage holes to prevent waterlogging. e plants
Table 3: Ratio of different data sets of the mass sediment flux between bare (*: mean value of three runs) and vegetated soil (mediumdensity configuration: 16% plant cover) from wind tunnel experiments (Burri et al. 2011b) and from the centre lane (clt) including
all 4 traps and the first trap of the centre lane (ct1) of the experiments 15, 21, and 33 of the field investigation.
Study type
data set
wind tunnel
total sediment flux [kg m
field investigation
ex.
ex.
ex.
ex.
ex.
ex.
15:
15:
21:
21:
33:
33:
vegetated soil
−2 −1
s ]
clt [number of grains]
ct1 [number of grains]
clt [number of grains]
ct1 [number of grains]
clt [number of grains]
ct1 [number of grains]
bare soil
∗
0.017
0.261
18
8
2
17
277
182
15’521
10’809
73’557
20’970
18’250
12’804
ratio
1:15
1:862
1:1’351
1 : 4’327
1:10’485
1:66
1:70
122
GRF Davos Planet@Risk, Volume 2, Number 2, April 2014
have been grown in there for about 4-5 months. Although
the substrate was well rooted, the stage of development
and stability of the soil matrix and pore structure were far
behind and the water cycle not as properly functioning
compared to the soil of the field investigation. ere, the
5 cm thick blue quartz sand layer of the vegetated plot was
on top of an alpine meadow whose soil has been developed for decades. Correspondingly, it complied much better with the requirements of the subsequently discussed
processes in view of stability and resistance against wind
erosion.
In dry sand, the angle of repose and, thus, the stability in general and of the surface grains in particular is
determined by their shape as well as by friction forces.
An increase in stability through weing processes is primarily due to adhesive binding (apparent cohesion) associated with interstitial liquid bridges between grains and,
therefore, directly related to the architecture of the matrix and pore structure of the sandy substrate (Barbasi et
al. 1999, Scheel et al. 2008). e more stable the substrate, the slower the desiccation process and, therefore,
the longer lasting the additional stability effect of the interstitial liquid bridges which is all the more decisive at
the wind-exposed surface.
e mechanisms underlying these processes and stabilising effects were not addressed in the present studies. However, based on previous research with different
substrates, it is likely that mycorrhizal hyphae, together
with the associated plant roots play a considerable part
in contributing; enmeshing sand grains and organic particles by acting as ”sticky string bags” (e.g. Degens et al.
1996; Rillig and Mummey 2006, Graf and Frei 2013). In
the course of the wind tunnel study of Burri et al. (2012
a, b) the first experimental demonstration was provided
that mycorrhizal fungi are able to increase soil resistance
to wind erosion. Although total root length of 2 monthold mycorrhized plants (Anthyllis vulneraria) was significantly smaller than of the non-mycorrhized ones, the
wind-induced soil loss of the mycorrhized root balls was
significantly reduced compared to the non-mycorrhized
control samples (Burri et al. 2011a).
In the vegetated plot of Latschüelfurgga the blue sand
was distributed above a naturally developed rhizosphere
with site-adapted plant species. Different to the red sand
on the plastic covering sheet – mimicking bare soil – and
to the set-up of the wind tunnel, it was in direct contact with the functional hydrologic cycle of the subjacent
soil’s matrix and pore structure which are more stable
and provide beer retention capacity than the sand itself.
Amongst other things, this superior stability is due to the
interactions of plant roots with soil organisms, in particular with their associated mycorrhizal fungi, which were
lacking not only in the control plot at Latschüelfurgga but
also in the wind tunnel experiment.
Related to the stability of the pore structure, the interactions between roots and soil micro-organisms, evaporation, transpiration as well as hydraulic li are certainly
key functions with respect to water flux and the distribution of moisture along soil depth.
Soil evaporation is a significant loss or depletion from
the water balance and maximized if there is a shallow
groundwater table, a hot and dry climate, a uniform finegrained soil, and bare surface ex-posed to sunlight and
wind. e shallower the water table is, the more continually the water will be supplied upward to the soil surface. In this case soil evaporation is controlled largely by
climatic conditions at the soil surface. is scenario was
met quite well on the vegetated plot in the alpine environment at Latschüelfurgga. Hence, a more or less continuous water flux during the potential wind erosion risk
periods may have been taken place. Consequently, the
near surface zone of the blue sand in the field investigation was probably rarely completely desiccated and the
additional stability through interstitial liquid bridges effective almost always, at least up to a certain degree. In
contrast, the sand surface of the planted (16% cover) configuration in the wind tunnel experiment was much drier
due to a lacking water table and missing of substantial radiation; the consequence being that the apparent cohesion
collapsed earlier, at a faster pace, and on a larger scale.
In addition to soil evaporation the transpiration of water within a plant coming from the roots and subsequently
lost as vapour through stomata in its leaves contributes
to the upward water movement too. Factors that affect
the plant driven evapo-transpiration include their growth
stage and age, percentage of soil cover, solar radiation,
humidity, temperature, and wind. Whereas the above
ground growth performance of the plants as well as the
percentage of soil cover were comparable in the field and
wind tunnel investigations, differences in solar radiation,
humidity, temperature, and wind were observed as well
as in rooting. It can, therefore, be taken for granted that
evapo-transpiration was by far lower in the wind tunnel
experiment, particularly due to insufficient light and radiation as well as the short duration of the wind erosive process (10 min.). As a consequence, in the wind tunnel experiment evapo-transpiration of plants was probably not
of importance with respect to the surface stability and resistance to shear forces.
Another process contributing to the upward water
movement is hydraulic li, a passive dislocation of water from roots into soil layers with lower water potential,
while other parts of the root system in moister soil layers are absorbing water. Usually, considerable amounts
of water are transferred from weer (deeper) layers to the
oen drier near surface zone of the soil during the night.
is partial rehydration of the upper soil layers provides
an additional source for transpiration the following day.
Lied water may also contribute to the availability of water soluble nutrients located most plentiful in the upper
soil layers and, therefore, indirectly influences survival
and growth performance of the plants and associated organisms. Hydraulic li may prolong or enhance fine-roots
activity in the subsurface layers by keeping them hydrated
and thus, buffer the rhizosphere organisms from effects of
soil drying during persistent periods of lacking precipitation (Bauerle et al. 2008, erejeta et al. 2007). A considerable positive influence on sand surface stability by
hydraulic li was most likely restricted to the vegetated
plot of the present field investigation. During the wind
GRF Davos Planet@Risk, Volume 2, Number 2, April 2014
tunnel experiment this process was probably not active.
Conclusively, it can be summarised that the findings
of the wind tunnel experiment and the data of the present
field investigation point into the same direction, emphasising the importance of plants in wind erosion control.
e data from the wind tunnel experiments may rather
represent a point of reference for the very first development stage of the re-colonisation process of a completely
unstructured and abiotic substrate. Despite the fact, that
for the first time an intact vegetation cover of live plants
was applied in the wind tunnel, there are still essential deficiencies in order to appropriately simulate natural conditions. Particularly concerned are soil formation related
to development stages and succession phases of the vegetation. e preliminary findings from the field investigations account primarily for an alpine environment having
undergone natural pedogenesis. Projected to restoration
of degraded soil, the findings from the field investigation
represent, therefore, a rather late stage of development.
Interactions between soil organisms and plant roots,
hydrological processes and the mechanical properties of
the above ground plant parts rank among the most decisive mechanisms in view of resistance against wind erosion. In respect of the laer, seasonal changes of plant
cover and the plant’s material properties need to be appropriately taken into consideration if field experiments
are envisaged or data of corresponding investigations are
analysed and compared. Even with respect to wind tunnel
experiments with live plants this might be an important
issue to think of.
In order to improve our knowledge on the performance of plants and vegetation types in wind erosion control and in view of adjusting conventional models for sediment transport and climate change at different scales, it
is necessary to properly include such processes in future
investigations, in the field as well as in the wind tunnel.
Consequently, it will be necessary to consider not only
one individual grass species but representatives of other
plant groups and combinations of species, in particular
with respect to root morphology.
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Citation
Graf, F. (2014): A Wind Erosion Case Study in an Alpine Environment (Davos, Switzerland) Compared to Wind Tunnel Experiments with Live Plants. In: Planet@Risk, 2(2): 117-124,
Davos: Global Risk Forum GRF Davos.