Meteorological Alleviation by a Windbreak Net at Reclaimed Land of Isahaya Bay

Abstract

The reclaimed land in Isahaya Bay is approximately 816 ha in area, including the central area (706 ha) and the Oe area (110 ha). The reclamation will be complete in 2007. It will be necessary to perform continued meteorological research on the lands, which are intended for agricultural usage after the completion of reclamation. These lands are not protected against the wind, and strong winds do frequently blow across this wide and horizontal area. In order to use this area as arable land, it will be necessary to introduce many windbreaks for the protection of these open fields. A windbreak net of 2.4 m height with an open space of 0.4 m between the bottom of the net and the ground was constructed on the open field and its length was 100 m. The windbreak net was made of blue polyethylene Russell net with 6 mm mesh. The wind speed was measured from a distance of -20H windward to 25H leeward and at heights of 6, 3, 2, 1 and 0.5 m, where H represents the height of the net on the ground, and the numerals the multiple distances from the net, and negative signs denote the windward side and positive signs the leeward. Wind speed, air temperature, relative humidity, and surface soil and leaf temperatures were measured from Nov. 2003 to Feb. 2004. Variations of horizontal, vertical and isopleths of the wind speed under conditions of both weak and strong winds were obtained. Wind speed decreased about 16% in the immediate leeward side, as compared with the standard wind speed at -20H at each height due to the low net density of 15%; however, the effective decrease was continued over a long distance. Air, surface leaf and surface soil temperatures with right-angle wind increased significantly in the leeward side under conditions of weak wind, and this increase continued for more than 25H. The temperatures increased and relative humidity decreased during the daytime for 20H in the leeward side. Temperature increase is useful for growth and development of crops, vegetables and other vegetation. However, the harmful decrease of temperatures by surface inversion was measured from the night to early morning under conditions of a fine or clear day with weak wind. As the net density was low and not sufficiently effective, it will be necessary to introduce considerably higher windbreak net and to reduce the open space under the net. Moreover it will be necessary to obtain accurate data using different windbreak nets in the open field.