On the Relationship Between Mean and Variance of Soil Moisture Fields1

Abstract: The objective of this work was to explain an apparent contradiction in the literature related to the relationship between mean and variance (or standard deviation) of soil moisture fields. Some studies found an increase in soil moisture variance with decreasing mean soil moisture, while others showed a decrease. The evidence of maximum variance in the mid‐range of mean soil moisture was also reported in the literature. In this paper, we focus on the effects of spatial variability of soil texture on the relationship between variance and mean of soil moisture during soil dry‐down processes. Soil texture influences soil moisture mean and variance through its direct effects on evaporation and drainage, which are two main factors controlling soil drying. A differential equation describing soil moisture dry down is proposed and studied. Our study shows that as mean soil moisture is greater than a threshold, variance increases with decreasing mean soil moisture. If mean soil moisture is less than the threshold, variance decreases with decreasing mean soil moisture. The threshold depends on soil texture and is between the field capacity and the wilting point. The soil moisture dry‐down equation is also applied to explain the apparent contradiction with regard to the relationship between mean and variance of soil moisture fields reported in the literature.     

我国水环境农业非点源污染防治研究简述

概述了我国农业非点源污染的危害、污染影响因素和污染防治措施方面的研究。     

昆山市人口指标的数学模型

数学模型常用于人口指标变化趋势的拟合, 以昆山市1985～2006年 7种人口指标为研究对象,采用马尔萨斯模型、Logistic模型和Keyfitz模型进行模拟,结合历史资料分析人口指标变化背后的社会经济因素。研究结果表明：从整体拟合效果来看,Logistic模型表现良好,但这很大程度上在于其三参数优势,而不是人口增长面临最大人口上限的压力,即使某一时段反映出人口上限,该值也可能在下一时段被突破;由于人口指标增长转折点的存在,使利用数学模型进行人口预测的可信度降低,但利用转折点分段分析,了解其代表的政治、经济因素的影响,可以为未来的规划决策提供科学参考;昆山的发展历程明显分为3个阶段,而不是以往认为的5个阶段.     

基于“流变-突变”理论的安全投入决策

运用"流变-突变"理论分析煤矿安全投入的效果,发现其随着时间的推移也表现出"安全流变-突变"的特征。笔者指出,安全投入应该是持续的、不断进行的,不能靠期初一次性安全投入解决,而且后续安全投入应该选择在最佳时段(即不安全度加速发展阶段)进行,最晚不能超过安全突变的预警点,这样才能使系统的安全流变阶段能够尽量延长,从而最大限度地发挥所有安全投入的效果,缓解我国煤矿安全投入需求增长与安全投入不足的矛盾。     

工业源重点调查单位分界点的确定

讨论了工业源重点调查单位分界点确定的思路和方法,并基于污染源普查更新调查中河南省的数据(数据时点为2010年)进行了实证研究,研究结果表明,所有污染物产排指标都呈现出高度右偏分布,适宜采用目录抽样法,将工业源区分为重点污染源和非重点污染源,对重点污染源进行全面调查,对非重点污染源进行抽样调查,分界点可以下调到80%。     

Modes of Variability of Annual and Seasonal Rainfall in Mexico

The aim of this study is to identify temporal and spatial variability patterns of annual and seasonal rainfall in Mexico. A set of 769 weather stations located in Mexico was examined. The country was divided into 12 homogeneous rainfall regions via principal component analysis. A Pettitt test was conducted to perform a homogeneity analysis for detecting abrupt changes in mean rainfall levels, and a Mann‐Kendall test was conducted to examine the presence of monotonically increasing/decreasing patterns in the data. In total, 14.4% of the annual series was deemed nonstationary. Fourteen percent of the samples were nonstationary in the winter and summer, and 9% were nonstationary in the spring and autumn. According to the results, the nonstationarity of some seasonal rainfall series may be associated with the presence of atmospheric phenomena (e.g., El Niño/Southern Oscillation, Pacific Decadal Oscillation, Atlantic Multidecadal Oscillation, and North Atlantic Oscillation). A rainfall frequency analysis was performed for the nonstationary annual series, and significant differences in the return levels can be expected for the scenarios analyzed. The identification of areas that are more susceptible to changes in rainfall levels will improve water resource management plans in the country, and it is expected that these plans will take into account nonstationary theory.     

Assessment of the Netherlands’ Flood Risk Management Policy Under Global Change

Climate change and sea level rise urge low-lying countries to draft adaption policies. In this context, we assessed whether, to what extent and when the Netherlands’ current flood risk management policy may require a revision. By applying scenarios on climate change and socio-economic development and performing flood simulations, we established the past and future changes in flood probabilities, exposure and consequences until about 2050. We also questioned whether the present policy may be extended much longer, applying the concept of ‘policy tipping points’. Climate change was found to cause a significant increase of flood risk, but less than economic development does. We also established that the current flood risk management policy in the Netherlands can be continued for centuries when the sea level rise rate does not exceed 1.5 m per century. However, we also conclude that the present policy may not be the most attractive strategy, as it has some obvious flaws.     

大别山区山洪灾害预警模型的建立与应用

根据近年来大别山地区高密度雨量站资料,利用GIS技术形成的更为精细的栅格点坡度和高度数据,得出雨量与山区坡度和高度的具体关系,并由此推算出两个子流域发生山洪灾害的雨量阈值;实时通过数据服务器获取前期逐时雨量资料,形成24 h和6 h雨量,根据雨量关系式求算面雨量,与山洪灾害雨量阈值进行比较,给出是否发生山洪灾害的预警信息;及时发布预警信息,将山洪灾害的预警信息在淮河流域气象中心进行实际应用。     

输煤皮带转载点密闭罩流场模拟与结构优化

为了研究输煤皮带转载点密闭罩内流场对吸尘效果的影响,采用Fluent软件对某转载点密闭罩内气固两相流动情况进行了数值模拟。通过气流分布和煤尘浓度分布的分析,发现溜槽转折处较大的气流偏转会增加煤尘起尘量和导致局部煤尘积聚,吸尘罩的开口角度对吸尘效果也有较大影响。为了疏导气流并加强吸尘效果,提出了溜槽转折处和吸尘罩的结构优化设计方案。研究表明,改进后气流分布更加均匀,吸尘效果有所提高,从而为转载点密闭罩的优化设计提供了参考。     

超低沸点挥发性有机物测定中的要点分析

采用大气浓缩仪-气相色谱法测定超低沸点挥发性有机物乙烷、乙烯、丙烷、丙烯、乙炔,选用PLOT-Q柱分析,控制真空罐内相对湿度约50%,通过优化测定条件使方法线性良好,5种低沸点化合物的检出限为0.08 nmol/mol～0.2 nmol/mol,样品保存时间不超过10 d。用该方法测定某地区环境空气中VOCs,结果检出率均为100%。