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排序方式: 共有4049条查询结果,搜索用时 625 毫秒
981.
S. W. Park S. Mostaghimi R. A. Cooke P W. McClellan 《Journal of the American Water Resources Association》1994,30(6):1011-1023
ABSTRACT: To quantify the effectiveness of best management practice (BMP) implementation on runoff, sediment, and nutrient yields from a watershed, the Nomini Creek watershed and water quality monitoring project was initiated in 1985, in Westmoreland County, Virginia. The changes in nonpoint source (NPS) loadings resulting from BMPs were evaluated by comparing selected parameters from data series obtained before, during, and after periods of BMP implementation. The results indicated that the watershed-averaged curve number, sediment, and nutrient (N and P) concentrations were reduced by approximately 5, 20, and 40 percent, respectively, due to BMP implementation. The nutrient yield model developed by Frere et al. (1980) was applied to the water quality parameters from 175 storms, but it failed to adequately describe the observed phenomena. Seasonal changes in nutrient availability factors were not consistent with field conditions, nor were they significantly different in the pm- and post-BMP periods. An extended period of monitoring, with intensive BMP implementation over a larger portion of the watershed, is required to identify BMP effectiveness. 相似文献
982.
ABSTRACT: The pebble count, a quick and simple technique for characterizing streambed materials, has long been used by geomorphologists, hydrologists, and river engineers. This paper describes how pebble counts have been used to monitor fine sediment (particles less then 6 mm in size) on the Boise National Forest. Data from two watersheds subjected to major wildfires and the failure of a dam are discussed. Following wildfires, pebble count data showed increases in streambed fines followed by improvement of the stream substrate with time as the watersheds recovered. For the dam failure, pebble count data showed an increase in fines in the stream below the failure and were used to track the distance of sediment movement downstream. Pebble counts may be best used where fine sediment on channel substrates are a concern, such as in granitic watersheds where coarse sands are a large component of bedload and land-disturbing activities introduce fine sediment into streams. Pebble counts are found to be a simple and rapid monitoring method that can be used to help determine whether or not land management activities or land disturbances are introducing fine sediment into streams. 相似文献
983.
Jeffery L. Vowell Chriswell G. Hutchens 《Journal of the American Water Resources Association》1987,23(2):337-339
ABSTRACT: An electronic controller was developed to facilitate water sampling at discrete stage heights in perennial mountain streams of southeast Oklahoma. The controller operates in conjunction with an automatic pumping sampler and a magnetic, relay transducer. In response to changes in stage, the transducer generates an electrical pulse when relays are closed by a magnetic float. The controller monitors these relay closures and signals the sampler to pump a single sample at each desired water level during both rising and falling stages. After a sample is taken, additional switch closures at that stage axe ignored until the relay immediately above or below is closed. Inadvertent switch closures caused by turbulent stage fluctuations are not translated to the sampler thus eliminating duplicate or multiple samples at a given stage. Field testing has proven this device to be a reliable component with a range of applications for improving water quality sampling systems. The controller features battery operation, low power consumption, circuit simplicity and can be easily constructed for less than $100, including parts and labor. 相似文献
984.
Bruce Anderson Donald F. Potts 《Journal of the American Water Resources Association》1987,23(4):681-690
ABSTRACT: Forest management activities in a second order drainage basin increased suspended sediment yields 7.7 fold in the first year following road construction, and two-fold following logging in the second year. Sediment supply limitations resulted in poor correlations between sediment concentrations with discharge. Sediment transport was strongly hysteretic, with the highest sediment concentrations occurring on the rising limbs of snowmelt hydrographs and individual peaks. In addition to discharge, hydrograph characteristics such as limb, dQ/dt, and the product of dQ/dt and limb aided in explaining variability of observed sediment concentrations. Sediment-turbidity relationships were strongly discharge dependent, reflecting the changing composition of the suspended load with stream power and sediment supplies. 相似文献
985.
ABSTRACT: The hydrologic impacts of livestock grazing schemes on selected plant communities and soils at Fort Stanton, New Mexico, were evaluated. Simulated rainfall was applied to 1 m2plots. On a mesa-top, infiltration rates for a grassland livestock exclosure and a pinyon pine-juniper community closely approximated each other and were significantly greater (P = 0.10) than either a moderate continuous or a heavy continuous treatment in a grassland community. Sediment concentration from the heavy continuous treatment was more than twice that of the other treatments. Infiltration rates on the hillside site were highest in a pinyon pine-juniper community receiving short duration grazing. Infiltration for this treatment was found to be significantly higher (P = 0.10) than that of a short duration grazing treatment, but not from a rest rotation grazing treatment on grassland. The short duration grazing treatment on a grassland had the highest sediment concentration, while the rest rotation grazing on a grassland and the short duration pinyon pine-juniper treatments were found to be similar. In the bottomland site, a fertilized and unfertilized treatment showed no significant difference in infiltration or sediment concentration, although twice as many animals were present on the fertilized treatment. 相似文献
986.
William L. Jackson Robert L. Beschta 《Journal of the American Water Resources Association》1984,20(4):527-533
ABSTRACT A flume study was conducted to examine (1)changes in the particle-size distribution of sediments in riffles due to the proportion of sand in transport and the total rate of bedload transport at the time the riffle is deposited and (2) the effect of high sand transport rates on the stability of gravel riffles. The median particle size of sediment deposited in the riffle was larger than that of the sediment in transport. Small but significant (a = 0.05) decreases in the median particle size of riffle sediments resulted as the sand-to-gravel ratio. Increased concentrations of sand in transport caused previously stable gravel riffles to undergo scour. These results, in combination with information from other studies, suggest that an alluvial channel with pool-riffle sequences and with sand and gravel beds may respond to an increased delivery of sand by reducing form roughness. Form roughness can be reduced by degrading riffles and filling pools. Subsequent responses may be increases in width-to-depth ratio and slope. 相似文献
987.
Marine sediment toxicity tests are widely applied in monitoring programs, yet relatively little is known about the comparability of data from different laboratories. The need for comparability information is increased in cooperative monitoring programs, where multiple laboratories (often with variable skill levels) perform toxicity tests. An interlaboratory comparison exercise was conducted among seven laboratories in order to document the comparability of sediment toxicity measurements during the Bight'98 regional sediment survey in southern California. Sediments from four stations in Los Angeles and Long Beach Harbors were tested using a 10-day survival test of the amphipod Eohaustorius estuarius. All laboratories successfully performed the sediment test and associated reference toxicant test. Statistically significant differences were found in mean amphipod survival rates among some laboratories for the field-collected sediments, but there was little evidence of a consistent bias among laboratories. Although the reference toxicant test indicated a five-fold variation in test sensitivity among laboratories, these results were not accurate predictors of interlaboratory performance for the sediment tests. The laboratories demonstrated excellent concordance (Kendall's W = 0.91) in ranking the field-collected sediments by toxicity. Agreement on classifying the sediments into categories (nontoxic, moderately toxic, and highly toxic) based upon the percent of survival was best for highly toxic sediments. An analysis of test precision based upon the variance among replicates within a test indicated that the measured survival rate for a sample may vary by up to 12 percentage points from the actual response. 相似文献
988.
黄河(喇嘛湾段)沉积物中铜、铅、锌、镉的化学形态研究 总被引:14,自引:0,他引:14
建立了黄河 (清水河段 )沉积物中铜、铅、锌、镉的化学形态分析方法 ,系统地研究了它们在其有效态中的聚集能力及在各种粒径沉积物中的分布 相似文献
989.
Patchy distribution fields: sampling distance unit of a zigzag survey and reconstruction adequacy 总被引:1,自引:0,他引:1
Kalikhman I 《Environmental monitoring and assessment》2002,80(1):1-16
A mathematical model was used to examine the effects of choosingvarious units of sampling distance of a zigzag survey on the adequacy of reconstructing patchy distribution fields. The modelsimulates fish or plankton patches (or gaps) of different shapesand spatial orientations, and an acoustic survey by zigzag or parallel transects along which a unit of sampling distance is set. Adequacy of the reconstructed fields to those originally generated is evaluated by calculating their correlations (r). A priori information on the autocorrelation radii for the field in the directions of the survey (Rs) and perpendiculardirection (Rp) allows optimisation of the survey design andthe algorithm of data analysis. A field can be reconstructed properly (r2 > 0.70) if the distance between transects D < (1.0–1.5)Rs and the unit of sampling distance d < (1.0–1.5)Rp. A posteriori determination of patchorientation allows reconstruction of the best field attainable onthe basis of the survey data. In cases of field movement, if thedimension of patches in the direction of movement exceeds that ofa surveyed area, a survey in the opposite direction gives bestresults; in contrast, if the dimension of moving patches is smaller than that of a surveyed area, it is reasonable to carryout a survey in the same direction. The criterion remains validwhen a survey is carried out by zigzag transects and a unit ofsampling distance is set along them. The results obtained indicate that, for a fixed transect spacing and a given number of sampling points on each full transect, zigzag pattern allowsless adequate reconstruction of an original distribution field (in cases of both immovable and movable fields) than corresponding parallel pattern. 相似文献
990.
Almost any type of sample has some utilitywhen estimating population quantities. The focus inthis paper is to indicate what type or combination oftypes of sampling can be used in various situationsranging from a sample designed to establishcause-effect or legal challenge to one involving asimple subjective judgment. Several of these methodshave little or no utility in the scientific area buteven in the best of circumstances, particularlycomplex ones, both probabilistic and non-probabilisticprocedures have to be used because of lack ofknowledge and cost. We illustrate this with a marbledmurrelet example. 相似文献