Importance of geological information for assessing drain flow in a Danish till landscape

There has, in recent years, been an increasing interest in developing nutrient load mitigation measures focussing on tile drains. To plan the location of such tile drain measures, it is important to know where in the landscape drain flow is generated and to understand the key factors governing drain flow dynamics. In the present study, we test two approaches to assess spatial patterns in drain flow generation and thereby assess the importance of including geological information. The approaches are the widely used topographical wetness index (TWI), based solely on elevation data, and hydrological models that include the subsurface geology. We set‐up an ensemble of 20 hydrological models based on 20 stochastically generated geological models to predict drain flow dynamics in the clay till Norsminde catchment in Denmark and test the results against TWI. We find that the hydrological models predict observed daily drain flow reasonably well. High drain flow volumes were found in stream valleys and in wetlands and lower drain flow volumes in the more hilly parts of the catchment. In spite of the apparent connection to the landscape, there was no statistically significant correlation between TWI and drain flow at grid scale (100 × 100 m). TWI was therefore not found to be a sufficient index on its own to assess where drain flow is generated, especially in the highlands of the catchment. The geology below 3 m was found to have a large impact on the drain flow, and correlations between sand percentage in the subsurface geology and drain flow volume were found to be statistically significant. Geological uncertainty therefore give rise to uncertainty on simulated drain flow, and this uncertainty was found to be high at the model grid scale but decreasing with increasing scale.     

2000年下半年全球地震活动

2000年下半年全球共发生Mw≥6．5地震27次，其中23次为浅源地震，深源地震4次。发生Mw≥7．0地震5次，最大震级达Mw8．0。11月16日和17日，在俾斯麦群岛和所罗门群岛交界地区，发生了3次Mw≥7．8地震。2000年，全球地震活动处于1976年以来的较高水平。     

我国北方干旱逐月变化特征分析

用1950～2000年我国629个站逐月降水资料,采用Z指数作为旱涝等级划分标准,把各地的旱涝情况划分成7个等级,把长江以北主要农业区格点化后．以各格点中达到干旱站点的多少为权重,求出格点中干旱发生的面积。通过对51年的资料分析,发现我国北方区干旱面积在不同月份有其不同的变化特征。其中1、2、4、7、8、9、11、12月八个月的干旱面积有不断扩大的趋势,3、5、10三个月的干旱面积处于平均状态,6月份的干旱面积有减少的趋势。     

Urban flood hazard mapping using change detection on wetness transformed images

ABSTRACT

The present study demonstrates the use of a new approach for delineating the accurate flood hazard footprint in the urban regions. The methodology involves transformation of Landsat Thematic Mapper (TM) imagery to a three-dimensional feature space, i.e. brightness, wetness and greenness, then a change detection technique is used to identify the areas affected by the flood. Efficient thresholding of the normalized difference image generated during change detection has shown promising results in identifying the flood extents which include standing water due to flood, sediment-laden water and wetness caused by the flood. Prior to wetness transformations, dark object subtraction has been used in lower wavelengths to avoid errors due to scattering in urban areas. The study shows promising results in eliminating most of the problems associated with urban flooding, such as misclassification due to presence of asphalt, scattering in lower wavelengths and delineating mud surges. The present methodology was tested on the 2010 Memphis flood event and validated on Queensland floods in 2011. The comparative analysis was carried out with the widely-used technique of delineating flood extents using thresholding of near infrared imagery. The comparison demonstrated that the present approach is more robust towards the error of omission in flood mapping. Moreover, the present approach involves less manual effort and is simpler to use.

Editor Z.W. Kundzewicz; Associate editor A. Viglione     

2002年上半年全球地震活动

2002年上半年,全球发生MW≥6．5地震16次,其中13次浅源地震,2次中源地震,1次深源地震;发生MW≥7．0地震6次。上半年MW≥7．0地震的地震矩释放是1976年以来的偏低水平。中国及邻区地震活动比较活跃：3月3日,在兴都库什地区发生了1次MW7．3中源地震;6月28日,在中国吉林汪清发生了1次MW7．3深源地震。统计检验表明,地震矩释放表现出持久性质。     

广义极值分布在重庆短历时极值降水中的应用

利用广义极值分布函数拟合1981—2016年重庆34个国家气象站短历时(1、3、6、12 h)极值降水序列,对拟合结果进行显著性水平检验,并给出不同重现期极值降水的空间分布。结果表明:广义极值分布函数能较好地拟合重庆地区的短历时极值降水。随着降水历时的延长,服从Weibull分布(Frechet分布)的站点数逐渐减少(增加)。各短历时不同重现期降水的空间分布具体表现为10 a以下及20 a以上基本相似,位于长江沿线以北的重庆西北部地区降水量明显大于重庆长江沿线以南地区,且渝东南降水的相对大值区位于彭水地区。随着重现期的增加,降水中心更加集中,渝东北的大值中心随着历时的延长向北移动。广义极值分布函数的形状参数的绝对值接近或超出0.5时,计算的高重现期(大于样本长度)极值降水存在较大偏差;当不同历时降水拟合的形状参数值具有明显差异时,高重现期降水可能出现与客观规律相悖的现象。     

近28a河南主要农业气象灾害及其影响

利用河南1978-2005年农作物受灾面积资料和新中国成立以来的产量资料,分析了河南主要农业气象灾害的变化特征和对农业生产的影响,结果表明,28 a来河南农作物受灾面积呈上升趋势,其阶段性变化为20世纪70年代后期至80年代前期较少,80年代中后期至21世纪初明显增多,最近几年又明显减少。气候条件对农业生产的影响以50年代最有利,80年代次之,70年代较差,60年代最差。     

北京夏季不同等级降水过程与旱涝关系的分析

利用北京地区近百年的逐日降水资料,对北京夏季（６￣８月）不同等级降水过程的时间演变及其与夏季旱涝的关系进行了分析。提出北京夏季的旱涝与暴雨过程的关系最为密切,也就是说北京夏季暴雨过程的多少直接反映出夏季反映出夏季旱涝情况;并分析了偏旱和偏涝年代不同降水过程出现的频次。指出,年代偏旱或偏涝与降水过程次数的多少关系不大,而与降水强度即暴雨过程的多少和暴雨的强度有密切的关系。因此旱涝的短期气候预测必须与     

广东干湿气候的特征

提出一种月、季干湿气候指数，并用于广东各地干湿气候的分析，结果表明，广东以４－９月为湿季，１０月到翌年１月为干季，２－３月为干转湿的过渡期，干季短而湿季长，干湿季节的互换呈纬向自北向南推南，３－４月呈北湿南干，９月呈北干南湿，干湿季的转换时间与自然季节的转换基本相符。     

一种干湿气候指数的计算方法

在科塔哥月干燥指数的基础，采纳了柯本在夏季多雨地区，草原气候（干）与林木气候（湿）界限的地区标准，结合广东地区气候特征，通过实际气候资料检验和修正，提出了一种适用于广东及其邻近地区的干湿气候指数计算公式，经过用广东省各地实际资料作年内的和冬，夏半年的，以及任意年，月的干湿指数的实际计算和分析，发现该指数能较清楚地反映了广东干湿气候变化特征，这对于农业生产和经济建设服务具有实际的意义。