民勤绿洲边缘带灌丛沙丘防风作用研究

Wind-protecting Effect of Shrub Dunes in Ecotone of Minqin Oasis

对民勤天然绿洲边缘带的固定、半固定及半流动风速灌丛沙丘实地观测表明,气流通过单个固定灌丛沙丘时,近地面平均风速降低4.4%,在965 m观测样线内降低57%,且风是连续降低的,愈靠近内侧降低得愈多,到绿洲内部农田边缘处时,平均风速仅有对照点的21%。深入了解灌丛沙丘对绿洲的防护机理,对于合理保护和恢复绿洲边缘带灌丛沙丘,避免绿洲遭受风沙侵袭具有重要意义。

The belt of oasis border is usually called as ecotone of desert and oasis, which is mainly composed of shrub dunes. These shrub dunes can be divided into stable dune, semi-stable dune, semi-shift dune and shift dune by coverage of vegetation. A lot of studies have indicated that these shrub dunes are nature roughness unit and they have significant roles on airflow near land surface. They can change airflow structure and interchange of air torrent at boundary layer near land surface. They can also increase resistance of land surface to moving airflow by enhancing aerodynamic roughness length. When airflow blow passed shrub dunes, wind speed will be decreased remarkably because the dunes resist wind. So, shrub dunes in ecotone are very important defending barrier to nature oasis. This research selected a sample line （965m） mainly composed of stable dune, semi-stable dune and semi-shift dune in ecotone of Minqing oasis and one of stable shrub dune as study object. Field observation results indicate that mean wind speed at layers near land surface decrease 4.4% through the sample stable shrub dune and decrease 57% through the sample line. The mean wind velocity is reduced continuously along the sample line. At the edge of cropland inner of oasis, the wind velocity is only 21% referring to reference observation point. But shrub dunes in ecotone of oasis are one kind of the most vulnerable and unstable ecosystem and they are easily vulnerable to become degeneration ecosystem so as to threaten the sustainable development of oasis. The evolutions of shrub dunes are mainly influenced by human activities and climate changes. If they degenerate to be shift dunes, they will change from shelter barrier to be dust bowl. So, attention should be paid not only to the inner but also to the outer of oasis about its protection. Shrub and grass belts should be built in the surroundings while windbreak grid built in the inner and base windbreak built at the edge of oasis. Water is the most important factor to the stableness of the ecotone. It is significant for the sustainable development of oasis to supply sufficient ecological water utilized to realize the stableness of the belts of oasis border, at the same time, all kinds of human destruction activities should be forbidden.