Download The effect of plot size on wind erosion Dr. József Lóki – Gábor

The effect of plot size on wind erosion∗
Dr. József Lóki – Gábor Négyesi ∗∗
Abstract
Those long-stretched-out plots which are parallel to the wind direction are not favourable in
terms of wind erosion. In Hungary, the plot size has been studied especially considering its
economic effect so far and the soil protection – wind erosion approach was missing during the
examinations.
Recently the significant changes in land use have given reasons for this kind of examinations.
Taking advantage of digital mapping and GIS software we have the opportunity for taking
soil-protection approach in plot size examinations. In a chosen sample area we have measured
the plot size changes by means of statistical and satellite image analyses. Moreover we have
measured how the plot size modifies the force of wind erosion.
Preliminaries
The process of wind erosion is influenced by a number of factors. On the one hand, there are
climatic factors and on the other hand, there are soil factors (soil granularity, soil structure,
organic-content, ground surface roughness and the moisture of the top soil, the length and the
direction of the deflation area) as well.
Chepil (Chepil W. S., 1945) was the first scientist who studied the effect of the length
of the erosion area. He pointed out that if a certain plot of arable land is situated parallel to the
wind direction wind erosion is expected to increase. Later, this area-length factor (together
with other wind erosion factors) was placed into the so called ‘wind erosion equitation’
(Skidmore 1965, Skidmore-Woodruff, 1968, Skidmore, 1987).
Earlier the plot size was considered – especially in terms of economy – by agronomists
in Hungary. They examined the size of area suitable for economical cultivation. Previously,
there was no precedent for studying the affect of plot size on wind erosion.
This paper demonstrates plot size changes by using statistics and satellite images. This
is particularly important as the size of the arable-lands cut up soon after the regime change
will increase again in order to economic rationality.
The size of deflation area
The length of the area affects the occurrence and power of soil erosion in two different
ways. First it influences the development of maximum wind force and it also supports or hinders the avalanche like increasing of the moving soil. Thus the erosion is getting heavier parallel with the length from the wind side of the area in the direction of the wind as long as it
reaches the suitable maximum of wind force and soil erodibility. Fryrear et a ll. (2001)
pointed out that the length factor also has a maximum distance, above which the erodable
amount of soil cannot increase. This distance – in case of a given wind force – depends on the
erodibility of the soil. If the soil has a good erodibility factor the above mentioned distance is
shorter than in the case of cohesive soils. The larger the flat land is the more likely that the
rather resistant soil will be damaged. The damage can also be increased due to evaporation by
∗
This research was supported by T 037249
University of Debrecen, Department of Physical Geography and Geoinformatics H-4010, DEBRECEN P.O.B.
9. HUNGARY
∗∗
which the top soil turns dry. The longer the plot is the stronger the wind force becomes and it
carries the soil particles blown out of the top soil with a higher velocity. By means of these
soil particles the erosion spread over like an avalanche. The larger the area is the more frequently the particles hit the ground and smaller and bigger particles are put in motion that is
why the degree of erosion and the damage in vegetation will be higher.
Originally the length of the area was regarded as the distance in the direction of the
prevailing wind. However, wind usually has more than one direction varying season-toseason or even within a day. In this case the most frequent wind direction is used. This is
counted as the quotient of the winds blowing parallel and perpendicular of dominant wind
direction. (The dominance of northern and northeastern winds is characteristic in the studied
Hajdúböszörmény small region.) Later studies (Skidmore, 1986) resulted in the construction
of tables containing the direction factors of wind erosion.
The value of L was simplified ignoring wind direction. Cole et all. (1983) applied the
following formula:
sec
w sec £
L= l cscl £
L≤
(l
2
+ w2
)
where l and w are the longer and shorter sides of the plot, respectively while £ is the
angle between the longer side and the prevailing wind direction.
Williams et al. (1984) applied the following formula in quantifying the deflation area
in the EPIC* wind erosion model:
L=
(
l cos π / 2 + α − ζ
l*w
) + w sin (π / 2 + α − ζ )
Where l where l and w are the longer and shorter sides of the plot, respectively while α
is the prevailing wind direction in radian (1 radian = 57.3°) in clockwise sense and ζ is the
angle in radian between the length of the area and the North direction in clockwise sense.
However, it is not sufficient to interpret plot influence on wind erosion only in the triangle of area length – width – wind direction as numerous factors may decrease the influence
of a large plot on wind erosion (Table 1.).
Table 1. Factors influencing the effect of plot size on wind erosion
precipitation
Natural factors
temperature
soil cultivation
Anthropogenic factors
plot size
sowing structure
plot protecting woods and
wind breakers
The role of precipitation and temperature is the most important among natural factors.
As moisture content plays an important role in the wind erosion resistance of the soils the
amount of precipitation in the most vulnerable spring period (March, April) and the tendency
showing in the analysis of the data of the spring months of several years are important. The
conclusions drawn from the analysis of the data of the spring months of the study area are
doubtful. A slight increase is observable in April but it is disregardly small its correlation does
not even reach 0.1. It has to be emphasized that both March and April precipitations exhibit
extreme fluctuation suggesting that the danger of dry spring periods have to be calculated
with. Temperature is important because sudden spring warming increase evaporation and thus
the drying of the top layers of the soils resulting in the danger of soil erosion. In annual sense
temperature increase produces negative evaporation balance resulting in the deterioration of
the soil structure through drying.
In areas prone to wind erosion soil cultivation should help the protection of the soil
against deflation besides the general demands (maintaining moisture content, seed-bed preparation, weed control cultivation of plant remnants and manure). Suitable cultivation of soils
bears in mind smallest possible disturbing, topsoil compaction and making the soil surface
rough. Fulfilling the last one is also very important therefore compaction after cultivation is
necessary in order to keep the moisture content of the soil and to make its surface rough. It is
also important to bear in mind that plots should have their longer side perpendicular to the
direction of prevailing (or most frequent) wind. Preference should be made for the soil preserving cultivations (direct sowing, ridge till, slot planting) presenting the least disturbing
effect.
Wind obstructions and wind holdings influence wind erosion in two ways. On the one
hand, they decrease wind speed in the wind shadow so that the movement of the soil is inhibited. On the other hand, they decrease the length of the plot thus decreasing the length of the
movement of the particles. Protection belts and walls present different protection depending
on their porosity, shape and wind speed. In calculating the soil loss in the case of the input
parameter of the deflation length the total length of the plot should be decreased by the length
ten times the height of the protecting wood belt. Naturally the effect of the wood belts depends on the distance between the belts and on their direction. In protecting against wind erosion small distance between wood belts is preferable. In the case of loose easily eroding soils
the forming of great plots is destructive as plot protecting wood belts become distant from
each other so that their effect is not effective in a significant part of the area. Planning the
wood belts the characteristic wind directions have to be regarded as well. Wind direction of
the spring months should be considered in those areas where wind direction is not constant as
wind erosion presents the greatest threat on soils in this time of the year.
Sowing structure should be constructed to cover the soils for as long as possible especially in the periods of strong storms. Plants sowed in the Spring suffer most as they are
greatly undeveloped at the time of the Spring winds. Direction of plant rows is also important
in reducing wind speed. Plant rows should be situated perpendicular to the prevailing wind.
Wind erosion losses can be reduced by applying ribboned or zonal sowing method of (Chepil
W.S., 1959). This means that Autumn eared and Spring sowed plants are sowed alternating
next to each other perpendicular to the wind direction so that the more developed plants protect the less developed ones. The allowable distance between the stubble or plant belts is
around 6-10 metres.
Changes in the plot size in Hungary
After the Second World War small-size-land structure formed with the dissecting of
the large-land-structure in our homeland, which contains 20-30 m narrow stripes. The structure of lands was very different, because everybody produced just on their own. Notable
change occurred in the years when huge plots were formed because of developing towards
machine cultivability. These, so-called industrial manufacturer systems produced just one
type of plant. The advantage of this was that the machines used in the cultivation may cultivate large, homogenous areas, and it made the growing cheaper. When the plots were formed,
the former wood belts – situated near roads – were cut down, and the roads were ploughed, so
that objects creating of mosaics of arable land disappeared. Hereby, nothing blocked the wind,
so it could put out its destructive work. The same plant type was produced in homogenous
areas, therefore in spring or autumn according to the sowing time the surface stayed uncovered.
The observations and soil using consequences of last decades prove that the manufacture systems have ecological and environmental limits too. Nyíri L. (1993) pointed that our
production sites are not suitable to form out too large (50-100 hectare) or even larger lands.
After the regime change of 1990 the variable plant growing where the area of the
manufactures was dissected into small plots reduced the danger of the development of wind
erosion. According to the 2003 data of the Central Statistic Office 1.6 % of the farms cultivate
75 % of the arable land in Hungary. However, the rate of economically not sustainable farms
is also great as 92.8 % of the farms cultivate less than 10 hectares. The number of farms having a larger area than 10 hectares is under 50000 while the average cultivated farm area is 8.2
hectares.
According to the subsequent data the concentration of land will continue. The number
of farms decreased from 985,5 thousand to 765,6 thousand from the year 2000 by 2003. Average sizes of farmers organizations (503 hectare) was less with about 30 % and the individual organisations was larger with 20 % than it was in 2000.
Arable land presented 75.87 %, forest only 3.60 % and grass land occupied 17.94 % of
the total study area in the year 2000. The average size of this land use unit was shown by Table 2.
Table 2. Average sample size of economic and individual organisations in the Hajdúböszörmény small area in 2000 (Source: Agriculture of Hungary in 2000)
Average area size of economical organisations (hectare)
arable
kitchen gar- fruit garden vine
grass
forest
reeds
fish lake
den
land
347,36
55,9
200,20
161,70
41,48
135,25
Average area size of individual organisations (hectare)
arable
kitchen gar- fruit garden vine
grass
forest
reeds
fish lake
den
land
4,90
0,05
0,34
0,10
8,68
1,76
5,28
0,01
Naturally, the average plot size does not mean that all of the areas are situated in one place.
For this, Tables 3. and 4. give some information, that plots of the area are dissected into small
pieces.
Table 3. Distribution of crop structure of individual organisations in the Hajdúböszörmény
small area in 2000 (Source: Agriculture of Hungary in 2000)
Individual organisations (total number: 2206)
-0,15 ha
0,150,511,1-5,0 5,1-10,0
10,150,1100,1300,00,50 ha 1,00 ha
ha
ha
50,0 ha 100 ha
300 ha
ha
41,76% 10,76% 6,52% 25,79% 7,62%
6,74%
0,57%
0,21%
0,03%
Table 4. Distribution of crop structure of economical organisation in the Hajdúböszörmény
small area in 2000) (Source: Agriculture of Hungary in 2000)
Economic organisations (Total number: 967)
-10 ha
10,150,1100,1300,1500,11000,1- 5000,1- 1000050,0 ha 100 ha
300,0
500,0
1000,0
5000,0
10000,0 ha
ha
ha
ha
ha
ha
7,02%
35,09% 10,53% 10,53% 1,75%
10,53% 24,56% -
At the same time, as it is cleared out from Table 4, that the quarter of economic organisations
(which are economically sustainable and viable) cultivate 1000-5000 hectare area. Obviously,
that these are not located like long narrow parcels next to each other, but are in one place and
in larger and homogeneous crop structure, respectively tillage system.
Resulted possession changes in the 1990s unfold in the decreasing of plot size. That is
confirmed well by LANDSAT satellite images (Figure 1.) from the sample area in different
years. Direction and size of the plot can be determined with the help of satellite images.
Summary
1992
Finding of the plot from the view of soil conservation-economy should be a very important
role because of the global climatic change and formation of big plots again in Hungary. Attention should be paid to the formation of the adequate crop structure, the suitable direction of
plots and the end of the arable land have to be closed with forest belts. Based on previous examinations (Szabó G., 2001) confirmed by our own enquiry the satellites and the statistical
data together are adaptable to the impoundment of the plot size.
References:
Chepil, W. S. (1945): Dynamics of Wind Erosion III: The Transport Capacity of the Wind. Soil Sci. v. 60.
pp.475-480.
Chepil, W. S. (1957): Width of field strips to control wind erosion. Kansas Agr. Expt. Sta. Tech. Bull. 92.
Chepil, W.S. (1959): Wind erodibility of farm fields. Journal of Soil and Water Conservation 14: pp. 214-219.
Cole, G.W. - Lyles, L. – Hagen, L.J. (1983): A simulation model of daily wind erosion soil loss. In: Trans.
ASAE 26. 1758-1765
Földmőveléstan, szerkesztette: Nyíri László, 1993, Mezıgazda Kiadó, Budapest
Fryrear, D.W. – Sutherland, P.L. – Davis, G. – Hardee, G. – Dollar, M. (2001): Wind Erosion Estimates with
RWEQ and WEQ. In.: Sustaining the Global Farm (Selected papers from the 10th International Soil Conservation Organization Meeting) pp.: 760-765.
Agriculture in Hungary in the year 2000. Publication of the Central Statistic Office. Budapest, 2000
Skidmore, E. L. – Woodruff, N. P. (1968): Wind erosion forces in the United States and their use in predicting
soil loss. Agriculture Handbook No. 346. U.S. Department of Agriculture, Washington, D. C.
Skidmore, E.L. (1986): Wind-erosion climatic erosivity. In: Climatic Change 9. pp. 195-208.
Szabó Gergely (2001): Land-use analysis comparing GIS and field data. Scientific publications of the Hungarian
Geographical Conference 2001. CD kiadvány
Williams, J.R. – Jones, C.A. – Dyke, P.T.l. (1984): A modeling approach to determining the relationship between
erosion and soil productivity. Transactions, American Society os Agricultural Engineers. 27. pp. 129-144.
Woodruff, N. P. – Siddoway, F. H. (1965): A wind erosion equation. Soil Sci. Soc. Am. Proc. 29(5) pp. 602-608.
1992
2001
1. figure
Changes in the size of arable land in the Hajdúhát