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61.
James P. Hughes Steven P. Millard 《Journal of the American Water Resources Association》1988,24(3):521-531
ABSTRACT: A common problem arises in testing for trends in water quality when observations are reported as “less than detection limit.” If a single detection limit is used for the entire study, existing non-parametric statistical methods, modified for ties, are applicable. If, however, the detection limit varies during the course of the study, resulting in multiple detection limits, then the commonly used trend detection methods are not appropriate. A statistic similar to Kendall's tau, but based on expected ranks, is proposed. Monte Carlo simulations show that the normal approximation to the distribution of this statistic is quite good, even for small samples and a large proportion of censored observations. The statistic is also shown to have greater power than the ad-hoc method of treating all observations less than the target censored observation as tied. 相似文献
62.
Temporal Trends in the Spatial Distribution of Impervious Cover Relative to Stream Location 下载免费PDF全文
J. Wickham A. Neale M. Mehaffey T. Jarnagin D. Norton 《Journal of the American Water Resources Association》2016,52(2):409-419
Use of impervious cover is transitioning from an indicator of surface water condition to one that also guides and informs watershed planning and management, including Clean Water Act (33 U.S.C. §1251 et seq.) reporting. Whether it is for understanding surface water condition or planning and management, impervious cover is most commonly expressed as summary measurement (e.g., percentage watershed in impervious cover). We use the National Land Cover Database to estimate impervious cover in the vicinity of surface waters for three time periods (2001, 2006, 2011). We also compare impervious cover in the vicinity of surface waters to watershed summary estimates of impervious cover for classifying the spatial pattern of impervious cover. Between 2001 and 2011, surface water shorelines (streams and water bodies) in the vicinity of impervious cover increased nearly 10,000 km. Across all time periods, approximately 27% of the watersheds in the continental United States had proximally distributed impervious cover, i.e., the percentage of impervious cover in the vicinity of surface waters was higher than its watershed summary expression. We discuss how impervious cover spatial pattern can be used to inform watershed planning and management, including reporting under the Clean Water Act. 相似文献
63.
随着城市生活水平的提高,室内装修装饰日益普及.各种新材料的引入,使得室内的各种有害气体总量日益增加.长期生活在这样的环境下,会对人体健康产生极为不利的影响.居民生活用煤气,在给人们生活带来便利的同时,作为一种可燃性、有毒气体,也具有极强的危害性.因此,研制一种能检测多种有害气体的实用仪器,具有很大的应用价值.本仪器为便携式多功能仪器,采用可充电电池供电,可以检测ppb的挥发性气体,显示气体浓度值,也可检测煤气,并具有声光报警功能. 相似文献
64.
J. B. Harcum Jim C. Loftis Robert C. Ward 《Journal of the American Water Resources Association》1992,28(3):469-478
ABSTRACT: The use of nonparametric tests for monotonic trend has flourished in recent years to support routine water quality data analyses. The validity of an assumption of independent, identically distributed error terms is an important concern in selecting the appropriate nonparametric test, as is the presence of missing values. Decision rules are needed for choosing between alternative tests and for deciding whether and how to pre-process data before trend testing. Several data pre-processing procedures in conjunction with the Mann-Kendall tau and the Seasonal Kendall test (with and without serial correlation correction) are evaluated using synthetic time series with generated serial correlation and missing data. A composite test (pre-testing for serial correlation followed by one of two trend tests) is evaluated and was found to perform satisfactorily. 相似文献
65.
Malcolm J. R. Clark Paul H. Whitfield 《Journal of the American Water Resources Association》1994,30(6):1063-1079
ABSTRACT: Current conventions for reporting analytical results from environmental samples brings the objectives of laboratory scientists into conflict with those of environmental scientists. The objective of chemical analyses is to provide estimates of the true composition of samples. Reported results must reflect the analytical uncertainty. Current conventions require left-censoring of those results below the Limit of Detection. The objective of statistical interpretation of environmental data is to provide estimates of the characteristics of ecosystems. Such statistical analyses are often confounded by left-censoring of analytical results. We review the different points of view and propose a compromise which recognizes these conflicting perspectives. 相似文献
66.
Bradford R. Crain 《Environmental management》1998,22(3):471-481
L and width 2w, then by a selection of viewing window is meant a choice of w, with the intent being to search for optimal viewing windows, with the goal in mind of improving variances of estimators
of population density, reducing sampling effort, while maintaining the property of unbiasedness. The notions of increasing
window sensitivity (IWS) and decreasing window sensitivity (DWS) are introduced, and a method of deriving confidence intervals
is discussed. 相似文献
67.
Summary. Detection of heterospecific predators and prey
via chemical cues is well known, but only a few studies have
examined the potential for such discrimination in cannibalistic
systems. In newts, Notophthalmus viridescens, adults are
opportunistic predators of conspecific larvae. I used a laboratory
bioassay to determine whether larvae and adults distinguish
between chemical stimuli from members of the
different age classes. Larvae distinguished between chemical
stimuli from larvae and adults by decreasing their activity
only following exposure to stimuli from adults. Decreased
activity is consistent with an antipredator response in many
prey species, including larval newts. In contrast, adults
increased their activity and increased time spent in open
areas in response to stimuli from larvae, but not to stimuli
from adults. Increased activity is consistent with a feeding
response; adults also showed increased activity and
increased time in open areas in response to chemical stimuli
from familiar heterospecific prey (brine shrimp). The proximate
cue that allowed the newts to distinguish between the
different age classes is not associated with short-term
dietary differences because all stimulus animals were fed the
same prey. Stimulus strength was controlled by diluting the
stimulus solutions according to the volume of the stimulus
animal. Therefore, there appear to be intrinsic differences in
the chemical signatures of larval and adult newts. 相似文献
68.
69.
Roy W. Koch Thomas G. Sanders H. J. Morel-Seytoux 《Journal of the American Water Resources Association》1982,18(5):815-821
ABSTRACT: The detection of change in a hydrologic varaible, particularly water quality, is a current problem. A method is presented for testing whether there has been a shift in the mean of a hydrologic variable based on the well established bivariate normal distribution theory. In this technique, the dependent, or target, and the independent, or control, variables are formed as weighted linear combinations of the mean values at a number of locations in a selected target and control area. The weighting factors are determined based on a mathematical programming technique which minimizes the conditional coefficient of variation thereby minimizing the number of observations required to detect a change of a preselected magnitude in the mean of the target area. The result is a situation where a savings in the number of observations required to detect a change is a consequence of adding more stations: the space-time tradeoff. Two applications of the technique are presented, the first using electrical conductivity (EC) data from two sets of river basins and the second using EC data from a set of basins as the target variable and annual discharge as the control. The results indicate that a significant savings in time can be achieved by using this method. 相似文献
70.