Small sample estimation for Taylor's power law

An analysis of counts of sample size N=2 arising from a survey of the grass Bromus commutatus identified several factors which might seriously affect the estimation of parameters of Taylor's power law for such small sample sizes. The small sample estimation of Taylor's power law was studied by simulation. For each of five small sample sizes, N=2, 3, 5, 15 and 30, samples were simulated from populations for which the underlying known relationship between variance and mean was given by ² = c^d. One thousand samples generated from the negative binomial distribution were simulated for each of the six combinations of c=1,2 and 11, and d=1, 2, at each of four mean densities, =0.5, 1, 10 and 100, giving 4000 samples for each combination. Estimates of Taylor's power law parameters were obtained for each combination by regressing log₁₀ s ² on log₁₀ m, where s ² and m are the sample variance and mean, respectively. Bias in the parameter estimates, b and log₁₀ a, reduced as N increased and increased with c for both values of d and these relationships were described well by quadratic response surfaces. The factors which affect small-sample estimation are: (i) exclusion of samples for which m = s ² = 0; (ii) exclusion of samples for which s ² = 0, but m > 0; (iii) correlation between log₁₀ s ² and log₁₀ m; (iv) restriction on the maximum variance expressible in a sample; (v) restriction on the minimum variance expressible in a sample; (vi) underestimation of log₁₀ s ² for skew distributions; and (vii) the limited set of possible values of m and s ². These factors and their effect on the parameter estimates are discussed in relation to the simulated samples. The effects of maximum variance restriction and underestimation of log₁₀ s ² were found to be the most severe. We conclude that Taylor's power law should be used with caution if the majority of samples from which s ² and m are calculated have size, N, less than 15. An example is given of the estimated effect of bias when Taylor's power law is used to derive an efficient sampling scheme.     

Simulating correlated count data

In this study we compare two techniques for simulating count-valued random n-vectors Y with specified mean and correlation structure. The first technique is to use a lognormal-Poisson hierarchy (L-P method). A vector of correlated normals Z is generated and transformed to a vector of lognormals X. Then, Y is generated as conditionally independent Poissons with means X _i . The L-P method is simple, fast, and familiar to many researchers. However, the method requires each Y _i to be overdispersed (i.e., σ² > μ), and only low correlations are possible with this method when the variables have small means. We develop a second technique to generate the elements of Y as overlapping sums (OS) of independent X _j ’s (OS method). For example, suppose X, X ₁, and X ₂ are independent. If Y ₁ = X + X ₁ and Y ₂ = X + X ₂, then Y ₁ and Y ₂ are correlated because they share the common component X. A generalized version of the OS method for simulating n-vectors of two-parameter count-valued distributions is presented. The OS method is shown to address some of the shortcomings of the L-P method. In particular, underdispersed random variables can be simulated, and high correlations are feasible even when the means are small. However, negative correlations cannot be simulated with the OS method, and when n > 3, the OS method is more complicated to implement than the L-P method.     

珠江三角洲小流域地下水化学特征及演化规律

珠江三角洲地下水环境日益恶化,已成为制约经济社会发展的重要因素。以珠海市具有典型特征的闭合小流域作为研究对象,分旱、雨两季采集地下水分析主要离子及D、18O同位素,全面系统地研究了地下水水化学的时空变异特征与演变规律。结果表明:研究区内地下水主要受大气降水及附近地表水体渗透补给,以蒸发及地下水径流排泄,季节变化对区域内水化学空间变异性影响较小。沿地下水流方向(补给区-径流区-排泄区),地下水化学类型主要从Ca-Na(Mg)-HCO3型向Na-Cl和Na-K-HCO3-CO3型演化,风化-溶滤、离子交换、海陆交互作用是控制当地地下水质演变的主要水文化学过程。     

Post-project analysis: The use of a network diagram for environmental evaluation of the Barekese Dam, Kumasi, Ghana

The paper presents an evaluation of post-project impacts of the Barekese Dam on three riparian communities downstream about 30 years after its construction. A network diagram, which incorporated a mathematical model, was used for impact identification and analysis. The expected environmental impacts, expressed as quantitative weighted impact scores, showed that the dam appeared to have exerted adverse impacts on the environmental quality of the communities. The impacts in the communities, however, increased with relative distances away from the dam and the river suggestive of cumulative impacts transmitted downstream. Strategic measures for improving environmental quality of the communities have been given.     

乌鲁木齐市大气TSP中28种元素的测定与变化规律的研究

报道了乌鲁木齐市一年的内大气ＴＳＰ中２８种元素含量的监测结果，污染程度及变化规律，为进一步加强环境管理和大气污染治理，改善环境质量提供科学依据。     

关于废水中污染物浓度日均值的思考

在环境污染防治工作从污染物浓度控制逐步向排污总量控制转变的过渡阶段 ,污染物浓度日均值仍然发挥着十分重要的作用。文章阐述了日均值的监测时段、监测频率及计算方法等技术要求 ,并结合排污总量监测 ,提出了一些建议     

Evaluation of Metric Precision for a Riparian Forest Survey

This article evaluates the performance of a protocol to monitor riparian forests in western Oregon, United States based on thequality of the data obtained from a field survey. Precision isthe criteria used to determine the quality of 19 field and 6 derived metrics. The derived metrics were calculated from thefield data. The survey consisted of 110 riparian sites on publicand private lands that were sampled during the summers of 1996 and 1997. In order to calculate metric precision, some of the field plots were re-measured. Metric precision was defined in terms of the coefficient of variability (CV) and standard deviation and then compared with a pre-defined data quality objective (DQO). A metric was considered precise if the CV met or exceeded the DQO. The geomorphology metrics were not precisewhile the forest stand inventory metrics and forest cover metrics, with some exceptions, were precise. The precision formany of the field and derived metrics compared favorably withthe level of precision for similar metrics reported in the literature. Recommendations are made to improve the precision for some metrics and they include changing the way precision is calculated, re-defining the field protocol, or improving field training.     

质疑：“红旗河”调水功能的可行性

据报道,“红旗河”方案确定的基本目标是由雅鲁藏布江、怒江、澜沧江、金沙江、雅砻江、大渡河调水600亿m³至新疆等干旱区,发展2亿亩农田（1亩=1/15 hm²）,建15万~20万km²绿带,以此彻底改善西北干旱区的生态与环境。“红旗河”通过上述河流的各调水点以上的年均径流量总和为1 444亿m³,占被调水河流总径流量的35%;“红旗河”计划年调水量约占可调水量1 444亿m³的42%。如果不考虑国际河流、调水口以下的水电站及工程的正常运行等因素,单纯就水资源而论,600亿m³总调水量是可行的。“红旗河”是跨流域调水的宏大设想,已经引起国内外的广泛关注,但这个宏大的设想也面临地质、技术、经济、社会、生态等领域的多重严峻挑战,存在极大的不确定性。特别是600亿m³的水能解决什么问题?在实施工程之前应该充分论证,采取有效措施防范和减轻这些风险带来的危害。从自然地理、资源环境和区域发展角度看,以下问题值得商榷：从主要受水区域新疆看,依据现有农田的净灌溉定额（大约600 m³/亩）,即使完全不考虑渠道（干渠及农田灌溉支渠）中途渗漏与水面蒸发,理论上最多发展1亿亩农田。同样,根据新疆为了维持塔里木河下游大海子水库以下的511 km²湿地所调用的水量估算,15万km²生态绿带建设每年至少需水1 100多亿 m³。600亿m³引水量既无法实现建造2亿亩农田的需要,也无法满足15万km²生态绿带的需要,二者同时实现更无可能。何况这600亿m³的水要经过6 000多km的长途跋涉,渗漏和蒸发非常大,最终能有多少水达到受水区?该项目计划建设期10 a、投资4万亿元,每亩农田（以2亿亩计算）2万元,每m³水费近66元。工程建成运行时,谁来为昂贵的水费买单?调水可能引起的环境影响和生态后果具有极大的不确定性,需要给予高度关注。如调入区是内流区,降水量小,蒸发量大,地形封闭,缺乏排水出路,环境条件特殊。大量引水,在灌溉不当的情况下,极易导致土壤次生盐渍化。跨流域调水工程不仅仅是一个复杂的水利工程,还是一个十分复杂的生态系统工程,更是一个十分复杂的社会经济工程。环境效应、生态后果和社会经济影响涉及复杂的地球物理、化学和生物过程,也涉及复杂的人地关系和谐平衡过程。“红旗河”同时涉及国际河流,潜在的地缘政治关系需要引起重视。在现有认识水平下,需要对这些问题进行深入系统研究,分析各系统之间的相互关系及影响规律,提出理性的评价系统和论证方案。此外,在“红旗河”的舆论造势中,还存在几个有悖科学认知的观点,如“改变中国气候格局”、“森林致水作用”、打破千百年来的“胡焕庸线”等。文章均进行了分析和澄清。     

不同尺度下人类活动对浑太河流域鱼类和大型底栖动物群落特征的影响

为了确定鱼类和大型底栖动物群落结构对多重人类活动压力响应是否一致,以浑太河流域为例,采用典范对应分析和冗余分析方法,以自然环境因子(包括海拔、坡度、流域面积、经纬度)为协变量,比较流域(各土地利用类型面积所占比例)、河岸带(各土地利用面积所占比例)和河段尺度(水质、水力形态、水文、栖息地质量、底质类型等)下人类活动因子对鱼类和大型底栖动物群落特征的影响. 结果表明:在流域尺度上,林地和农村聚落用地面积所占比例解释了8.09%的鱼类群落特征空间变异,林地、旱地、城镇建设用地和农村聚落用地面积所占比例解释了18.28%的大型底栖动物群落特征空间变异;在河岸带尺度上,城镇建设用地面积所占比例解释了4.26%的鱼类群落特征空间变异,没有环境因子对大型底栖动物群落特征产生显著影响;在河段尺度上,电导率、ρ(TDS)、ρ(NH₄+-N)、ρ(COD_Mn)、河宽、QHEI(a qualitative habitat evaluation index,栖息地评估指数)、ρ(DO)、ρ(TN)、硬度和pH解释了36.60%的鱼类群落特征空间变异,电导率、ρ(TDS)、ρ(NH₄+-N)、ρ(COD_Mn)解释了14.65%的大型底栖动物群落特征空间变异.河段尺度人类活动因子对鱼类群落特征的直接影响最为显著,其次为流域尺度,河岸带尺度的影响最小;流域尺度和河段尺度人类活动因子对大型底栖动物群落特征影响大致相同. 河段尺度的水力形态和水质状况主要受流域尺度土地利用变化的影响,其次是河岸带尺度. 土地利用变化对水力形态的影响较水质更大.      

Institutional quality effects on separate waste collection: some evidence from Italian provinces

Separate waste collection in Italy comes under the responsibility of local authorities, provinces, while national laws set quantitative targets to be achieved over time. Overall, just a few provinces have reached the thresholds set by the latest law (Legislative Decree 152/2006) and some territorial differentiation has been detected. The aim of this paper is twofold: to verify the effectiveness of Legislative Decree 152/2006 in promoting higher levels of separate collection and to test whether institutional quality (considering the following indicators: voice and accountability, regulatory quality, rule of law and corruption) affects provinces’ efficiency in the separate collection process. For this purpose, we implement the stochastic frontier analysis (SFA). Results suggest that the effectiveness of Legislative Decree 152/2006 has been affected by provinces’ institutional quality. In particular, the presence of institutional constraints did not allow the most virtuous provinces to achieve the targets set by law.