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261.
James F. Saunders Marylee Murphy Martyn Clark William M. Lewis 《Journal of the American Water Resources Association》2004,40(5):1339-1349
ABSTRACT: Historical flow records are used to estimate the regulatory low flows that serve a key function in setting discharge permit limits through the National Pollutant Discharge Elimination System, which provides a nationwide mechanism for protecting water quality. Use of historical records creates an implicit connection between water quality protection and climate variability. The longer the record, the more likely the low flow estimate will be based on a broad set of climate conditions, and thus provides adequate water quality protection in the future. Unfortunately, a long record often is not available at a specific location. This analysis examines the connection between climate variability and the variability of biologically based and hydrologically based low flow estimates at 176 sites from the Hydro‐Climatic Data Network, a collection of stream gages identified by the USGS as relatively free of anthropogenic influences. Results show that a record of 10 to 20 years is necessary for satisfactory estimates of regulatory low flows. Although it is possible to estimate a biologically based low flow from a record of less than 10 years, these estimates are highly uncertain and incorporate a bias that undermines water quality protection. 相似文献
262.
Kenneth E. Hyer Douglas L. Mayer 《Journal of the American Water Resources Association》2004,40(6):1511-1526
ABSTRACT: Surface water impairment by fecal coliform bacteria is a water quality issue of national scope and importance. In Virginia, more than 400 stream and river segments are on the Commonwealth's 2002 303(d) list because of fecal coliform impairment. Total maximum daily loads (TMDLs) will be developed for most of these listed streams and rivers. Information regarding the major fecal coliform sources that impair surface water quality would enhance the development of effective watershed models and improve TMDLs. Bacterial source tracking (BST) is a recently developed technology for identifying the sources of fecal coliform bacteria and it may be helpful in generating improved TMDLs. Bacterial source tracking was performed, watershed models were developed, and TMDLs were prepared for three streams (Accotink Creek, Christians Creek, and Blacks Run) on Virginia's 303(d) list of impaired waters. Quality assurance of the BST work suggests that these data adequately describe the bacteria sources that are impairing these streams. Initial comparison of simulated bacterial sources with the observed BST data indicated that the fecal coliform sources were represented inaccurately in the initial model simulation. Revised model simulations (based on BST data) appeared to provide a better representation of the sources of fecal coliform bacteria in these three streams. The coupled approach of incorporating BST data into the fecal coliform transport model appears to reduce model uncertainty and should result in an improved TMDL. 相似文献
263.
Naomi E. Detenbeck Colleen M. Elonen Debra L. Taylor Leroy E. Anderson Terri M. Jicha Sharon L. Batterman 《Journal of the American Water Resources Association》2004,40(3):705-720
ABSTRACT: In 1998 and 1999, third‐order watersheds in high mature forest (HMF) and low mature forest (LMF) classes were selected along gradients of watershed storage within each of two hydrogeomorphic regions in the Lake Superior Basin to evaluate threshold effects of storage on hydrologic regimes and watershed exports. Differences were detected between regions (North and South Shore) for particulates, nutrients, and pH, with all but silica values higher for South Shore streams (p < 0.05). Mature forest effects were detected for turbidity, nutrients, color, and alkalinity, with higher values in the LMF watersheds, that is, watersheds with less that 50 percent mature forest cover. Dissolved N, ammonium, N:P, organic carbon, and color increased, while suspended solids, turbidity, and dissolved P decreased as a function of storage. Few two‐way interactions were detected between region and mature forest or watershed storage, thus threshold based classification schemes could be used to extrapolate effects across regions. Both regional differences in water quality and those associated with watershed attributes were more common for third‐order streams in the western Lake Superior drainage basin as compared with second‐order streams examined in an earlier study. Use of ecoregions alone as a basis for setting regional water quality criteria would have led to misinterpretation of reference condition and assessment of impacts in the Northern Lakes and Forest Ecoregion. 相似文献
264.
265.
Ramsis B. Salama 《Water, Air, & Soil Pollution: Focus》2005,5(1-2):27-44
Perth groundwater resources are obtained from three major aquifers that occur beneath the Perth metropolitan area: the Superficial
aquifer, Leederville aquifer and Yarragadee aquifer. Each aquifer has a unique seasonal water level pattern controlled by
soils, geomorphology and geology. Landuse is mainly responsible for variations in recharge; however, the hydraulic properties
control aquifer response and water level pattern to a greater degree. Groundwater in the three aquifers is generally of very
good quality except in localised areas. Salinity increases with depth and in direction of groundwater flow in the three aquifers.
The best water quality is in the Superficial aquifer in the Wanneroo well field area. The geochemistry and stable isotope
signatures from the three major aquifers revealed distinct water types that suggest very little hydraulic connection or mixing
of waters between these aquifers at the present abstraction and recharge regimes. The results also show that the Leederville
and Yarragadee aquifers were recharged during earlier cooler times while the Superficial aquifer is being recharged at present. 相似文献
266.
267.
地下水水质评价的多元线性回归分析模型研究 总被引:4,自引:0,他引:4
运用回归分析理论和方法,建立了一个基于多元线性回归分析法的地下水水质评价模型,并将该模型用于遵义市海龙坝地下水水质评价.结果表明,建立的模型较符合本研究区的实际情况. 相似文献
268.
浅谈高职高专院校中的语文素质教育 总被引:2,自引:0,他引:2
一方面阐释语文素质教育在高职高专教育中的重要性及其作用;另一方面阐释了作为一名大学语文教师应如何在教学中体现"素质教育",培养出具有较高综合素质的职业技术人才. 相似文献
269.
Summary This paper is a comparative study of several antennas commonly used in cellular telephones. These include a monopole, a helix-monopole
and a patch antenna. Each one of these structures is modeled and numerically tested using finite-difference time-domain simulation
and human models based on magnetic-resonance images, which allow for inclusion of details of the human body in the simulation.
The testing procedure involves antenna simulation in the proximity of the human head. The behavior of each antenna is evaluated
for variable distances from the head geometry (0, 1, 2, 4, 8 and 16 mm). Continuous waveform, representative of the sources
used in mobile telephones, (250 mW, 1.8 GHz) is used as the form of the antenna excitation. The simulation outputs used as
measures for this comparative study include transmitting and receiving antenna characteristics and the specific absorption
rate (SAR). The SAR levels for the head tissues are calculated for and with accordance to the two currently accepted standards:
Federal Communications Commission (FCC) and International Commission on Non-Ionizing Radiation Protection (ICNIRP). The computed
SAR levels within each of the considered tissues vary for the three antennas under investigation and are within the determined
health safety standards. Results suggest that the patch antenna may be the structure of choice when considering safety standards,
as its radiation yields the lowest local SAR in the head tissues. 相似文献
270.
The Water Fluxes of the Yellow River to the Sea in the Past 50 Years, in Response to Climate Change and Human Activities 总被引:12,自引:1,他引:12
Jiongxin X 《Environmental management》2005,35(5):620-631
Since the 1970s, the water fluxes to the sea of the Yellow River have declined significantly. Based on data of precipitation, air temperature, the measured and “natural” river flow, the water diversion and consumption, and the areas of erosion and sediment control measures over the drainage basin, water fluxes to the sea of the Yellow River are studied in relation with the influences of changing climate and human activities. The Yellow River basin can be divided into different water source areas; multiple regression indicates that the variation in precipitation over different water source areas has different effect on water fluxes to the sea. In the period between 1970 and 1997, averaged air temperature over the whole Yellow River increased by about 1.0°C, from 16.5°C to 17.5°C, a factor that is negatively correlated with the water yield of the Yellow River. Water diversion and consumption has sharply increased and resulted in a significant decline in the water fluxes to the sea. Since the 1960s, erosion and sediment control measures have been practiced over the drainage basin. This factor, to a lesser degree, is also responsible for the decrease in water fluxes to the sea. A multiple regression equation has been established to estimate the change in water fluxes to the sea caused by the changes in precipitation, air temperature, water diversion and consumption, erosion, and sediment control measures, indicating that the contribution of water diversion and consumption to the variation in annual water flux to the sea is 41.3%, that of precipitation is 40.8%, that of temperature is 11.4%, and that of erosion and sediment control measures is 6.5%. 相似文献