草地退化对土壤理化性质的影响

文章以张家川县不同退化程度草地为研究对象,探讨草地退化对土壤理化性质的影响。结果显示:土壤密度和含水量均随着草地退化程度的增加而下降,有机碳和速效磷则随着草地退化程度增加呈现出先上升后下降的趋势。同时,不同退化程度的草地0~2.5 cm层土壤全氮和全磷含量基本均高于深层,土壤全钾含量在2.5~5 cm层高于其他层。总之,草地退化影响着土壤理化性质。     

配电网自动化应用中的典型误区与对策

论述了现有配电网自动化系统的基本特征、广义特征及发展中遇到的信息系统悖论问题,分析配电网自动化及其信息系统实施的必要条件,针对重庆城乡"二元化"结构提出因地制宜的实现方式和实用化条件,考虑了网络、设备、通道、投资、客户、运维等多方面因素,旨在基于现有理论和经验分析并避免配电网自动化应用中的典型误区。从自动化及相关系统的实施条件、实现方式和实用化等方面进行深入剖析,以新的角度重新认识配电网自动化,对技术、管理和理念等方面问题逐一解析,力图破解配电网自动化领域的信息系统悖论,最终加快实现配电运维水平大幅度提升。     

TDC3000 DCS系统控制方案组态初探

从实用的角度探讨了TDC3000 DCS系统控制方案组态的方法,提出了控制方案组态的基本原则,阐明了控制方案组态中的几条经验性规律.通过几个典型示例,说明了TDC3000 DCS系统控制方案的组态相当灵活,可以用多种方式来实现同一控制方案.     

非典型条件句的虚拟语气分析

虚拟语气是英语的一种重要语法,由if引导的非真实条件从句是典型的虚拟语气结构．但是,虚拟语气涉及的范围很广,笔者将诸多的不含典型条件句虚拟语气归类为以单词、短语、句子及其他方法表现出来的4种形式．掌握非典型条件句的虚拟语气是学好虚拟语气的重要条件,也是学好英语语法的关键之一．     

Preliminary results on nitric oxide emission from a southern African savanna ecosystem

During October to December 1994, we measured diel and seasonal variability in nitric oxide (NO) exchange by a southern African savanna ecosystem (Grasslands Research Station, Marondera, Zimbabwe). A set of automated dynamic chambers was installed in three sub-sites: Miombo forest, natural grassland, and cropped soils (maize, sorghum, groundnut). The latter received a single application of commercialy available in-organic or cowdung fertilizer. The dynamic chamber system is described in detail, including correction of the resulting data for wall losses and gas-phase reactions of the NO-NO₂-O₃ triad. During the dry season, net vertical NO fluxes were close to the detection limit (0.44 ngN m^-2s^-1) regardless of vegetation type. During the transition from dry to wet season strong "pulsing" effects of the NO flux were observed: NO emissions increased by a factor of up to 60 within hours. During the wet season, NO emissions varied between the different sub-sites of the ecosystem; corresponding means are in ranking order of fertilized agricultural plots (27.2 ngN m^-2 s^-1), unfertilized agricultural plots (8.5 ngN m^-2 s^-1 ), grassland (4.4 ngN m^-2 s^-1 ), and Miombo forest (1.5 ngN m^-2 s^-1). Soil moisture was found to be the dominant factor controlling the NO fluxes. Whenever it was not limiting, the diel behaviour of NO emission followed the daily variation of soil temperature measured 1 cm below the surface.     

基于高维Gaussian Copula函数的区域农田灌溉需水分析

以广东省农田灌溉需水为例,构建了分区降水的8维联合分布模型,提出了给定全省降水频率的全省及分区农田灌溉需水分析思路,采用同频、典型年和最大权函数3种方法对区域需水、分区降水频率及需水进行了分析计算和对比。结果显示:Gaussian Copula函数能够很好地模拟广东省8个分区年降水联合分布,单变量最优分布主要为广义极值分布和广义正态分布。3种方法之间的全省农田灌溉需水相差甚小,但是各分区需水差别相对较大。典型年法推求的分区降水频率差异最明显,最大权函数法推求的大部分分区降水频率处于同频位置附近,其中分区Ⅷ的降水频率在特枯和特丰时明显偏离同频位置,而且其置信区间范围明显大于其它分区,表明分区Ⅷ不确定性更大。由于基于高维Copula函数联合分布模拟既考虑了分区降水的独立分布,又考虑了它们之间的相依性,而且能够给出分区需水的置信范围,因此认为最大权函数法推求区域农田灌溉需水更为合理。     

土壤水分对玉米蒸腾特性的影响

针对干旱半干旱地区的农业用水短缺问题,以节水灌溉及水资源的合理利用为目标,利用包裹式茎流计对甘肃省会宁县玉米的蒸腾进行连续观测,并同步监测玉米地不同埋深下的土壤水势、土壤含水率及周围气象因子,研究土壤水分对玉米蒸腾特性的影响。结果显示:(1)在不同天气条件下,玉米蒸腾速率的日变化情况大致相同,均呈"几"字型的变化曲线。(2)不同埋深下土壤水势及含水率的日变化幅度不大,均呈缓慢下降趋势。土壤水势及含水量在晴天的变化情况比阴雨天的对称性更好;7月至8月,气温升高,玉米蒸腾耗水增加,土壤水势及含水率逐渐下降,20 cm深处的土壤水势变化幅度最大;20～30 cm深处的土壤水势与蒸腾速率的相关性稍好于10～20 cm深处的土壤水势与蒸腾速率的关系,这主要跟玉米的根系分布有关;各埋深下土壤水势与玉米蒸腾累积量的相关性都呈极显著正相关;30 cm埋深处土壤水分对蒸腾作用的影响比5 cm深处土壤水分对蒸腾作用的影响大。(3)对比发现蒸腾速率与土壤水势的相关性比与土壤含水率的好。结果表明,土壤水分是影响玉米蒸腾耗水差异的重要原因。     

改良型透水铺装对弱透水土质地区的水质控制试验

为改善透水铺装在弱透水土质地区的应用,通过增加渗透导管的方式改良了常规透水铺装的土基层。通过人工降雨试验对改良型透水铺装及其不同结构层对弱透水土质地区典型雨水径流中COD、TN、TP和Cu~(2+)等污染物的去除效果进行了分析,并与相同条件下的常规透水铺装进行了对比。结果表明,改良型透水铺装对4种污染物的去除效果有明显增强,在60 min降雨历时条件下,降雨重现期为2 a时改良型透水铺装对4种污染物的去除能力分别提升了11.87%、27.61%、9.17%和29.02%,且降雨重现期越短越有利于透水铺装对COD、TN、TP和Cu~(2+)的去除。     

基于MikeFlood的组合情景洪水风险分析

针对模拟河道洪水淹没时,干、支流同频率暴雨组合情景不能合理反映洪水漫堤真实情景的缺陷,考虑到洪水汇流淹没过程中干、支流实际降水情况的差异,构建基于干、支流不同频率组合方案下洪水淹没情景的MikeFlood耦合水动力模型。以典型丘陵地貌地区修水为研究对象,模拟研究区域内干流以及主要支流分别发生20年、50年、100年不同频率暴雨时的24种组合情景,选取其中几种情景与干、支流发生同一种频率暴雨的典型暴雨情景进行对比。结果表明,选取的组合情景与典型情景相比,其淹没水深的分布、大小均有较大的变化,可多样性地反映洪水来临时洪水漫堤的情景,为丘陵地区的洪水风险预报和紧急避洪转移工作提供决策依据和技术支撑。     

A decade of fertilizer research on groundnuts (Arachis hypogaea L) in the savannah zone of Nigeria

Field experiments on fertilizer requirements of groundnuts carried out between 1970 and 1980 were reviewed and discussed. Results of the various studies seem to suggest that existing recommendations which are based on short term annual trials carried under non-intensive or semi-intensive traditional farming systems are becoming inadequate under a system of continuous intensive cultivation. Besides phosphorus and sulphur which have long been known to be deficient in the Nigerian savannah soils, the availability of the other nutrients in these soils can no longer be taken for granted. Potash and micronutrient deficiencies, notably those of B and Mo, hitherto rare, are now showing up with increasing frequency.As a P source, single superphosphate appears to be the most suitable fertilizer for groundnuts in the Nigerian savannah at present partly because it also contains S and partly because of its solubility. Nitrogen fertilizer application tends to depress pod and kernel yield although it enhances haulm production. Groundnut haulm constitutes an important livestock feed in the West African savannah and the role of fertilizer N may have to be viewed in this context. Calcium as a nutrient does not appear to cause any serious fertility problem. Where gypsum application has produced large increases in kernel and haulm yields the dominant effect has been attributed to the sulphate component. Magnesium response has so far not been reported on groundnuts in Nigeria.It is suggested that soil fertility research in the savannah zone of West Africa should move away from short-term annual trials to long-term studies designed to tackle problems that are likely to arise under continuous cultivation in these poorly buffered sandy soils.