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71.
Porous fence is a kind of artificial windbreak that has many practical applications. The threshold wind velocities at different distances downwind from porous fences were measured and the corresponding characteristics of particle movement observed to assess their shelter effect. It is found that the fence’s porosity is the key factor that determines the resulting shelter effect. The area near a fence can be typically classified into five regions, each with a different mode of particle movement. Dense fences, and especially solid fences, favor the accumulation of sand upwind of the fences. Fences with porosities of 0.3–0.4 produce the maximum threshold wind velocity; those with porosities of 0.3–0.6 (depending on the fence height) provide the maximum effective shelter distance. It is confirmed that the fence porosities of 0.3–0.4 that have been proposed for practical application in previous research are the most effective for abating wind erosion. 相似文献
72.
本文分析了大气中CO2浓度增加的事实,原因及其发展趋势,结果认为,大气中CO2浓度上升对我国农业的发展产生了一定的不利影响。因此,为控制大气中CO2的上升,必须采取相应的有效对策。 相似文献
73.
我国南方山地和丘陵的荒漠化问题 总被引:1,自引:1,他引:1
沙漠化问题及其带来的巨大危害与后果已经引起人们的关注。然而, 我国南方许多山地与丘陵区正在广泛、迅速发展的山地荒漠化和谷地沙石化过程, 同样不能忽视。它使这些地区的自然环境变劣、生态平衡严重失调, 土地退化和生产能力衰退或枯谒, 最终导致整个自然环境向恶性循环发展的严重恶果。我国南方山地荒漠化和谷地沙石化的土地面积估计约1.0-1.3亿公顷。主要分布在云南、四川、贵州、江西等十多个省(区), 其中云南的小江流域, 四川的金沙江河谷及岷江上游; 广西的南流江一带, 江西的兴国县和鄱阳湖区等尤为严重和典型。我国南方山地荒漠化的主要原因是人类不合理的经济活动所引起, 而不利的气候和特殊的地质、地貌条件促使其不断加剧和扩展之势。 相似文献
74.
本文用1951—1984年的资料对500百帕、65°N以北的高度场作自然正交展开,用模糊聚类对典型场归类。指出,极涡月平均场有三种形态,1月份平均场形态较特殊;大尺度距平场也有三种形态,4月份距平场形态较特殊。此外,研究了典型场的时间权重系数与厄尔尼诺、南方涛动的关系。指出,厄尔尼诺现象发生的当年4月、10月极地大尺度距平场有明显的异常特征。这种极地环流异常已明显影响到我国4月份、10月份的气温场。因而,可能是厄尔尼诺年我国气温4月偏低、10月偏高的一个重要原因。 相似文献
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78.
GPS observations of the ionospheric F2-layer behavior during the 20th November 2003 geomagnetic storm over South Korea 总被引:3,自引:1,他引:3
The ionospheric F2-layer peak density (NmF2) and its height (hmF2) are of great influence on the shape of the ionospheric
electron density profile Ne (h) and may be indicative of other physical processes within the ionosphere, especially those
due to geomagnetic storms. Such parameters are often estimated using models such as the semiempirical international reference
ionosphere (IRI) models or are measured using moderately priced to expensive instrumentation, such as ionosondes or incoherent
scatter radars. Global positioning system (GPS) observations have become a powerful tool for mapping high-resolution ionospheric
structures, which can be used to study the ionospheric response to geomagnetic storms. In this paper, we describe how 3-D
ionospheric electron density profiles were produced from data of the dense permanent Korean GPS network using the tomography
reconstruction technique. These profiles are verified by independent ionosonde data. The responses of GPS-derived parameters
at the ionospheric F2-layer to the 20th November 2003 geomagnetic storm over South Korea are investigated. A fairly large
increase in the electron density at the F2-layer peak (the NmF2) (positive storm) has been observed during this storm, which
is accompanied by a significant uplift in the height of the F2 layer peak (the hmF2). This is confirmed by independent ionosonde
observations. We suggest that the F2-layer peak height uplift and NmF2 increase are mainly associated with a strong eastward
electric field, and are not associated with the increase of the O/N2 ratio obtained from the GUVI instruments aboard the TIMED satellite. It is also inferred that the increase in NmF2 is not
caused by the changes in neutral composition, but is related to other nonchemical effects, such as dynamical changes of vertical
ion motions induced by winds and E × B drifts, tides and waves in the mesosphere/lower thermosphere region, which can be dynamically
coupled upward to generate ionospheric perturbations and oscillations. 相似文献
79.
Extreme weather events have played an important role in driving the ecosystem dynamics in high altitude areas,but the underlying mechanism remains unclear.To understand if and how the soil processes of an ecosystem react to extreme drought,we manipulated a once-in-a-century meteorological extreme drought in an alpine meadow on the Tibetan Plateau,which is also known as the "forerunner of global weather changes".The extremity was determined by statistical extreme weather events with respect to a historical reference period from April to September during 1962-2004,where the local historical precipitation data was calculated and intensified to 100-year recurrent drought event with GumbelⅠdistribution.The indicators we measured included soil microbial biomass C/N/P and soil enzymatic activities of phosphatase(AP) disbounding organic phosphate,cellobiohydrolase(CBH),βglucocidase(BG),N-releasing enzyme N-acetylglucosaminidase(NAG) as well as soil respirations,during and after the treatments.It was found that the manipulated event induced a rapid shift in microbial biomass and activities,indicating a lower resistance of the underground process.However,the microbial and biochemical parameters saw rapid recovery after the event,which meant the soil processes enjoyed high resilience.The high responsiveness and lag-time effects of the soil indicators rendered new horizons for us to evaluate the interaction between the extremes and the ecosystem stability.Our study indicated that the once-in-a-century extreme drought induced very short term response in the soil biotic process,and the soil processes worked to buffer against such events under the observation period. 相似文献
80.
甲醛对Nessler法和次氯酸酚盐法测定养殖水体总氨氮的影响 总被引:1,自引:0,他引:1
水体中含一定浓度的甲醛 ,用 Nessler法测得的总氨氮 (TAN)浓度远远高于实际浓度 ,差异百分数最高达 4 5 0 .0 6 % ;用次氯酸酚盐法测得的总氨氮 (TAN)浓度则明显低于实际浓度 ,差异百分数最高可达 84 .77%。甲醛浓度范围为 10~ 4 0 mg/ L、TAN浓度为 1~ 10 mg/ L,通过双因素有重复观察值的交叉实验 ,得到如下矫正式 :(1)用 Nessler法测 TAN时 ,Y=4 .0 5 2 9- 0 .0 72 4 x1+ 0 .2 0 6 4x2 ,标准误差为 1.3381;(2 )用次氯酸酚盐法测 TAN时 ,Y=0 .1196 + 0 .0 345 x1+ 1.0 0 2 2 x2 ,标准误差为 0 .4 6 2 7。 Y为 TAN实际浓度 (mg/ L) ,x1为甲醛浓度 (mg/ L) ,x2 为测得的 TAN浓度(mg/ L )。比较含氨氮水体与不含氨氮水体中的甲醛浓度 ,方差分析结果表明 ,没有显著差异。 相似文献