降雨对河岸生态系统杨树树干液流及其环境控制的影响

降雨能够改变土壤水分状况进而促进林木蒸腾,然而场降雨量及其持续时间对林木树干液流及其环境控制机制的影响尚不明确。为此,在华北半干旱半湿润区的北京市顺义区共青林场,选取位于河岸生态系统不受土壤水分胁迫的欧美杨(Populus×euramericana)人工林为研究对象,在2019年和2021年生长季,使用TDP热扩散法测量树干液流,同步监测气象及土壤含水量等环境因子。根据对该区长期(2016—2017年、2019年和2021年)降雨数据统计分析结果,将2次降雨脉冲间隔超22.5 h的事件划分2场独立的降雨事件。按照降雨事件雨量及历时,将其中位数±1.5倍标准误的事件定义为常见事件,而将累积概率大于90%的事件定义为极端事件。结果表明:(1)太阳辐射是唯一显著控制该杨树人工林生长季树干液流的环境因子(偏相关系数r_p=0.539),饱和水汽压差、风速和土壤含水量均与树干液流不相关(p>0.533),降雨事件发生前后这一环境控制特征没有发生变化;(2)雨后树干液流随着场降雨量的增加而降低(R²=0.78,p=0.004),但与降雨事件历时无显著相关关系;(3)树干液流在常见降雨事件和极端事件后,在半小时尺度上随时间变化无显著差异(p≥0.264),但4类降雨事件后主导的环境控制因子却不完全相同,太阳辐射和饱和水汽压差总能显著促进半小时尺度的树干液流(r_p≥0.374),而土壤含水量仅在常见和极端的强降雨历时事件后,显著促进雨后半小时尺度液流(r_p≥0.215)。风速显著抑制常见场降雨量事件后半小时尺度的树干液流(r_p=-0.258),却能显著促进常见和极端场降雨历时事件后半小时尺度的树干液流(r_p≥0.183)。研究成果为进一步深入揭示降雨特征对树干液流及其生物物理控制机制的影响,以及改进气候变化下生态水文过程的模拟与评估提供参考。     

Coping with imbalanced data problem in digital mapping of soil classes

An unsolved problem in the digital mapping of categorical soil variables and soil types is the imbalanced number of observations, which leads to reduced accuracy and the loss of the minority class (the class with a significantly lower number of observations compared to other classes) in the final map. So far, synthetic over- and under-sampling techniques have been explored in soil science; however, more efficient approaches that do not have the drawbacks of these techniques and guarantee retention of the minority classes in the produced map are essentially required. Such approaches suggested in the present study for digital mapping of soil classes include machine learning models of ensemble gradient boosting, cost-sensitive learning and one-class classification (OCC) of the minority class combined with multi-class classification. In this regard, extreme gradient boosting (XGB) as an ensemble gradient learner, a cost-sensitive decision tree (CSDT) within the C5.0 algorithm, and a one-class support vector machine combined with multi-class classification (OCCM) were investigated to map eight soil great groups with a naturally imbalanced frequency of observations in northwest Iran. A total of 453 profile data points were used for mapping the soil great groups of the study area. A data split was done manually for each class separately, which resulted in an overall 70% of the data for calibration and 30% for validation. The bootstrapping approach of calibration (with 10 runs) was performed to produce multiple maps for each model. The 10 bootstraps were evaluated against the hold-out validation dataset. The average values of accuracy measures, including Kappa (K), overall accuracy (OA), producer's accuracy (PA) and user's accuracy (UA), were explored. In addition, the results of this study were compared with a previous study in the same area, in which resampling techniques were used to deal with imbalanced data for digital soil class mapping. The findings show that all three suggested methods can deal well with the imbalanced classification problem, with OCCM showing the highest K (= 0.76) and OA (= 82) in the validation stage. Also, this model can guarantee the retention of the minority classes in the final map. Comparing the present approaches with the previous study approach demonstrates that the three newly suggested methods can remarkably increase both overall and individual class accuracy for mapping.     

Dynamic characteristics and static response analysis of interval parameters structure

Determining the interval parameters of structural dynamic characteristics and the extreme value of static response can solve some structural uncertainties. The dynamic characteristics and static response of structures are always closely linked with the parameters. Finding the intrinsic link between them can make the calculation of the dynamic characteristics and the extreme value of interval structure simple and effective. Through function derivation of elastic modulus E and mass density ρ, it can be found that they are related to structural dynamic characteristics and static response. And the dynamic characteristics of trusses are taken as an example to calculate the maximum displacements of loading direction and unloading direction, their parameters are compared and the relation between elastic modulus and the displacement of loading direction are verified.     

海阳市灾害性天气特征及防御对策

应用海阳市近50年的气象观测资料和出现的气象灾害记录资料,分析了影响海阳市的干旱、暴雨、大风、热带气旋(台风)、冰雹、霜冻、寒潮、连阴雨等灾害性天气,指出了灾害性天气的特征,并提出了相应的防御对策。     

宁夏近50年极端强降水事件的时空变化分析

[目的]研究近50年宁夏极端强降水事件的时空变化特征。[方法]根据宁夏20个站点1961～2010年的逐日降水资料,采用百分位的方法定义宁夏极端降水事件的阈值,选取极端降水事件的降水量、频率、强度等多个指标,并利用线性回归和M-K非参数统计检验法分析了宁夏极端降水事件的时空分布及变化趋势特征。[结果]采用百分位法确定的宁夏近50年平均极端降水阈值大致呈现从南到北逐渐减小的分布状况,阈值较大的区域主要位于宁夏海原以南的固原地区,较小的区域位于宁夏的同心、盐池以北的大部地区,与宁夏气候平均年降水量的分布极为相似;近50年宁夏大部分地区的极端降水频率、极端降水量以减少趋势为主,而极端降水强度呈增大趋势。对麻黄山、六盘山2个典型站的极端降水事件分析发现,麻黄山极端降水频率与极端降水量呈减少趋势,极端降水强度为增大趋势,六盘山极端降水频率、极端降水量与极端降水强度均呈减少趋势;麻黄山年极端降水频率在1994年之前呈减少趋势,1994年以后呈增加趋势,突变发生在2003年;年极端降水强度1984年以来为增加趋势,突变时间为1984年,六盘山从1978年至今为减小趋势。[结论]该研究为宁夏气候突变分析提供理论依据。     

沙尘天气对兰州市城区环境空气质量影响研究

以2013年3月8 ～15日兰州市发生的系统性沙尘天气为例,分析了兰州市PM10、TSP以及能见度等污染物及气象因子对兰州市城区环境空气质量的影响.结果表明,此次沙尘天气甘肃始发地在河西走廊酒泉地区,主要受强冷空气影响,沙尘在风力影响下远距离输送,8日17：00在酒泉市发生时,大气最低能见度3.4 km,可吸入颗粒物最大浓度14.74 mg/m3,23：00影响到兰州等地,对甘肃省大部分地区影响较大,尤其对兰州市环境空气质量影响较大;受沙尘天气的影响,8 ～15日兰州市环境空气质量7d为重污染,1d为中度污染,首要污染物为可吸入颗粒物;能见度和可吸入颗粒物有明显的的负相关关系,湿度对能见度和颗粒物浓度的拟合存在影响.     

格点天气预报数据与机理模型耦合的冬小麦干旱预警方法

为了开展大范围的冬小麦干旱预警,以中国北方冬小麦区为实例,构建了土壤水分动态预报模型,结合未来10 d高精度天气要素预报、土壤自动水分观测和冬小麦发育期观测数据,建立了北方冬小麦区干旱预警系统。利用该系统对2018年4—5月进行逐日的冬小麦干旱预警,对干旱预警产品的分析表明:系统对未来10 d土壤相对湿度预报的决定系数在0. 63～0. 91之间,均方根误差在5. 6%～18. 2%之间,预报时效越近,准确率越高。从不同的干旱等级预测准确率看,对于干旱等级较高的重旱和特旱预报准确率较高,轻旱和中旱的预报准确率略低。该系统基本满足冬小麦干旱预警需求,对国家级农业气象部门大范围农业干旱监测和预警业务是有益的补充。     

湖北省一次暴雨落区预报偏大原因分析

利用常规观测资料、NCEP再分析资料和EC等数值模式预报资料,分析了湖北省2017年6月23日暴雨到大暴雨天气过程,得出此次暴雨过程预报落区偏大、范围偏北的原因。此次过程的天气形势背景为发生在副高外围西南急流出口区左侧、切变线和风速涌线南侧的暖区暴雨,范围小、雨带集中;副高脊线偏西偏北,西南急流中心轴偏北偏东,急流北界偏南、偏西分量较大,不利于江汉平原到鄂东北出现大范围暴雨天气;过于信赖EC形势预报导致了此次暴雨预报出现偏差,暖区暴雨可多参考华东模式;当上下层西南急流都很强盛时,特别需要警惕在暴雨过程前期西南急流加强过程中可能出现的暖区暴雨。