THE CHARACTER OF WINDBREAKS AND THEIR INFLUENCE ON MITIGATION OF SOIL EROSION

Windbreaks create efficient soil protection against wind erosion particularly at the time when soil cover is not protected by vegetation cover of cultivated plant. The objective of this research was to find correlation between qualitative parameters of windbreaks and their efficiency in terms of wind speed reduction. The wind speed measurement was carried out by 4 stations along windbreak. The station contains 2 anemometers at heights 0.5 and 1 m above the surface. The character of windbreak was described by photogrammetry method as the value of optical porosity from the photo documentation of the windbreak at the time of ambulatory measurement. Important dependency between the value of optical porosity and efficiency of windbreak emerged from the results. An important protective effect of windbreak on soil was proven on the leeward side of the windbreak in the belt corresponding with approximately six times the height of the windbreaks.


INTRODUCTION
Wind erosion is a dynamic process, where soil particles are detached and relocated by erosive forces of wind.Wind erosion starts at the time, when wind forces exceed the threshold value of soil resistance to erosion (Pasák et al. 1984).The speed and extent of this type of erosion is influenced by geologic, climatic and anthropogenic factors (Pelt et al. 2010).This process is the result of the whole complex of interactions of wind speed, rainfall, surface roughness, soil texture, soil aggregation, soil moisture, agricultural activities, vegetation cover and the size of the estate (Janeček et al. 2012).
One of the ways how to permanently prevent soil loss removal is to reduce wind speed and the intensity of wind erosion by windbreaks (Janeček 2012).In dry areas, suitably distributed windbreaks at 5% of the area can reduce wind speed by 30-50% and soil losses even by 80% (Bird et al. 1992).However, an optimal distribution and com-position of the windbreak is very complicated process and has not been so far quite clearly described (Středa et al. 2008).
Windbreak determines any woody vegetation of linear character, which protect soil against erosion and does not have impact only on erosion processes, but also on micro climate of the close surroundings -temperature, soil moisture, evapotranspiration, soil temperature etc. (Litschmann, Rožnovský 2005).The ability of windbreak to fulfill its function in the landscape is given by its external and internal structure.The external structure consists of width, height, shape and orientation.The internal structure consists of the amount and arrangement of branches, leaves and trees or shrubs trunks (Brandle et al. 2004).
Permeability of windbreaks is generally defined according to porosity.Windbreaks are classified into: wind porous (porosity ca.60%), wind medium porous and non-porous (porosity ca.20%) (Abel et al. 1997, Janeček et al. 2005).The structure of the windbreak is affected by the number of tree rows, distance between each woody plants, foliage density and structure of branching, which is established by used woody plants creating the windbreak (Kuhns 1998).The parameters of the height and the porosity of windbreak can be used for definition of wind-break´s structure.Porosity of windbreaks is usually distinguished as real (aerodynamic) and optical.Aerodynamic porosity is defined as a ratio between an average wind speed measured at the windward side of the windbreak and an average speed at an open space (Litschmann, Rožnovský 2005, Guan et al. 2003).The optical porosity is considered to be a ratio of the background which is visible from the vertical direction to the windbreak (Burke 1998).As the determination of aerodynamic porosity is very difficult, the parameter of optical porosity is mostly used (Vigiak et al. 2003).For the evaluation windbreak´s efficiency, the optical porosity determined on the basis of photographs is more often used (Kenney 1987, Litschmann et al. 2007).
According to Heisler and DeWalle (1988) windbreaks of low and medium porosity have significantly higher efficiency in comparison with windbreaks of higher porosity.Windbreaks of low porosity have more frequent occurrence of turbu-lent flow with higher wind speed at leeward side than windbreaks with medium porosity (Cornelis et al. 2000).Brandle and Hintz (1987) state if windbreak reduces the wind speed to one half, drag force of the wind is one eighth of the original value.Cornelis and Gabriels (2005) determined the optimal value of optical porosity in the range of 20-35%.
The effect of windbreaks on wind speed reduction is stated in the range of 20 to 35times the height of the windbreaks at the leeward side (Heisler et DeWalle 1988, Abel et al. 1997, Vézina 2001, Vigiak et al. 2003, Brandle et al. 2004, Janeček et al. 2012).The authors relate the reduction of windbreak´s efficiency to the value of optical porosity.However, Středa (2008) did not confirm the dependency of optical porosity and windbreak´s height.
Terrain measurement of the windbreak´s efficiency was carried out by stationary anemometric stations.Currently the optical porosity was evaluated from the photographs taken during the measurement.
The ambulatory measurement of the efficiency of windbreak was taken during favourable meteorological conditions, i.e. at the wind speed higher than 4m.s-1 and at the vertical direction of the wind to the windbreak.The wind speed was measured by anemometers Vantage Pro 2 by Davis.The extent of measurement is stated by producer from 0.5 to 89m.s-1 with the accuracy of ± 1 m.s-1 or ± 5%.Anemometers were interconnected with the device WIND DATALOGER by AP-EL Applied electronics, which serve for data collection and communication with the computer.The anemometers were attached to steel rod at the heights of 0.5m and 1m above soil surface, alternatively above plant cover.At the windward side one measuring station was placed in the distance of the 3H (H=height of the windbreak) and at the leeward side 3 measuring stations were placed in the distances of 3H, 6H and 9H.
The optical porosity was determined on the basis of photographs acquired by digital camera NikonD5100.The 30 meters long representative section was depicted in the windbreak by stakes.The photos of the marked out part were taken during measurement both at the windward and leeward side of the windbreak, always in the vertical axis to the windbreak.The photos were taken from the tripod at the height of 1.6m.For the evaluation of the optical porosity these programs were used: GIMP (version 2.8.2),ArcGIS for Desktop (ArcMap 10.2) and Table Processor Excel 2013.Firstly, the photos were processed in the graphical software GIMP.According to the accessible tools a graphical modification of the photography (highlighting of the vegetation cover from the background) in order to create binary picture was carried out.That conversion was very important for the determination of the cover and the background of the windbreak (black Grid=cover, white Grid=background).Those modified photographs were analyzed in the ArcGIS.For the analysis of binary picture a square grid with 6 -7 rows and 12 columns was used.For the lower row of the windbreaks, the dimension of one square of the grid was 2.5 x 2.5m (Figure 1).For the analysis of the upper row of the windbreaks, more detailed grid was used; i.e. each square 2.5 x 2.5m was further divided into 16 smaller squares.More detailed method for the upper row was used to increase accuracy of total optical porosity determination, where the height of the windbreak in each columns of evaluation was taken into account.The squares in the highest row with the optical porosity of 100% were not included into determination of the total optical porosity and did not affect the value of the total optical porosity of the windbreak.
Statistical evaluation of the relationship between wind speed and optical porosity was realized in the MS Excel program.

RESULTS AND DISCUSSION
Measurements were performed at the windbreak in Dobrovíz (4 times) and in Středokluky (3 times) at different phenological phases of woody plants with corresponding value of the optical porosity (OP in %).
Table 1 shows direct proportion between the optical porosity and wind speed reduction at the leeward side mainly at the sites 3H and 6H; for low values of porosity also at the site 9H.Correlation coefficient is ranging between 0.849 and 0.940 (Tab 2), which corresponds with findings of Heisler and DeWalle (1988).
The highest efficiency of the windbreak was expressed at the lowest values of optical porosity ranging between 12-23% (Fig 2).For the values of optical porosity 37 and 41%, lower wind speed reduction is apparent at the leeward side.The measurements showed that the lowest wind speed reduction was for the optical porosity of 53%.At this porosity value, the wind speed was higher than at the windward side at the site L9H.The correspondence of measurement is evident for the optical porosity 17 and 18%, as both curves of wind speed have practically similar progress.
The highest wind speed reduction was measured for the values of optical porosity between 12 and 23%.On the contrary, the lowest wind speed reduction was again for the value of optical porosity 53%.Measurements at the height 1m above surface did not prove similar trend for the values of optical porosity 17 and 18%.The measurement of wind speed at the height 0.5 and 1m above surface or vegetation cover did not show any statistically important relationship.Wind speed measurement at different heights should have shown the lowest wind speed at the ground and with the rising height the speed should have been increasing similarly like the progress of par-abolic function.However, the established correlation coefficient 0.036 for the anemometers at the evaluated heights did not prove this dependency.
No evident wind speed reduction was found for the value of OP 53% in the distance 9H, contrarily wind speed was higher there than at the windward side.Středa et al. (2008) came to the same conclusion.Presented results show that for the values of optical porosity 17, 18, and 23% the wind speed reduction is ranging between 40 to 60%.
Our terrain measurements showed that the protective impact of windbreak is not expressed at the value of OP 53% in the distance 9H which is in contrast to the findings of Vigiak et al. (2003) and Brandle et al. (2004).Cornelis and Gabriels (2005) state, that the value of optical porosity is considered to be between 20-35% for the maximal wind speed reduction.Based on our carried out measurements we can state that the values lower than 20% (18, 17 and 12%) are possible to be considered as optimal values of OP.

CONCLUSION
The efficiency of windbreaks was evaluated by the anemometric stations and the measured values were related to the optical porosity gained from the photographs taken during the measurement.The measurement was done for 2 windbreaks.Determined values of the optical porosity were ranging between 12 -53%.
The measurement proved direct correlation for wind speed reduction at the leeward side and the optical porosity value.The correlation coefficients were ranging between 0.849 and 0.940.The highest wind speed reduction was found for the values of optical porosity up to 30%.For the distances 3H and 6H (multiples of the windbreak´s height) at the leeward side the highest efficiency of windbreak was found.It the distance 9H behind the windbreak the wind speed reduction was not found for the highest value of the optical porosity (53%).
Conclusive correlation between the values of wind speed at the height of 0.5 and 1m was not found.

SUMMARy
The ambulatory measurements of wind speed around the windbreak at windward and leeward side were carried out in different distances from the windbreak at the multiples of its height.At the time of the measurement the photographs from the representative part of the windbreak were taken, from which the optical porosity (OP) was determined.The measurement was carried out at different phenological phases, and thus also at different OP values ranging between 12 to 53%.We found direct correlation between the OP and the efficiency of windbreaks with correlation coefficient ca.0.9.The highest wind speed reduction was recorded in the distances up to 6H at the leeward side.For the high value of OP (53%) no wind speed reduction was found in the distance 9H at the leeward side.

Figure 1 .
Figure 1.Modification of the photography and evaluation of the optical porosity

Table 1 .
Average wind speeds and reduction of the wind speed behind the windbreak Note: NA -Data not available (complete data are not available due to failure at the sensor), SD -Standard deviation.

Table 1 .
Correlation between the optical porosity and reduced wind speed