Assessing soil management practices in terms of soil erosion control
by using field experiments and model simulations
J. Schmidt, N. Seidel, M. Schindewolf
Abstract
In this paper the effectiveness of soil erosion control by alternative management
practices is investigated by comparative model simulations taking different
management practices into account. The EROSION 3D model (SCHMIDT, J. 1996,
VON WERNER, M. 1995) is applied in order to simulate soil loss and deposition as
well as sediment transport into surface water courses on the watershed scale.
EROSION 3D is an event-based model using the momentum flux approach by
SCHMIDT, J. (1991). The model simulations show that despite of the conversion of
conventional into conservation tillage erosion rates still exceed tolerable amounts
resulting in the formation of ephemeral gullies at locations where surface runoff
accumulates. Only the partial existence of a permanent vegetation cover (e.g. a
grassed waterway) is able to avoid these kinds of linear erosion features and allows
to limit soil loss within the entire catchment to a tolerable amount.
1
Introduction
The use of soils for agriculture requires to eliminate the natural vegetation cover and
to replace it by cultivated plants. Thus the protection of soils with respect to the
impact of wind and water is - if not completely suspended - at least occasionally
interrupted. As a consequence soil erosion leads to irreversible soil losses, which in
turn reduces soil productivity and impairs water quality because of the contamination
by sediments and particle-bound pollutants.
Above all accelerated soil erosion is a consequence of the missing permanent
ground cover of most arable lands. Under the criterion of soil protection it is crucial to
adapt soil management in a way that temporal gaps in soil cover can be avoided.
Recent management practices pursue this goal, by abandoning the plow completely.
At present following variants exist: The most advanced system is the direct seeding
or no till variant, where the seeds are directly sown by cutting a nearly invisible slot
through an existing cover of plants or plant residues. The difference to the so-called
conventional or conservation tillage practices is the fact that the top soil is not
loosened up at all so that the natural soil structure is preserved. In the conservation
tillage variant the soil is loosened up superficially at least some centimetres using a
rotary tiller, however, without – as by the conventional management – turning around
the top horizon completely. In order to cover the soil surface plant residues (mulch) or
intermediate cover crops are used in combination with conservation as well with
conventional tillage. A disadvantage of conservation and non tillage management
practices is the high susceptibility for parasites and plant diseases which results in a
high need for pesticides. In order to evaluate the environmental benefits of
conservation and non tillage systems it is of crucial importance to estimate their
effectiveness in terms of soil erosion control.
2
Study areas
The study focusses on two subcatchments of the Striegis River in Saxony/Germany.
The “Klatschbach” catchment is approximately 6 km² in size and part of the Mulde
Lösshügelland (loess hills). Loess thickness averages 3 m (BERNHARDT, 1986) and
the most common soils are Gleysols and Stagnic Umbrisols partly covered by
colluvial deposits of up to 2 m thickness at bottom slopes (SLfUG 2004a, Soil
Survey). The Klatschbach catchment is dominated by agricultural land use (arable
land: 69 %, grassland: 18 %, forest: 8 %, settlements and roads: 5 %).
The „Oberreichenbach” catchment is approximately 9 km² in size and is situated in
the foothills of the Erzgebirge (Ore Mountains/Sxony). The loess cover is less than
2 m thick (BERNHARDT, 1986). Cambisols and Stagnic Umbrisols, partly covered by
colluvial deposits, are the dominant soil types, but Gleysols can be frequently found
in the flood plains. Land use distribution is as follows: arable land: 59 %, grassland:
16 %, forest: 14 %, settlements and roads: 11 %.
3
Methods
3.1
Rainfall and surface runoff simulator
The rainfall simulator used for this study consists of three linked rainfall modules
allowing the irrigation of a 3x1 m plot (SCHINDEWOLF, M. & J. SCHMIDT 2011).
Each of the identical rainfall modules is equipped with a VeeJet nozzle, which
provides intermitted rain by oscillating. The VeeJet 80/100 nozzle (Spraying
Systems) represents a quasi standard in rainfall simulations due to its drop size
distribution and drop velocities which are comparable with natural heavy rainstorms
(AUERSWALD ET AL., 1992; FOSTER ET AL., 1982; HASSEL AND RICHTER,
1992; KAINZ ET AL., 1992). By varying the nozzle pressure the drop size distribution
and fall velocities can be altered to some degree.
The rainfall intensity is regulated by the nozzle’s turning speed and by the time the
nozzle remains at the reversal points (AGASSI AND BRADFORD, 1999).
The experimental plot is bordered by a metal frame, draining runoff and sediment
funnel at the frame’s lower end. In order to keep the small plot simulations
comparable to the large plot experiments despite of the limited plot length a runoff
feeding device is installed at the upper end of the plot. Via a drain water pump the
runoff adding module can be supplied with water and suspended sediments. Thus
the experiment simulates the last 3 m of a slope segment with a variable virtual
length of up to 40m. The use of sediment-loaded water facilitates the adjustment of
sediment concentration and detachment rate when runoff passes the plot.
Figure 1: Rainfall simulator
3.2
The EROSION 3D Model
The model EROSION 3D, used within this study, was developed with the intention to
create an easy-to-use tool for erosion prediction in soil and water conservation
planning and assessment (SCHMIDT ET AL., 1992, VON WERNER, M. 1995).
The EROSION 2D/3D model is predominantly based on physical principles. The
model simulates the detachment of soil, the transport respectively deposition of
detached soil particles by overland flow, incl. the grain size distribution of the
transported sediment and the sediment delivery into downstream water courses
caused by single events (SCHMIDT, 1992). The infiltration rate is estimated by an
infiltration subroutine based on the modified approach of GREEN and AMPT (1911)
assuming that rain water penetrates the soil in a piston-like flow and saturates the
available pore space completely. Because the Green and Ampt approach
presupposes a rigid soil matrix the temporal variability of soil structure due to tillage,
slaking and sealing, shrinking and swelling, biological activities etc. has to be
considered by an additional empirical parameter which allows calibrating the
saturated hydraulic conductivity on the basis of measured data. In the EROSION
2D/3D model this parameter is called skinfactor which is determined by iterative
calibration of simulated infiltration rates. Values of skinfactor <1 reduce the simulated
infiltration rate, in order to take the effects of soil slaking and sealing as well as
anthropogenic compaction into account. Values of skinfactor >1 cause an increase of
infiltration rate, e.g. for the consideration of macroporeflow. If skinfactor = 1 infiltration
rate is obviously not affected by either slaking and sealing or macropores.
The basic assumption of the detachment and transport subroutine is that the erosive
impact of overland flow and droplets is proportional to the momentum fluxes exerted
by the flow and the falling droplets respectively. The resistance to erosion of the soil
is expressed as the critical momentum flux. In this approach, the sum of all mobilizing
forces (overland flow impact, impact of the droplets) acting on soil particles is
compared to the sum of those forces which prevent the particles from being detached
and transported. Erosion occurs if the sum of mobilizing forces is greater than that of
the resisting forces. In cases where the opposite applies, no particles are eroded
from the soil surface.
Erosion is limited either by the amount of sediment that can be detached from the soil
surface or by the transport capacity of the flow. In order to transport detached
particles the vertical component within the flow must counteract the settling of the
particles for deposition.
The application of EROSION 3D requires information on site-specific relief, soil and
rainfall conditions. This information is supplied to the model using the following
parameters:
-
Relief parameters: x,y,z coordinates (digital elevation model)
-
Rainfall parameters: Date of rainfall event (dd.mm), rainfall duration, rainfall
intensity. Generally the model uses standard 10min intervals for precipitation
intensity.
-
Soil parameters: Texture, bulk density, content of organic matter, initial soil
moisture, erosional resistance, hydraulic roughness of the soil surface and
percentage ground cover.
The effects of different types of land use and agricultural management practices on
erosion are accounted for by varying the values for erosional resistance, hydraulic
roughness, skin factor and percentage soil cover. Suggested values for these input
parameters can be estimated from a digital database (DPROC) which includes
specific parameters for a broad range of soils, surface conditions, and management
options.
EROSION 3D uses a grid-cell data representation of the watershed. The values of all
input parameters are assumed to be spatially uniform below the scale of grid
resolution.
The model produces raster-based, quantitative estimates of soil loss, soil deposition,
and the sediment delivery into the surface water system. The following data are
provided for each grid cell:
Parameters related to area:
-
Erosion and deposition for a chosen grid cell (mass/unit area)
-
Erosion, deposition and net erosion for the watershed draining into a chosen
grid cell (mass/unit area)
Parameters related to cross-section of flow:
-
Runoff (volume/unit width)
-
Sediment delivery (mass/unit width)
-
Sediment concentration (mass/unit flow volume)
-
Particle-size distribution of the transported sediment (percentages of clay, silt
and sand by mass).
The predicted spatial distribution of erosion and deposition can be plotted as a
colored map (see Fig. 2, 3).
3.3
Land use scenarios for model simulations
The simulation runs are based on different land use scenarios which cover different
types of land use and management practices. In order to define the distribution of the
basic types of land use within the investigated catchments, remote sensing data were
evaluated (Orthophotos, Coloured Infrared Biotope Mapping). The following
scenarios were arranged in order to allow direct comparisons of land use and
management impact:
Scenario “Conventional Tillage”
In this scenario, the arable land is plowed in the traditional way. Soil condition is
characterized as crusted because of the incomplete ground cover as well as the
weak structural soil stability. Crops correspond to the real crop rotation of the year
2004.
Scenario “Conservation tillage”
In this scenario, plowless or non-turning tools are used for primary tillage. The most
important effect of these tillage techniques is the conservation of stable soil
aggregates and fast draining macropores. In order to reduce or even prevent soil
crusting, crop residues are left on the soil surface forming a permanent ground cover
of 30 %. Crops correspond to those of scenario A.
Scenario “Grassland / Direct Drilling (no tillage)”
“Grassland and “Direct Drilling” are combined in scenario C because of comparable
conditions regarding soil management. On the one hand, mechanical impact on the
soil structure is reduced to an absolute minimum, maintaining the permeability of
soils at the highest level possible. Moreover, all-season ground cover prevents soil
crusting. On the other hand, there is no recurring loosening of top soils as in the case
of conventional or conservation tillage. Thus, there is no temporal increase in
infiltration capacity due to tillage operation. Crops again correspond to those of
scenario “Conventional Tillage”.
Scenario “Forest”
In this scenario, the catchments are totally covered by mixed woodland (potential
natural vegetation), apart from settlements and roads. Because of the high biological
activity and the absence of any tillage impact, infiltration capacity is high.
4
Results and Discussion
4.1
Experimental results
The artificial rainfall experiments are designated to estimate specific parameters of
the EROSION 3D model as erosional resistance, hydraulic roughness and skin factor
which are affected predominantly by tillage practice.
According to MICHAEL (2000) resistance to erosion varies over a broad range.
Regarding the regional soil types of Saxony there is a variance of 4.E-5 [N/m²] - 2E-2
[N/m²] mainly controlled by soil texture. Since this study is focussed on loessinfluenced soils the measured data on erosional resistance do not differ as much.
Resistance to erosion is lowest on conventional tillage (Ø 0.00113 N/m²) plots and
highest on non tillage plots (Ø 0.00384 N/m²). This general trend of decreasing
resistance of erosion with increasing tillage intensity is also confirmed by
experimental results of KNAPEN et al. (2007a) and MICHAEL (2000). Under
conservation tillage resistance to erosion varies over a broad range (0.00053-0.008
N/m²) indicating that this tillage practice results in highly variable field conditions
referring to particle detachment. Apparently conservation tillage is not a well defined
practice in particular concerning working depth and residue cover, which varies in
between 5 and 50%.
Hydraulic roughness increases from conventional tillage (Ø 0.002 s/m1/3) to
conservation tillage (Ø 0.025 s/m1/3) and further to non tillage (Ø 0.184 s/m1/3). This
increase is associated with an increase in soil cover proving that soil cover is the
most decisive factor for hydraulic roughness.
Non tillage practices result in significantly higher skinfactors (Ø 9.83) indicating a
higher amount of macropores open to the soil surface and less soil sealing
respectively higher aggregate stability compared to conventional (Ø 2.0) and
conservation tillage (Ø 0.76). Compared to conventional tillage the lower average
skinfactors of conservation tillage result from subsurface compaction due to the
absence of loosening (KOCH et al., 2008).
4.2
Modelling results
Based on the scenarios described above model results were evaluated regarding the
following tillage practices: conventional tillage, conservation tillage and non tillage
(SEIDEL, N. 2008, SEIDEL, N. and SCHMIDT, J. 2009).
At first Fig. 2 shows the simulated distribution of erosion and deposition within the
Klatschbach catchment referring to the conventional tillage practice scenario. Yellow
to red colours mark areas affected by erosion, green to blue colours indicate areas
susceptible to deposition. As expected erosion is almost negligible under forest and
on grassland (≤ 0,25 kg/m ²) whereas on arable land soil loss increases up to > 25
kg/m ².
Figure 2: Simulated erosion and deposition within the Klatschbach catchment
(Saxony) referring to the conventional tillage scenario assuming a 100years storm
event in Mai and moderate initial soil moisture conditions
Apart from the widespread sheet erosion the Klatschbach catchment exhibits
numerous rill erosion features, which result from cumulating surface runoff in slope
depressions. Due to its enormous erosion and transportation capacity the cumulated
runoff in the depression lines results in high erosion rates (> 25 kg/m ²).
Deposition of eroded sediments occurs, where surface runoff is decelerated due to
decreasing slope gradient or increasing hydraulic roughness caused by land use
changes. In both cases decreasing runoff velocities result in a reduction of sediment
transport capacity. This happens in particular at the transition from arable fields to
forest and/or from fields to grassland. However, the major parts of the sediments
enter the surface water system because sufficiently broad buffering strips are
missing.
Corresponding to Fig. 2 the spatial distribution of erosion and deposition in case of
conservation tillage is presented in Fig. 3. As the comparison of both maps shows, a
conversion from conventional to conservation tillage results in a clear reduction of
erosion. While conventional tillage is accompanied with more than 25 t/hectars of soil
loss, sheet erosion accounts for maximally 2,5 t/hectars in case of conservation
tillage due to the additional amount of mulch cover.
Despite of the conversion of management practises from conventional to
conservation tillage, sheet erosion still range between 0.25 and 25 kg/m². On closer
consideration the erosion map shows that substantial soil losses occur along
depression lines in spite of conservation tillage. That shows that additional erosion
control measures as grassed water ways or buffer strips are necessary for both
scenarios conventional and conservation tillage in order to avoid excessive soil
losses and sedimentation of surface water courses.
By reducing cultivation intensity furthermore (grassland/non tillage scenario) erosion
can be reduced again significantly, so that additional passive erosion control
measures (e.g. grassed water ways or buffer strips) are no longer necessary. Under
forest no considerable soil erosion takes place.
Figure 3: Simulated erosion and deposition within the Klatschbach catchment
(Saxony) referring to the conservation tillage scenario assuming a 100years storm
event in Mai and moderate initial soil moisture condition.
5
Conclusions
The model simulations enable regional authorities to locate soil erosion risks within
the catchment area and to identify the main source areas contributing predominantly
to sediment delivery to surface water courses. The simulated land use scenarios
show that soil erosion risks can be reduced considerably by changing tillage practice.
Referring to conventional tillage soil loss decreases by 96.5 %, if soil management is
converted to conservation tillage. Furthermore, in case of non tillage or grassland the
reduction in soil loss amounts up to 99,8%. Under forest there is nearly no soil loss at
all.
5
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Authors
Prof. Dr. Jürgen Schmidt
Soil and water conservation unit
Agricola Str. 22
09599 Freiberg
jschmidt@tu-freiberg.de
Dipl. Geoökol. Nicole Seidel
Soil and water conservation unit
Agricola Str. 22
09599 Freiberg
seidel.nicole@web.de
Dipl. Geogr. Marcus Schindewolf
Soil and water conservation unit
Agricola Str. 22
09599 Freiberg
marcus.schindewolf@tbt.tu-freiberg.de