Research On Spatial Patterns Of Shelterbelts In Typical Sand Area In Xinjiang,China

The ecological environment is very fragile in the sandy area of Xinjiang. Water resources shortages, low vegetation coverage, serious sand dunes activation in this region. Many wind-sand disasters in here, such as gale, sandstorm, blowing sand and dust. Wind-sand disasters affected the production and living of the people, economic development and political stability of Xinjiang. Sand hazards also affected the ecological environment of the country. This study selected two typical sandy areas in Xinjiang: 150 regiment in north Xinjiang and Hotan region in south Xinjiang. The collaborative configuration of shelter forest system was studied at regional scale. It has very important theoretical significance and practical significance to the regional wind and sand, wind erosion hazard research and governance.Because of the characteristics of the wind-sand disasters were local movement and long distance diffusion, remote sensing and geographic information system technology, combined with wind tunnel test, FLuent numerical simulation and field observation were used in the study. The climate change, characteristics of sand and dust weather and vegetation spatial and temporal variation characteristics were analyzed in the sandy area of Xinjiang(1980-2014), and the current situation, causes and temporal and spatial variation of wind-sand disasters have been studied at the regional scale. In order to study the spatial distribution of the shelter forest system on the regional scale, the dust source path was analyzed, and the effects of different underlying surface conditions on the aeolian sand movement were also analyzed. To analysis the variation of shelterbelt porosity, dominant factor model and mechanism model of porosity has been built on the scale of forest. The physical mechanisms responsible for wind erosion were used to investigate the optimal porosity of shelterbelts for wind speed reduction. Collaborative allocation optimization model between natural vegetation and shelter forest and optimal allocation model of crops and farmland protection forest were analyzed at the local scale. Windbreak system configuration mode which is based protection forest, natural vegetation and crop is constructed in the sandy area of Xinjiang. The results were as follows:1) The annual average wind speed decreased in north and south Xinjiang during 1960-2014. Seasonal wind speed and annual average wind speed decreased after 1995. The feedback effects of changes in vegetation on mean wind speed appeared to vary in region when different time scales are used to examine them. The most useful time scale was generally found to be 20 ten-days and the optimal time lag was 8 ten-days. Wind and vegetation factors directly affected the occurrence of dust storm. The main reason for the decrease of dust storm was the increase of the vegetation coverage and the decrease of the strong wind on the regional scale in recent years.2) The grain characteristic of falling dust and different height surface and their relationship were analyzed by the field observation data in Mosuowan. Sediment settling velocity gradually decreased from west to east in the study area, especially in April, a clear decreased trend presented at the height of 0.2m, 0.5m and 1.5m. The surface soil tended to be thicker in dune, interdune lowland and farmland. The surface dust was easy to be blown into the air in these areas. So these areas turned into the dust source. The surface soil tended to be refined in the natural vegetation area of the desert and the interior of the shelter forest. The shelter forest system can restrain the sand and dust. The shelter forest system can reduce the wind speed and wind erosion. The shelter forest system can also intercept and degrade the sand and dust. These areas were difficult to surface dust blown into the air.3) Belt scale: The physical mechanisms responsible for wind erosion were used to investigate the optimal porosity of shelterbelts for wind speed reduction and a mathematical model relating porosity of shelterbelts to relative wind speed was constructed. The results show significant correlation among total shelterbelt porosity, crown area, average crown height, and average clear bole height(r2 = 0.968, p = 0.000). A sharp inflection point was found near a porosity of 0.35 based on the emission mechanisms. The protection zone integral was used to measure the effectiveness of the protection in the numerical simulation process. The protection area refered to the integral area of forest wind attenuation curve and the linear u/u0=100%. The results showed that the shelterbelt optimal porosity was 0.35.4) Regional scale: For allocation of shelter forest system in regional scale, dust source and path were analyzed in Hotan region in south Xinjiang and 150 regiment in north Xinjiang. Five types of transmission paths in Hotan are: southwest, west, northwest, north and northeast. The airflow trajectories had large difference between two altitudes, and it had high speed transmission and mainly westerly flows in 1500 m. The northeast route had high dust weather, which occupy 67.6%, 44% and 48% separately. The transmission path near the ground were 6 in March in the 150 regiment, the northern route has the highest proportion(46%). Sand-dust weather in northwest path was highest in the 150 regiment in 2011-2014, airflow and sand-dust weather with the increase of the month, the first increase and then decrease in the west path and the northwest path.5) Local scale: Wind performed a significant regularity when existed obstacles such as shelterbelts and natural vegetations, which was that the synergistic action of shelterbelts and natural vegetations could reduce wind speed through analysis of relative wind speed at two heights(0.5m and 1.5m). Using software programming, we obtained the optimal model of collaborative allocation of shelterbelts and natural vegetations in the smallest relative wind speed and in different relative wind speeds. For example: in the height of 0.5m, relative wind velocity was 0.5, optimal model of collaborative allocation was that the shelterbelts had 30.95m3 canopy volume, 0.47m2 basal area, 3.79m2 row and plant spacing, and the natural vegetations had 1.44 m average height and 0.21 coverage. Jujube forest significantly affected the flow field and near ground wind speed at different wind speed and porosity. Jujube can improve the windbreak effect of 1.28%-29.05%.6) The wind tunnel experiment shows that the difference between small wind speed(8m/s) flow field and large velocity(14m/s) flow field was not obvious. Influence of the forest belt on the wind speed differences under different structures, in the forest after 2H at canopy height, 8 lines of forest and 4 lines of forest than 6 lines of forest appeared more turbulent. While in the sky at 20 cm above the forest, the accelerating turbulent region appeared in 6 lines of forest. Plurality of forest belts with differences in flow field under different configuration of forest obviously in the same wind speed. The core forest of uplift zone of resistance, 4 lines and 6 lines smaller than 8 lines, 10 lines, 12 lines and 14 lines. The change of flow field in different structure under different forest configuration in the near surface. The distance between shelterbelts significantly affected the flow field distribution in the uplift resistance. The wind speed reduction rate model and wind tunnel test results were used to construct the optimization pattern of farmland protection forest in the local scale. When the initial speed was 10 m/s, optimal allocation model of farmland shelterbelt network was that core forest line was 14, field belt line was 6, shelterbelt distance 1 were 20 H and shelterbelt distance 2 were 11 H.