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991.
在三维随机海浪数值模拟的基础上,获得了波峰的空间分布。为了验证由不同的波峰定义方法得到的波峰长度和波峰方向角等特征量的准确性,对直接利用波峰边界上距离最远的两点之间距离定义波峰长度和利用椭圆拟合波峰和以椭圆的要素定义波峰长度2种不同方法得到的波峰长度统计结果进行了比较,发现由2种方法定义得到的波峰特征量的值基本一致,尤其是波峰长度个数的统计值极为接近,由2种方法得到的统计值精确度相当。这表明,不同定义方法对边界不规则波峰长度统计值的影响较小。  相似文献   
992.
直立植被粗糙度和阻力分解的风洞实验研究   总被引:31,自引:10,他引:21  
直立植被覆盖的床面上,由于直立植被消耗风动量,裸露床面上的剪切力因之降低,从而抑制风蚀。在风洞中用圆柱形木棒模拟直立植被,研究了地表总阻力系数CD、直立植被阻力系数Cf和粗糙度z0及其影响因素。实验结果表明,直立植被高度和密度对地表总阻力系数CD和粗糙度z0都有影响,但是程度不同。地表总剪切力分解为作用在直立植被上的剪切力和作用在裸露地表上的剪切力,表达式为τ=Fr/S+τs·S'/S。随着植被密度和高度增加,作用在植被上的剪切力增加,而作用在裸露地表上的剪切力减少。当侧影盖度Lc≥0.03时,τs可以忽略不计,用CD=2u*2/ uz2+CfLc计算植被阻力系数Cf。计算出的直立植被阻力系数Cf与1/√Lc之间成线性关系,而与AR无关。这与Marshall在风洞中通过直接测量粗糙元的阻力计算出的粗糙元阻力系数遵循类似的规律。地表总阻力系数CD与粗糙度z0之间较好的相关性说明,只要采用适当的测量方法,用曲线拟合方法获得直立植被覆盖地表总阻力系数CD和粗糙度z0是可靠的。  相似文献   
993.
集约式养殖凡纳滨对虾体长与体重的关系   总被引:9,自引:0,他引:9  
研究了集约化养殖凡纳滨对虾Litopenaeus vannamei体长与体重的关系,通过全过程采样测试并经回归分析得到不同生长阶段的体长、体重关系式。研究发现当体长大于12.5cm时相同体长的雌虾要明显重于雄虾(p〈0.01)。比较地膜养殖池与沙底养殖池对凡纳滨对虾生长的影响,结果表明当养殖天数大于90d时,地膜养殖池对虾的生长明显优于沙底养殖池对虾(p〈0.05)。  相似文献   
994.
提出一种提高短基线相对精度的测定方法。 试验表明,在测角误差影响等同或小于测距误差影响的情况下,该方法能使短基线的相对精度提高4~6倍。  相似文献   
995.
《国际泥沙研究》2020,35(2):134-145
Understanding diffusion and solute transport,is regarded as a main activity in environmental management,sustainable development and sedimentation processes because of their role in diffusion of contaminants.The current paper presents an experimental study which was done in a parabolic channel with permeable and impermeable bed conditions in order to estimate the transverse mixing coefficient(TMC) and the maximum solute mixing length(SML) for different channel bed roughness coefficients and discharges.The cross section of the channel was 0.5 m wide and 0.3 m deep with a parabolic shape(i.e.similar to furrow irrigation).Three levels of bed roughness coefficient of 0.02,0.04,and 0.06,and three discharges of 5,10,and 15 L/s were considered.Bed infiltration channel was applied at a low level and medium level as well as a case without infiltration.In the current experiments,sodium chloride was used as a soluble tracer and was injected into the water at mid depth in the upstream cross section.The tracer concentration profile in the water and velocity profile were measured at eight cross sections in the channel.The results showed that the values of SML ranged between 20 and 46 m in impermeable and permeable beds channel beds respectively.The assessment of usual discharge values,infiltration,and roughness coefficient of a furrow(agricultural field) indicated that the SML of the furrows were less than20 m.In the current research,the average dimensionless mixing coefficient was found to be 0.17 in the nonrectangular channel.Eventually,equations were developed to explain the TMC and SML in a permeable parabolic channel that could be beneficial in prediction of fertilizer transport in furrow irrigatio n.  相似文献   
996.
Based on the analyses on amplitudes of historical variation of temperature and precipitation inthe past 500 years and latest 100 years,according to the regional climate change scenarios forChina estimated by composite GCM,the potential impacts of climate change on cropping systemsin China in future are simulated and assessed using the cropping system model developmentspecially for the Chinese cropping patterns.It is shown that under the projected future climatechange by 2050 the most parts of the present double cropping area would be replaced by thedifferent triple cropping patterns while the current double cropping area would shift towards thecentral part of the present single cropping area.More explicitly,the northern boundary of triplecropping area would shift from its current border at the Changjing River to the Huanghe River,ashift of more than 5 degrees of latitude.And the shift of multiple cropping areas leads to asignificant decrease of single cropping area.Furthermore,considering the changes mentioned above in combination with the likely negativebalance of precipitation and evapotranspiration and,therefore,increase of moisture stress(i.e.less water availability),as well as the possible increase of heat stress disaster and decrease of LGS(length of growing season),the potential implication of climate change for agriculture in China arealso analyzed roughly in this paper.As a result,however,it is still very difficult to reach a specific conclusion that the futureclimate change will he favorable or unfavorable to farm in China because of the complicated Chinesefarming patterns,the complex-various social and economic environment of agriculturaldevelopment and,especially,a great scientific uncertainties in the investigation/prediction ofclimate change.  相似文献   
997.
Simultaneousinversionofvelocitydistribu┐tionandinterfacepositionsSONG-LINLI1)(李松林)NING-YUANWU2)(吴宁远)ZHAN-LONGSONG1)(宋占隆)JIN-...  相似文献   
998.
Recent publications from field and simulation studies indicate that runoff per unit area decreases as the length of the slope being observed is increased. This scale effect has been observed and documented for slopes with a uniform infiltration capacity as well as for slopes along which infiltration capacity is variable. This paper presents the design and testing of a laboratory flume for the study of the processes that lead to this scale effect, particularly for the case of slopes covered with crops. The features of the flume include reliable experimental control of soil water content prior to rainfall, high intensity rainfall without erosion, uniform crop growth along the slope, and the option of varying the infiltration rate independently of rainfall and soil characteristics. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   
999.
Vegetation density on foredunes exerts an important control on aeolian sediment transport and deposition, and therefore on profile development. In a long‐term monitoring field experiment, three plots were planted with regular grids of reed bundles in three different densities: 4, 2 and 1 bundles per m2. This study reports on the differences in profile development under the range of vegetation densities. Topographic profiles were measured between May 1996 and April 1997. Results indicate important differences in profile development for the three reed bundle densities: in the highest density plot a distinct, steep dune developed, while in the lowest density a more gradual and smooth sand ramp was deposited. When the stems had been completely buried, differences in profile evolution vanished. After a second planting of reed stems in January 1997 the process was repeated. In May 1997, all plots had gained a sand volume ranging from 11·5 to 12·3 m3 m?1, indicating that the sediment budget is relatively constant, regardless of the particular profile evolution. The field evidence is compared with simulations of profile development, generated by the foredune development model SAFE. The model successfully reproduces the overall profile development, but in general, the equations used for vegetation–transport interaction overestimate the effect of vegetation. This causes some deviations between field and model results. Several reasons for this are discussed. Based on the experiments reported here, recommendations are given for further research. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   
1000.
研究了我国西部走滑型地震的地表破裂长度L和位移量D与震级M_s和复发时间间隔T之间的定量关系。结果表明:震级M_s和复发间隔T不仅与L值有关,还同时与D值相关;乘积LD值可用来估算震级M_s,比值D/L与复发间隔T密切相关;对于M_s或LD值给定的地震,D/L值大(即L值小或D值大)的地震,有较长的复发间隔;当破裂长度L一定时,有较长复发间隔的地震,M_s或LD值较大;当位移量D相同时,M_s或LD值较大地震的复发间隔较短  相似文献   
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