北方水稻低温冷害指标持续适用性检验与比较

农业气象指标的可持续适用性检验与比较,对提升农业气象业务能力意义重大。应用北方地区代表县(市)1960—2010年逐日气温资料和历年水稻产量资料,计算冷害温度指标界定水稻冷害发生的正确率,对中国北方水稻延迟型冷害和障碍型冷害常用的指标进行应用效果检验,并比较适用性差异。结果表明,界定水稻延迟型冷害的5—9月气温指标稳定适用,正确率达到78%左右,可持续性好;积温差额指标也比较稳定,可以继续应用;但是积温距平指标判定冷害的正确率较低,多数地方不宜继续使用。界定水稻障碍型冷害的日平均气温、日最低气温及其持续时间指标比较稳定可靠,多数县(市)可以继续应用,但少数县市应暂停使用;日最低气温指标界定冷害的正确率略高于日平均气温指标。多年来,这些指标尽管要素和原理不变,但在适用性及可持续性方面存在一定的年代和区域差异,应选择使用并及时修订过时的指标,以适应气候变化和水稻栽培。     

A comparison of stream bank erosion processes on forested and moorland streams in the Balquhidder Catchments,central Scotland

Stream bank erosion rates measured over a two-year period on a moorland and a forested stream in the Institute of Hydrology's Balquhidder Paired Catchments in central Scotland were compared. Bank erosion rates are generally higher on the mainstream of the moorland catchment and highest in wintger on both streams. Bank erosion is correlated with the incidence of frost: minimum temperatures measured on stream banks of the forested stream were an average of 3·7°C higher than on stream banks both outside the forest and on the moorland stream. This makes the incidence of frost on forested stream banks half as frequent. Volumes of material eroded from the mainstreams were combined with bulk density measurements and it is estimated that erosion of the mainstream banks is contributing 1·5 and 7·3 per cent of the sediment yield of the forested and moorland catchments, respectively. Analysis of the vertical distribution of erosion on the banks of both streams suggests an undercutting mechanism which is more pronounced in the moorland stream. The influence of trees on bank erosion and possible implications for the management of forest streams are discussed. © 1997 by John Wiley & Sons, Ltd.     

Soil slopes under combined horizontal and vertical seismic accelerations

Conventional methods of designing earth structures are based on pseudo-static stability analysis employing a horizontal seismic coefficient. This paper discusses the stability and permanent displacement of a slope subject to combined horizontal and vertical accelerations. A log-spiral failure mechanism is used. It is shown that seismic force has a significant effect on stability and permanent displacement of slopes. The parametric study reveals that vertical acceleration may play an important role on stability and permanent displacement if the corresponding horizontal acceleration is large. © 1997 John Wiley & Sons Ltd.     

高易溶盐非饱和伊犁原状黄土的临界状态特性

为探究伊犁地区高易溶盐含量黄土的临界状态特性,针对不同易溶盐含量的非饱和伊犁原状黄土,开展了控制吸力和净围压的三轴固结剪切试验。研究结果表明：在常吸力固结剪切条件下,非饱和伊犁原状黄土的强度随含盐量的增加存在峰值;含盐量一定,不同吸力非饱和土的q-p或e-p临界状态线均为平行直线,q- 临界状态线可归一为饱和土临界状态线表示的一条直线,其斜率随含盐量的增加呈幂函数降低;非饱和土e-p临界状态线的斜率 大于饱和土的斜率 ,两者均随含盐量的增加先减后增,相同有效净平均应力下非饱和土和饱和土的临界状态孔隙比之比 与气体饱和度1- 可以归一化;q-p平面上初始屈服曲线和后继屈服曲线均可近似假定为对称于 线的椭圆,吸力和应力对屈服曲线的硬化作用是等向的。     

Flash-flood and bedload dynamics of desert gravel-bed streams

Comparatively little is known about the hydrology of desert flash-floods despite the extent of the world's drylands. There is even less known about their sedimentary behaviour and particularly about the movement of coarse material as bedload. The results of an intense field monitoring programme carried out on an ephemeral gravel-bed stream in the northern Negev Desert are presented. In this semi-arid setting, flow duration analysis indicates that the channel is hydrologically active for 2% of the time, or about seven days per year, and that overbank flow can be expected for only 0·03% of the time—about three hours per year. Multipeaked flood hydrographs are the norm, reflecting many factors including the arrival of separate slugs of discharge from contributing subcatchments. The passage of the initial flood bore is surprisingly slow, but the rising limb of the flood hydrograph is rapid with a median time of rise of 10 minutes, in keeping with expected flash-flood behaviour. Bedload flux is high, averaging 2·67 kg s⁻¹ m⁻¹ during the period that the channel carries flow. This gives very high bedload sediment yield despite the infrequent and short duration of flood flows and matches the high yield of suspended sediment. The relationship between bedload flux and boundary shear stress is simple, in contrast with perennial gravel-bed streams, and the exponent of the log–log relationship is 1·52. Of great value is that the behaviour of the Nahal Eshtemoa corroborates a pattern established by the authors previously in a smaller tributary stream. © 1998 John Wiley & Sons, Ltd.     

Stress–strain model for clays with anisotropic void ratio distribution

The porosity of soils is considered to be a directional measure and its distribution is characterized by a functional form. This form has been used to extend the critical state soil mechanics framework to include the effects of structure in soils. A new internal plastic energy dissipation formulation has been proposed to account for fabric arrangement. New expressions for the yield locus, and the plastic stress–strain response of structural soils have been derived. The applicability of the concepts to model the plastic stress–strain behaviour of a number of soils is illustrated. The advantage of the new model is very well identified in modelling the stress–strain behaviour of K₀ consolidated and natural clays. © 1998 John Wiley & Sons, Ltd.     

水稻对砷吸收的机理及控制砷吸收的农艺途径研究进展

全世界约一半人口以大米为主食,亚洲人口主食对水稻的依赖程度甚至超过90%。当前全球各地均存在不同程度的砷(As)污染,水稻容易在籽粒中积累砷,从而使砷通过食物链进入人体,威胁人体健康。水稻中砷含量水平为几个到几百个ng/g不等,砷从土壤进入水稻的过程涉及复杂的物理化学过程和形态转化,最终主要以砷酸、亚砷酸及砷的巯基、甲基配位等形态储存于大米中。田间水管理、施肥以及添加土壤改良剂等方法都可以控制稻田农田生态系统中水稻对砷的吸收,但是每种技术都有其优势和局限性。水稻农田生态系统中砷生物地球化学及水稻对砷的吸收和代谢等诸多因素都影响着水稻及谷粒中砷的浓度。综合考虑农艺活动对土壤中pH、氧化还原条件、有机质结构和共存元素等因素的影响,考虑不同的地域特征和经济因素,是在生产实践中实现控制水稻对砷吸收的关键。综合运用多种农艺方法进行水稻耕作是未来控制水稻吸收砷的重要途径;新型农艺方法在控制水稻吸收砷过程中的应用,气候变化对大米吸收砷的影响,以及非破坏原位与活体分析技术在砷形态分析中的应用,是未来在全球尺度上更科学有效地控制大米中的砷含量、降低人体砷暴露风险的关键,也是未来的重点发展方向和艰巨挑战。     

膜下滴灌条件下不同灌溉量对番茄根系分布和产量的影响

采用分层挖掘法,对膜下滴灌5 426 m³·hm^-2（W1)、4 419 m³·hm^-2（W2）、3 700 m3·hm^-2（W3）3个灌溉量下番茄根系分布特征和产量进行了研究。结果表明：随灌水量的减少,番茄根系表现出向深层发展的趋势,符合“水大根小”的特征,根长、根表面积、根长密度随着土壤深度的增加均呈“单峰型”曲线：即灌水量愈少,根长、根表面积、根长密度的峰值愈位于土壤的深层;番茄根冠比也随着灌水量的减少而呈增大的趋势,W2处理、W3处理分别比W1处理提高了26.53%和70.71%;适度的控制灌溉水量可以促进根系的生长发育,扩大水分吸收空间,有利于番茄增产,4 419 m³·hm^-2灌溉量是膜下滴灌最适宜灌溉水量。     

Topographic differentiation simulation of crop yield and soil and water loss on the Loess Plateau

De-farming slope farmland has been an effective measure in recent years for the improvement of the eco-environment and the mitigation of soil and water loss on the Loess Plateau. This paper, taking the Yangou Basin as a case study and using day-by-day meteorological data of Yan’an station in 2005, simulated and analyzed the quantitative relation between crop yield, soil and water loss and topographic condition with the aid of WIN-YIELD software. Results show that: 1) topographic gradient has important influence on crop yield. The bigger gradient is, the lower the crop yield. Yields of sorghum and corn decrease by 15.44% and 14.32% respectively at 25° in comparison to the case of 0°. In addition, yields of soya, bean and potato decrease slightly by 5.26%, 4.67% and 3.84%, respectively. The influences of topographic height and slope aspect on crop yield are slight. 2) Under the same topographic condition, different crops’ runoff and soil loss show obvious disparity. Topographic gradient has important influence on soil and water loss. In general, the changing trend is that the soil and water loss aggregates with the increase of gradient, and the maximal amount occurs around 20°. The influence of topographic height is slight. Topographic aspect has a certain effect, and the fundamental characteristic is that values are higher at the aspect of south than north. 3) Topographic gradients of 5° and 15° are two important thresholds. The characteristic about soil and water loss with the variation of topographic gradients show that: the slope farmland with gradient less than 5° could remain unchanged, and the slope farmland more than 15° should be de-farmed as early as possible.