Aerodynamic Parameters of Windbreak Based on its Optical Porosity

The paper summarizes the results of wind velocity measurement in differentdistances from windbreak in combination with optical porosity (OP) determination indifferent periods. Experimental data were obtained by direct measurement of wind speedand OP analyses of images of four windbreaks in Southern Moravia (part of Czech Republic).Wind speed at 2 m above the soil surface in a defined distance from windbreakhad been measured since 2006. Thirty images of different phenological stages of selectedwindbreaks from 2006 to 2010 were analyzed. Windward or leeward images were convertedinto black and white spectrum. All available measurements were used for a mapcreation. The highest values of OP (up to 50%) are achieved in non-vegetation period.Due to the high OP variability in height of terminal tree branches “OP reduced” wasalso assessed (just squares to 2/3 height of windbreaks were evaluated). The wind speedreduction on the leeward side relatively strongly correlated with OP value. The highestcorrelation was found out when the wind speed measurement at 50 m on leeward side wasused. The dependence decreases with increasing distance. Full foliage in summer (10%OP) reduces a wind speed about 60% at 50 m and about 30% at 150 m on the leeward.These values for non-foliaged windbreaks decrease to 80 and 90%. Maximum distanceof windbreak effect on wind speed reduction was found out by the extrapolation of thecurves constructed using the regression equation of wind speed reduction in dependenceon OP and different distances from the windbreak. Regardless of OP value, the reductioneffect disappears at a distance of 250 m. The quietest zone of evaluated windbreak withan average height of windbreaks 15–18 m was detected in the area about four times theheight (4H). The effect of windbreaks decreases with increasing porosity.