50 m 射电望远镜的时间标准问题的讨论

定量分析了原子钟的时间频率特性产生的守时时差,估算了原子时频标准的守时时差对有关物理量测量所产生的影响．给出了现代实用型原子钟的性能指标比较表,和用现代无线电手段传递、比对时频标准信号达到的指标．论述了高精度时间频率标准在大地、深空间探测、VLBI及毫秒脉冲星计时应用测量中的重要地位、作用．阐述了50 m射电望远镜的科学目标、“嫦娥”1号探月卫星任务对时间标准提出的高精度要求和选用原则．为了实现其科学目标和任务,必须建设与其研究目标相适宜的、标准尽可能高的原子时频标准,才能获得高质量的数据信息和高效能的研究成果．并对建设怎样的时间标准等问题进行讨论和提出具体建议．     

Assessing crop yield benefits from in situ rainwater harvesting through contour ridges in semi-arid Zimbabwe

Rainwater harvesting through modified contour ridges known as dead level contours has been practiced in Zimbabwe in the last two decades. Studies have shown marginal soil moisture retention benefits for using this technique while results on crop yield benefits are lacking. This paper presents results from a field study for assessing the impact of dead level contours on soil moisture and crop yield carried out from 2009 to 2011 within the Limpopo River Basin. The experiments were carried out on two study sites; one containing silt loam soil and another containing sandy soil. Three treatments constituting dead level contoured plots, non-contoured plots and plots with the traditional graded contours were used on each site. All the three treatments were planted with a maize crop and managed using conventional farming methods. Planting, weeding and fertiliser application in the three treatments were done at the same time. Crop monitoring was carried out on sub plots measuring 4 m by 4 m established in every treatment. The development of the crop was monitored until harvesting time with data on plant height, leaf moisture and crop yield being collected. An analysis of the data shows that in the site with silt loam soil more soil moisture accumulated after heavy rainfall in dead level contour plots compared to the control (no contours) and graded contour plots (P < 0.05). However the maize crop experienced an insignificantly (P > 0.05) higher yield in the dead level contoured treatment compared to the non-contoured treatment while a significantly (P < 0.05) higher yield was obtained in the dead level contoured treatment when compared with a graded contoured treatment. Different results were obtained from the site with sandy soil where there was no significant difference in soil moisture after a high rainfall event of 60 mm/day between dead level contour plots compared to the control and graded contour plots. The yield from the dead level contoured treatment and that from the graded contoured treatment were comparable and both not significantly (P > 0.05) higher than that from the non-contoured treatment. This suggests that adopting dead level contours as an in situ rainwater harvesting technique results in crop yield benefits in fields with soil type conditions that enable runoff generation but is not likely to have benefit in soils with low runoff generation.     

一个新的正规定则

证明了单位圆内亚纯函数族F,若不正规,则存在一个函数列在开平面上的任意紧子集上一致收敛于一个非常数的亚纯函数,并应用它得到一个新的正规定则:设F为区域D内的一亚纯函数族,若对每一f∈F,fnf′≠1(n≥2),则F在区域D内正规.     

地下水埋深对作物产量与水分利用效率的影响及作物系数变化

采用排水式蒸渗仪试验,研究不同地下水埋深对冬小麦和春玉米产量和水分利用效率的影响及其作物系数的变化,表明冬小麦和春玉米生育期的地下水位分别控制在1.5 m和1.0 m以下时产量和水分利用效率最为适宜,作物系数随地下水埋深的变化而变化,幅度可达到0.5～0.6,作物系数最大值处于1.2 m埋深处。研究结果可为地下水埋深较...     

热力学方法在土体动力Ramberg—Osgood模型中的应用研究

从Ramberg—Osgood模型的骨架曲线和滞回曲线出发,假定塑性中心的移动为直线,构造土体动力耗散函数,从热力学基本定律出发研究了土体动力耗散特性及动力变形机理．提出了反映筑坝堆石料动力特性的第1阌值应变和第2阈值应变,两个阈值应变主要受最大动剪切模量系数、指数及无黏性土的内摩擦角的影响．第1和第2阈值应变间屈服面...     

陇东黄土区地质灾害气象预警方法与应用——以泾川县为例

在地质灾害详细调查的基础上,基于GIS平台以地质灾害点密度为依据生成初步地质灾害易发性区划图.从诱发地质灾害的内部因素出发将地层岩性、地形坡度、地貌类型、植被覆盖率、多年降雨量分级后,叠加到初步易发性区划图中,生成了地质灾害气象预警区划基图.以降雨作为诱发地质灾害的主导外在因素,重新对泾川县进行了地质灾害气象预警区划,...     

Net economic effects of achieving maximum economic yield in fisheries

Improving the economic performance of fisheries is becoming increasingly important in fisheries management, and in some cases, maximum economic yield (MEY) is set as a key management target. However, recent critics of MEY as a management target have argued that a decline in the level of fishing activity necessary to achieve the target will result in a subsequent loss of economic activity elsewhere in the economy and, potentially, a net loss to society. In this paper, an input–output framework is used to estimate the net economic effects of achieving MEY in several Australian fisheries for which there is information on their short- and long-term performances when moving towards MEY. While overall losses were found in the short term, achieving MEY was found to result in a net economic benefit to society in the longer term. Local coastal communities in particular were found to benefit, although some losses were incurred elsewhere in the economy.     

Cyclic Shearing Deformation Behavior of Saturated Clays

The apparatus for static and dynamic universal triaxial and torsional shear soil testing is employed to perform stress-controlled cyclic single-direction torsional shear tests and two-direction coupled shear tests under unconsolidated-undrained conditions. Through a series of tests on saturated clay, the effects of initial shear stress and stress reversal on the clay’s strain-stress behavior are examined, and the behavior of pore water pressure is studied. The experimental results indicate that the patterns of stress-strain relations are distinctly influenced by the initial shear stress in the cyclic single-direction shear tests. When the initial shear stress is large and no stress reversal occurs, the predominant deformation behavior is characterized by an accumulative effect. When the initial shear stress is zero and symmetrical cyclic stress occurs, the predominant deformation behavior is characterized by a cyclic effect. The pore water pressure fluctuates around the confining pressure with the increase of cycle number. It seems that the fluctuating amplitude increases with the increase of the cyclic stress. But a buildup of pore water pressure does not occur. The de- formations of clay samples under the complex initial and the cyclic coupled stress conditions include the normal deviatoric deforma- tion and horizontal shear deformation, the average deformation and cyclic deformation. A general strain failure criterion taking into account these deformations is recommended and is proved more stable and suitable compared to the strain failure criteria currently used.     

灰色灾变模型在蒋家沟泥石流年输沙量短期预测中的应用

根据蒋家沟泥石流年输沙量最近8年的实测资料,运用灰色系统理论的灾变预测方法建立了泥石流年输沙量的灾变预测模型,对未来可能出现的年输沙量超过200万立方米的灾变年份进行了预测,并引入等维新息模型,使短期灾变预测得以连续进行。     

THE GROUNDWATER RESOURCES AND ITSSUSTAINABLE DEVELOPMENTIN THE SOUTH EDGE OF TARIM BASIN

1 INTRODUCTIONSustainability as present understand comes fromword protection strategy commission, which firstly putforward some wide excepted environmental sustainability principles and three important identifications forlife support system: soil, air and water. Then, in thereport of Our Common Future (WCED, 1987), theconcept of sustainable development is put to an outstanding level. Sustainable groundwater resources development is then a great issue in the recent years andthe future (…